A
microorganism (from the ,
mikrós, "small" and ,
organismós, "organism"; also spelled
micro organism or
micro-organism) or
microbe is an
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
that is
microscopicMicroscopic or Micro is a term 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:...
(usually too small to be seen by the naked human eye). The study of microorganisms is called
microbiologyMicrobiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryotes such as fungi and protists, and prokaryotes. Viruses, though not strictly classed as living organisms, are also studied...
, a subject that began with
Anton van LeeuwenhoekAntonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, the 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
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
of his own design.
Microorganisms are very diverse; they include
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, fungi,
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, and
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s; microscopic plants (called
green algaeThe 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
animalsMicro-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
planktonPlankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification...
and the planarian. Some microbiologists also include
virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . 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
microscopicMicroscopic or Micro is a term 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 namibiensisThiomargarita 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, generally wide, but sometimes up to ....
, are
macroscopicMacroscopic is a word commonly used to describe physical objects that are measurable and observable by the naked eye.When applied to phenomena and abstract objects, it describes existence in the world as we perceive it, often in contrast to experiences or theories considering objects of geometric...
and visible to the naked eye.
Microorganisms live in all parts of the
biosphereThe biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth. From the broadest biophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements...
where there is liquid
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, including
soilSoil 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. It is composed of particles of broken rock that have been altered by chemical and...
,
hot springA hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas.-Definitions:...
s, on the
oceanAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% 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
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
and deep inside rocks within the Earth's
crustCrust may refer to:Geology and soil science:*Crust , the outer solid layer of a planet*Continental crust*Oceanic crust*Soil crust*the dough or pastry shell of pies, pizzas, etc.Music:*Crust punk, a hardcore punk / extreme metal fusion...
. Microorganisms are critical to nutrient recycling in ecosystems as they act as
decomposerDecomposers are organisms that eat the dead or decaying organisms, and in doing so carry out the natural process of decomposition. The 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 nitrogenNitrogen fixation usually refers to the biological process by which nitrogen in the atmosphere is converted into ammonia. This process is essential for life because fixed nitrogen is required to biosynthesize a basic building block of life, e.g. nucleotides for DNA and amino acids for proteins...
, they are a vital part of the
nitrogen cycleNitrogen is the most abundant element in the atmosphere and is essential to all life. The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a cycle that includes gaseous components.Earth's atmosphere is...
, and recent studies indicate that airborne microbes may play a role in
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
and weather.
Microbes are also exploited by people in
biotechnologyBiotechnology is technology based on biology, agriculture, food science, and medicine. Modern use of the term usually refers to genetic engineering as well as cell- and tissue culture technologies...
, both in traditional
food and beverage preparationFermentation 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. A more restricted definition of fermentation is the chemical conversion of sugars into ethanol...
, and in modern technologies based on
genetic engineeringGenetic engineering, recombinant DNA technology, genetic modification/manipulation and gene splicing are terms that apply to the direct manipulation of an organism's genes. Genetic engineering is different from traditional breeding, where the organism's genes are manipulated indirectly...
. However,
pathogenic microbesPathogenic microbes are microbes that are pathogens and thus cause infectious diseases. This article is dedicated to human pathogenic microbes....
are harmful, since they invade and grow within other organisms, causing
diseaseA disease or medical condition isan abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs...
s that kill millions of people, other animals, and plants.
Evolution
Single-celled microorganisms were the first forms of life to develop on Earth, approximately
3–4 billion years agoTo help compare orders of magnitude of different times this page lists times between 1017 seconds and 1018 seconds See also times of other orders of magnitude.* Shorter times...
. Further evolution was slow, and for about 3 billion years in the
PrecambrianThe Precambrian is an informal name for the span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic timescale. It spans from the formation of Earth around 4500 Ma to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first...
eon, all organisms were microscopic. So, for most of the history of
life on EarthLife on Earth: A Natural History by David Attenborough is a groundbreaking 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
amberAmber is fossilized tree resin , which has been appreciated for its color and natural beauty since neolithic times. Good quality amber is used for the manufacture of ornamental objects and jewelry...
that is 220 million years old, which shows that the
morphologyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
of microorganisms has changed little since the
triassicThe Triassic is a geