Microorganism

Microorganism

Overview

A microorganism or microbe is a microscopic
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 that comprises either a single cell (unicellular), cell clusters, or no cell at all (acellular). The study of microorganisms is called microbiology
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...

, a subject that began with Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

's discovery of microorganisms in 1675, using a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 of his own design.

Microorganisms are very diverse; they include bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, fungi, archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, and protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s; microscopic plants (green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

); and animals
Micro-animals
Micro-animals are animals that are microscopic and thus cannot be seen with the naked eye. Mostly these microorganisms are multicellular but none are vertebrates. Microscopic arthropods include dust mites, and spider mites, while microscopic crustaceans include copepods and the cladocera...

 such as plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 and the planarian.
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Encyclopedia

A microorganism or microbe is a microscopic
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 that comprises either a single cell (unicellular), cell clusters, or no cell at all (acellular). The study of microorganisms is called microbiology
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...

, a subject that began with Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

's discovery of microorganisms in 1675, using a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 of his own design.

Microorganisms are very diverse; they include bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, fungi, archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, and protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s; microscopic plants (green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

); and animals
Micro-animals
Micro-animals are animals that are microscopic and thus cannot be seen with the naked eye. Mostly these microorganisms are multicellular but none are vertebrates. Microscopic arthropods include dust mites, and spider mites, while microscopic crustaceans include copepods and the cladocera...

 such as plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 and the planarian. Some microbiologists also include virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, but others consider these as non-living. Most microorganisms are unicellular  (single-celled), but this is not universal, since some multicellular organisms are microscopic
Microscopic
The microscopic scale is the scale of size or length used to describe objects smaller than those that can easily be seen by the naked eye and which require a lens or microscope to see them clearly.-History:...

, while some unicellular protists and bacteria, like Thiomargarita namibiensis
Thiomargarita namibiensis
Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, in general, wide, but sometimes up to...

, are macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...

 and visible to the naked eye.

Microorganisms live in all parts of the biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

 where there is liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, including soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, hot spring
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

s, on the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 floor, high in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 and deep inside rocks within the Earth's crust
Crust
Crust may refer to:* Crust * The Crust, television seriesPhysical sciences:* Crust , at least continent-wide structure* Soil crust, local biology-sensitive structureFood:* Crust, dense surface layer of bread...

. Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposer
Decomposer
Decomposers are organisms that break down dead or decaying organisms, and in doing so carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and...

s. As some microorganisms can fix nitrogen
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

, they are a vital part of the nitrogen cycle
Nitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...

, and recent studies indicate that airborne microbes may play a role in precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 and weather.

Microbes are also exploited by people in biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...

, both in traditional food and beverage preparation
Fermentation (food)
Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

, and in modern technologies based on genetic engineering
Genetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...

. However, pathogenic microbes are harmful, since they invade and grow within other organisms, causing disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

s that kill humans, other animals and plants.

Evolution



Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago. Further evolution was slow, and for about 3 billion years in the Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 eon, all organisms were microscopic. So, for most of the history of life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

 the only forms of life were microorganisms. Bacteria, algae and fungi have been identified in amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...

 that is 220 million years old, which shows that the morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 of microorganisms has changed little since the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period.

Most microorganisms can reproduce rapidly, but not as much as when the environment is cold. And microbes such as bacteria can also freely exchange genes by conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

, transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

 and transduction
Transduction (genetics)
Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...

 between widely-divergent species. This horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...

, coupled with a high mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

) to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of 'super-bugs
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...

' — pathogenic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that are resistant to modern antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s.

Pre-microbiology


The possibility that microorganisms exist was discussed for many centuries before their actual discovery in the 17th century. The existence of unseen microbiological life was postulated by Jainism
Jainism
Jainism is an Indian religion that prescribes a path of non-violence towards all living beings. Its philosophy and practice emphasize the necessity of self-effort to move the soul towards divine consciousness and liberation. Any soul that has conquered its own inner enemies and achieved the state...

 which is based on Mahavira
Mahavira
Mahāvīra is the name most commonly used to refer to the Indian sage Vardhamāna who established what are today considered to be the central tenets of Jainism. According to Jain tradition, he was the 24th and the last Tirthankara. In Tamil, he is referred to as Arukaṉ or Arukadevan...

’s teachings as early as 6th century BCE. Paul Dundas notes that Mahavira asserted existence of unseen microbiological creatures living in earth, water, air and fire. Jain scriptures also describe nigoda
Nigoda
In Jainism cosmology, the Nigoda is a realm existing in which the lowest forms of life reside in endless numbers, and without any hope of release by self-effort. Jain scriptures describe nigodas which are sub-microscopic creatures living in large clusters and having a very short life and are said...

s which are sub-microscopic creatures living in large clusters and having a very short life and are said to pervade each and every part of universe, even in tissues of plants and flesh of animals. However, the earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the Roman
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 scholar Marcus Terentius Varro
Marcus Terentius Varro
Marcus Terentius Varro was an ancient Roman scholar and writer. He is sometimes called Varro Reatinus to distinguish him from his younger contemporary Varro Atacinus.-Biography:...

 in a 1st century BC book titled On Agriculture in which he warns against locating a homestead near swamps:
In The Canon of Medicine
The Canon of Medicine
The Canon of Medicine is an encyclopedia of Galenic medicine in five books compiled by Ibn Sīnā and completed in 1025. It presents a clear and organized summary of all the medical knowledge of the time...

(1020), Abū Alī ibn Sīnā (Avicenna) hypothesized that tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

 and other diseases might be contagious

In 1546, Girolamo Fracastoro
Girolamo Fracastoro
Girolamo Fracastoro was an Italian physician, poet, and scholar in mathematics, geography and astronomy. Fracastoro subscribed to the philosophy of atomism, and rejected appeals to hidden causes in scientific investigation....

 proposed that epidemic
Epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...

 diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or even without contact over long distances.

All these early claims about the existence of microorganisms were speculative and were not based on any data or science. Microorganisms were neither proven, observed, nor correctly and accurately described until the 17th century. The reason for this was that all these early studies lacked the microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

.

History of microorganisms' discovery


Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

 was one of the first people to observe microorganisms, using a microscope of his own design, and made one of the most important contributions to biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

. Robert Hooke
Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

 was the first to use a microscope to observe living things; his 1665 book Micrographia
Micrographia
Micrographia is a historic book by Robert Hooke, detailing the then thirty year-old Hooke's observations through various lenses. Published in September 1665, the first major publication of the Royal Society, it was the first scientific best-seller, inspiring a wide public interest in the new...

contained descriptions of plant cells.

Before Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery why grapes could be turned into wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...

, milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

 into cheese
Cheese
Cheese is a generic term for a diverse group of milk-based food products. Cheese is produced throughout the world in wide-ranging flavors, textures, and forms....

, or why food would spoil. Leeuwenhoek did not make the connection between these processes and microorganisms, but using a microscope, he did establish that there were forms of life that were not visible to the naked eye. Leeuwenhoek's discovery, along with subsequent observations by Lazzaro Spallanzani
Lazzaro Spallanzani
Lazzaro Spallanzani was an Italian Catholic priest, biologist and physiologist who made important contributions to the experimental study of bodily functions, animal reproduction, and essentially discovered echolocation...

 and Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

, ended the long-held belief that life spontaneously appear
Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

ed from non-living substances during the process of spoilage.

Lazzaro Spallanzani found that boiling broth would sterilise
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...

 it and kill any microorganisms in it. He also found that new microorganisms could only settle in a broth if the broth was exposed to the air. Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 expanded upon Spallanzani's findings by exposing boiled broths to the air, in vessels that contained a filter to prevent all particles from passing through to the growth medium, and also in vessels with no filter at all, with air being admitted via a curved tube that would not allow dust particles to come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment. This meant that the living organisms that grew in such broths came from outside, as spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s on dust, rather than spontaneously generated within the broth. Thus, Pasteur dealt the death blow to the theory of spontaneous generation and supported germ theory
Germ theory of disease
The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases...

.

In 1876, Robert Koch
Robert Koch
Heinrich Hermann Robert Koch was a German physician. He became famous for isolating Bacillus anthracis , the Tuberculosis bacillus and the Vibrio cholerae and for his development of Koch's postulates....

 established that microbes can cause disease. He found that the blood of cattle who were infected with anthrax
Anthrax
Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...

 always had large numbers of Bacillus anthracis
Bacillus anthracis
Bacillus anthracis is the pathogen of the Anthrax acute disease. It is a Gram-positive, spore-forming, rod-shaped bacterium, with a width of 1-1.2µm and a length of 3-5µm. It can be grown in an ordinary nutrient medium under aerobic or anaerobic conditions.It is one of few bacteria known to...

. Koch found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, and this caused the healthy animal to become sick. He also found that he could grow the bacteria in a nutrient broth, then inject it into a healthy animal, and cause illness. Based on these experiments, he devised criteria for establishing a causal link between a microbe and a disease and these are now known as Koch's postulates
Koch's postulates
Koch's postulates are four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890...

. Although these postulates cannot be applied in all cases, they do retain historical importance to the development of scientific thought and are still being used today.

Classification and structure



Microorganisms can be found almost anywhere in the taxonomic
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 organization of life on the planet. Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

 are almost always microscopic, while a number of eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s are also microscopic, including most protists, some fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

, as well as some animals
Micro-animals
Micro-animals are animals that are microscopic and thus cannot be seen with the naked eye. Mostly these microorganisms are multicellular but none are vertebrates. Microscopic arthropods include dust mites, and spider mites, while microscopic crustaceans include copepods and the cladocera...

 and plants. Viruses are generally regarded as not living and therefore are not microbes, although the field of microbiology
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...

 also encompasses the study of viruses.

Prokaryotes


Prokaryotes are organisms that lack a cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 and the other membrane bound organelles. They are almost always unicellular, although some species such as myxobacteria
Myxobacteria
The myxobacteria are a group of bacteria that predominantly live in the soil. The myxobacteria have very large genomes, relative to other bacteria, e.g. 9-10 million nucleotides. Sorangium cellulosum has the largest known bacterial genome, at 13.0 million nucleotides...

 can aggregate into complex structures as part of their life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...

.

Consisting of two domains
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, the prokaryotes are the most diverse and abundant group of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and inhabit practically all environments where some liquid water is available and the temperature is below +140 °C. They are found in sea water, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, air
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, animals' gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

s, hot spring
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

s and even deep beneath the Earth's crust in rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s. Practically all surfaces which have not been specially sterilized are covered by prokaryotes. The number of prokaryotes on Earth is estimated to be around five million trillion trillion, or 5 × 1030, accounting for at least half the biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

 on Earth.

Bacteria




Bacteria are practically all invisible to the naked eye, with a few extremely rare exceptions, such as Thiomargarita namibiensis
Thiomargarita namibiensis
Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, in general, wide, but sometimes up to...

. They lack membrane-bound organelles, and can function and reproduce as individual cells, but often aggregate in multicellular colonies. Their genome is usually a single loop of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

, although they can also harbor small pieces of DNA called plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

s. These plasmids can be transferred between cells through bacterial conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

. Bacteria are surrounded by a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

, which provides strength and rigidity to their cells. They reproduce by binary fission or sometimes by budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...

, but do not undergo sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

. Some species form extraordinarily resilient spores
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...

, but for bacteria this is a mechanism for survival, not reproduction. Under optimal conditions bacteria can grow extremely rapidly and can double as quickly as every 10 minutes.

Archaea



Archaea are also single-celled organisms that lack nuclei. In the past, the differences between bacteria and archaea were not recognised and archaea were classified with bacteria as part of the kingdom Monera
Monera
Monera is a superseded kingdom that contains unicellular organisms without a nucleus , such as bacteria....

. However, in 1990 the microbiologist Carl Woese
Carl Woese
Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...

 proposed the three-domain system
Three-domain system
The three-domain system is a biological classification introduced by Carl Woese in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria...

 that divided living things into bacteria, archaea and eukaryotes. Archaea differ from bacteria in both their genetics and biochemistry. For example, while bacterial cell membranes are made from phosphoglycerides
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...

 with ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 bonds, archaean membranes are made of ether lipid
Ether lipid
Ether lipids are lipids in which one or more of the carbon atoms on glycerol is bonded to an alkyl chain via an ether linkage, as opposed to the usual ester linkage.-Types:...

s.

Archaea were originally described in extreme environments, such as hot spring
Hot spring
A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.-Definitions:...

s, but have since been found in all types of habitats. Only now are scientists beginning to realize how common archaea are in the environment, with crenarchaeota
Crenarchaeota
In taxonomy, the Crenarchaeota has been classified as either a phylum of the Archaea kingdom or a kingdom of its own...

 being the most common form of life in the ocean, dominating ecosystems below 150 m in depth. These organisms are also common in soil and play a vital role in ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 oxidation.

Eukaryotes


Most living things which are visible to the naked eye in their adult form are eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s, including human
Human
Humans are the only living species in the Homo genus...

s. However, a large number of eukaryotes are also microorganisms. Unlike bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, eukaryotes contain organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s such as the cell nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

, the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

 and mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

 in their cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

. The nucleus is an organelle which houses the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 that makes up a cell's genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

. DNA itself is arranged in complex chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s.
Mitochondria are organelles vital in metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 as they are the site of the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

 and oxidative phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...

. They evolved from symbiotic bacteria and retain a remnant genome. Like bacteria, plant cell
Plant cell
Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...

s have cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

s, and contain organelles such as chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s in addition to the organelles in other eukaryotes. Chloroplasts produce energy from light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, and were also originally symbiotic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

.

Unicellular eukaryotes are those eukaryotic organisms that consist of a single cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 throughout their life cycle. This qualification is significant since most multicellular
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

 eukaryotes consist of a single cell called a zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

 at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple cell nuclei
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 (see coenocyte
Coenocyte
A coenocyte is a multinucleate cell. It can result from multiple nuclear divisions without accompanying cell divisions, or from cellular aggregation followed by dissolution of the cell membranes inside the mass. Coenocytes are found in fungi and some protists, such as algae and slime mold. Some...

). However, not all microorganisms are unicellular as some microscopic eukaryotes are made from multiple cells.

Protists


Of eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify. Several algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 are multicellular protists, and slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms. The number of species of protozoa is uncertain, since we may have identified only a small proportion of the diversity in this group of organisms.


Animals



Mostly animals are multicellular, but some are too small to be seen by the naked eye. Microscopic arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s include dust mites and spider mite
Spider mite
Spider mites are members of the Acari family Tetranychidae, which includes about 1,200 species. They generally live on the under sides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed...

s. Microscopic crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s include copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s and the cladocera
Cladocera
Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently...

, while many nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s are too small to be seen with the naked eye. Another particularly common group of microscopic animals are the rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s, which are filter feeders that are usually found in fresh water. Micro-animals reproduce both sexually and asexually and may reach new habitats as eggs that survive harsh environments that would kill the adult animal. However, some simple animals, such as rotifers and nematodes, can dry out completely and remain dormant for long periods of time.

Fungi



The fungi have several unicellular species, such as baker's yeast (Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

) and fission yeast (Schizosaccharomyces pombe
Schizosaccharomyces pombe
Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length...

). Some fungi, such as the pathogenic yeast Candida albicans
Candida albicans
Candida albicans is a diploid fungus that grows both as yeast and filamentous cells and a causal agent of opportunistic oral and genital infections in humans. Systemic fungal infections including those by C...

, can undergo phenotypic switching
Phenotypic switching
Phenotypic switching is switching between two cell-types. An example is Candida albicans, which, when it infects host tissue, switches from the usual unicellular yeast-like form of into an invasive, multicellular filamentous form. This switching between two cell-types is known as...

 and grow as single cells in some environments, and filamentous hyphae
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

 in others. Fungi reproduce both asexually, by budding or binary fission, as well by producing spores, which are called conidia
Conidium
Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...

 when produced asexually, or basidiospore
Basidiospore
A basidiospore is a reproductive spore produced by Basidiomycete fungi. Basidiospores typically each contain one haploid nucleus that is the product of meiosis, and they are produced by specialized fungal cells called basidia. In grills under a cap of one common species in the phylum of...

s when produced sexually.

Plants



The green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

 are a large group of photosynthetic eukaryotes that include many microscopic organisms. Although some green algae are classified as protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s, others such as charophyta
Charophyta
The Charophyta are a division of green algae, including the closest relatives of the embryophyte plants. In some groups, such as conjugating green algae, flagellate cells do not occur. The latter group does engage in sexual reproduction, and motility does not involve flagella, since they are...

 are classified with embryophyte
Embryophyte
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plants. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees,...

 plants, which are the most familiar group of land plants. Algae can grow as single cells, or in long chains of cells. The green algae include unicellular and colonial flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...

s, usually but not always with two flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

 per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales
Charales
Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus Chara in 1753.-Description:...

, which are the algae most closely related to higher plants, cells differentiate into several distinct tissues within the organism. There are about 6000 species of green algae.

Habitats and ecology


Microorganisms are found in almost every habitat
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...

 present in nature. Even in hostile environments such as the poles
Geographical pole
A geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...

, desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s, geyser
Geyser
A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The word geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb geysa, "to gush", the verb...

s, rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s, and the deep sea
Deep sea
The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter...

. Some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Extremophiles have been isolated from rocks as much as 7 kilometres below the Earth's surface, and it has been suggested that the amount of living organisms below the Earth's surface may be comparable with the amount of life on or above the surface. Extremophiles have been known to survive for a prolonged time in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

, and can be highly resistant to radiation, which may even allow them to survive in space. Many types of microorganisms have intimate symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 organism (parasitism
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

). If microorganisms can cause disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 in a host they are known as pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s.

Extremophiles


Extremophiles are microorganisms which have adapted so that they can survive and even thrive in conditions that are normally fatal to most life-forms. For example, some species have been found in the following extreme environments:
  • Temperature
    Temperature
    Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

    : as high as 130 °C (266 °F), as low as -17 °C
  • Acid
    Acid
    An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

    ity/alkalinity
    Alkalinity
    Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...

    : less than pH
    PH
    In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

     0, up to pH 11.5
  • Salinity
    Salinity
    Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

    : up to saturation
  • Pressure
    Pressure
    Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

    : up to 1,000-2,000 atm
    Atmosphere (unit)
    The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

    , down to 0 atm (e.g. vacuum
    Vacuum
    In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

     of space
    Outer space
    Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

    )
  • Radiation
    Ionizing radiation
    Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

    : up to 5kGy
    Gray (unit)
    The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....



Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

, crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 and atmosphere, their specific evolutionary adaptation mechanisms to their extreme environment can be exploited in bio-technology, and their very existence under such extreme conditions increases the potential for extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

.

Soil microbes


The nitrogen cycle
Nitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...

 in soils depends on the fixation of atmospheric nitrogen
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

. One way this can occur is in the nodules in the roots of legumes that contain symbiotic bacteria of the genera Rhizobium
Rhizobium
Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes and Parasponia....

, Mesorhizobium
Mesorhizobium
Mesorhizobium is a genus of Gram-negative soil bacteria. At least one, the nitrogen fixing species, Mesorhizobium loti, forms symbiotic root nodules with plants in the genus Lotus. Strain MAFF303099 of M. loti has been fully sequenced....

, Sinorhizobium
Sinorhizobium
Sinorhizobium/Ensifer is a genus of nitrogen-fixing bacteria , two of which have been sequenced.-Etymology:...

, Bradyrhizobium
Bradyrhizobium
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen they must use nitrogen compounds such as nitrates....

, and Azorhizobium
Azorhizobium
Azorhizobium is a genus of Gram-negative soil bacteria. They fix nitrogen in symbiosis with plants in the genus Sesbania. Strain ORS571 of A. caulinodans has been fully sequenced....

.

Symbiotic microbes


Symbiotic microbes such as fungi and algae form an association in lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

. Certain fungi form mycorrhiza
Mycorrhiza
A mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant....

l symbioses with trees that increase the supply of nutrients to the tree.

Importance


Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

 and nitrogen cycle
Nitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...

, as well as fulfilling other vital roles in virtually all ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s, such as recycling other organisms' dead remains and waste products through decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...

. Microbes also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of Biosphere 2
Biosphere 2
Biosphere 2 is a structure originally built to be an artificial, materially-closed ecological system in Oracle, Arizona by Space Biosphere Ventures, a joint venture whose principal officers were John P. Allen, inventor and Executive Director, and Margret Augustine, CEO...

 on an improper balance of microbes.

Use in food



Microorganisms are used in brewing
Brewing
Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...

, winemaking
Winemaking
Winemaking, or vinification, is the production of wine, starting with selection of the grapes or other produce and ending with bottling the finished wine. Although most wine is made from grapes, it may also be made from other fruit or non-toxic plant material...

, baking
Baking
Baking is the technique of prolonged cooking of food by dry heat acting by convection, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones. It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, quiches, cookies and crackers. Such items...

, pickling
Pickling
Pickling, also known as brining or corning is the process of preserving food by anaerobic fermentation in brine to produce lactic acid, or marinating and storing it in an acid solution, usually vinegar . The resulting food is called a pickle. This procedure gives the food a salty or sour taste...

 and other food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

-making processes.

They are also used to control the fermentation
Fermentation (food)
Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

 process in the production of cultured dairy product
Dairy product
Dairy products are generally defined as foods produced from cow's or domestic buffalo's milk. They are usually high-energy-yielding food products. A production plant for such processing is called a dairy or a dairy factory. Raw milk for processing comes mainly from cows, and, to a lesser extent,...

s such as yogurt and cheese
Cheese
Cheese is a generic term for a diverse group of milk-based food products. Cheese is produced throughout the world in wide-ranging flavors, textures, and forms....

. The cultures also provide flavour and aroma, and inhibit undesirable organisms.

Use in water treatment


Specially-cultured microbes are used in the biological treatment of sewage and industrial waste effluent, a process known as bioaugmentation
Bioaugmentation
Bioaugmentation is the introduction of a group of natural microbial strains or a genetically engineered variant to treat contaminated soil or water....

.

Use in energy



Microbes are used in fermentation to produce ethanol, and in biogas
Biogas
Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...

 reactors to produce methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

. Scientists are researching the use of algae to produce liquid fuels
Algae fuel
Algae fuel might be an alternative to fossil fuel and uses algae as its source of natural deposits. Several companies and government agencies are funding efforts to reduce capital and operating costs and make algae fuel production commercially viable...

, and bacteria to convert various forms of agricultural and urban waste into usable fuels
Cellulosic ethanol
Cellulosic ethanol is a biofuel produced from wood, grasses, or the non-edible parts of plants.It is a type of biofuel produced from lignocellulose, a structural material that comprises much of the mass of plants. Lignocellulose is composed mainly of cellulose, hemicellulose and lignin...

.

Use in science


Microbes are also essential tools in biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...

, biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

, and molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

. The yeasts (Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

) and fission yeast (Schizosaccharomyces pombe
Schizosaccharomyces pombe
Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length...

) are important model organism
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...

s in science, since they are simple eukaryotes that can be grown rapidly in large numbers and are easily manipulated. They are particularly valuable in genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

, genomics
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...

 and proteomics
Proteomics
Proteomics is the large-scale study of proteins, particularly their structures and functions. Proteins are vital parts of living organisms, as they are the main components of the physiological metabolic pathways of cells. The term "proteomics" was first coined in 1997 to make an analogy with...

. Microbes can be harnessed for uses such as creating steroids and treating skin diseases. Scientists are also considering using microbes for living fuel cells, and as a solution for pollution.

Use in warfare



In the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, diseased corpses were thrown into castles during siege
Siege
A siege is a military blockade of a city or fortress with the intent of conquering by attrition or assault. The term derives from sedere, Latin for "to sit". Generally speaking, siege warfare is a form of constant, low intensity conflict characterized by one party holding a strong, static...

s using catapults or other siege engine
Siege engine
A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and...

s. Individuals near the corpses were exposed to the deadly pathogen and were likely to spread that pathogen to others.

Human digestion



Microorganisms can form an endosymbiotic
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...

 relationship with other, larger organisms. For example, the bacteria that live within the human digestive system contribute to gut immunity, synthesise vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s such as folic acid
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

 and biotin
Biotin
Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...

, and ferment complex indigestible carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s.

Diseases and immunology



Microorganisms are the cause of many infectious diseases. The organisms involved include pathogenic bacteria
Pathogenic bacteria
Pathogenic bacteria are bacteria that cause bacterial infection. This article deals with human pathogenic bacteria.Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic...

, causing diseases such as plague
Bubonic plague
Plague is a deadly infectious disease that is caused by the enterobacteria Yersinia pestis, named after the French-Swiss bacteriologist Alexandre Yersin. Primarily carried by rodents and spread to humans via fleas, the disease is notorious throughout history, due to the unrivaled scale of death...

, tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

 and anthrax
Anthrax
Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...

; protozoa, causing diseases such as malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, sleeping sickness and toxoplasmosis
Toxoplasmosis
Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-blooded animals, including humans, but the primary host is the felid family. Animals are infected by eating infected meat, by ingestion of feces of a cat that has itself...

; and also fungi causing diseases such as ringworm, candidiasis
Candidiasis
Thrush redirects here. For the hoof infection see Thrush .Candidiasis or thrush is a fungal infection of any of the Candida species , of which Candida albicans is the most common...

 or histoplasmosis
Histoplasmosis
Histoplasmosis is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease primarily affects the lungs...

. However, other diseases such as influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

, yellow fever
Yellow fever
Yellow fever is an acute viral hemorrhagic disease. The virus is a 40 to 50 nm enveloped RNA virus with positive sense of the Flaviviridae family....

 or AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

 are caused by pathogenic viruses, which are not usually classified as living organisms and are not therefore microorganisms by the strict definition. , no clear examples of archaean pathogens are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease
Periodontal disease
Periodontitis is a set of inflammatory diseases affecting the periodontium, i.e., the tissues that surround and support the teeth. Periodontitis involves progressive loss of the alveolar bone around the teeth, and if left untreated, can lead to the loosening and subsequent loss of teeth...

.

Importance in ecology



Microbes are critical to the processes of decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...

 required to cycle nitrogen and other elements back to the natural world.

Hygiene


Hygiene is the avoidance of infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 or food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 spoiling by eliminating microorganisms from the surroundings. As microorganisms, particularly bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, are found practically everywhere, this means in most cases the reduction of harmful microorganisms to acceptable levels. However, in some cases it is required that an object or substance be completely sterile, i.e. devoid of all living entities and virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es. A good example of this is a hypodermic needle
Hypodermic needle
A hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body or extract fluids from it...

.

In food preparation microorganisms are reduced by preservation methods (such as the addition of vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

), clean utensils used in preparation, short storage periods or by cool temperatures. If complete sterility is needed, the two most common methods are irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

 and the use of an autoclave
Autoclave
An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, although a precursor known as the...

, which resembles a pressure cooker.

There are several methods for investigating the level of hygiene in a sample of food, drinking water, equipment etc. Water samples can be filtrated through an extremely fine filter. This filter is then placed in a nutrient medium. Microorganisms on the filter then grow to form a visible colony. Harmful microorganisms can be detected in food by placing a sample in a nutrient broth designed to enrich the organisms in question. Various methods, such as selective media or PCR, can then be used for detection. The hygiene of hard surfaces, such as cooking pots, can be tested by touching them with a solid piece of nutrient medium and then allowing the microorganisms to grow on it.

There are no conditions where all microorganisms would grow, and therefore often several different methods are needed. For example, a food sample might be analyzed on three different nutrient mediums designed to indicate the presence of "total" bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 (conditions where many, but not all, bacteria grow), mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

s (conditions where the growth of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 is prevented by e.g. antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s) and coliform
Coliform Index
The coliform index is a rating of the purity of water based on a count of fecal bacteria. Coliform bacteria are microorganisms that primarily originate in the intestines of warm-blooded animals...

 bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 (these indicate a sewage contamination).

See also

  • Bacterium
  • Biological warfare
    Biological warfare
    Biological warfare is the use of biological toxins or infectious agents such as bacteria, viruses, and fungi with intent to kill or incapacitate humans, animals or plants as an act of war...

  • Culture collection
  • Cyanobacteria
  • Fungi
  • Microbial intelligence
    Microbial Intelligence
    Microbial intelligence is the intelligence shown by microorganisms. The concept encompasses complex adaptive behaviour shown by single cells, and altruistic and/or cooperative behavior in populations of like or unlike cells mediated by chemical signalling that induces physiological or behavioral...

  • Nanobacterium
    Nanobacterium
    Nanobacterium is the unit or member name of a proposed class of living organisms, specifically cell-walled microorganisms with a size much smaller than the generally accepted lower limit size for life...

  • Petri dish
    Petri dish
    A Petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...

  • Prokaryote
    Prokaryote
    The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

  • Protozoa
    Protozoa
    Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

  • Soil contamination
    Soil contamination
    Soil contamination or soil pollution is caused by the presence of xenobiotic chemicals or other alteration in the natural soil environment....

  • Staining
    Staining (biology)
    Staining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image. Stains and dyes are frequently used in biology and medicine to highlight structures in biological tissues for viewing, often with the aid of different microscopes...

  • Virus
    Virus
    A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...


External links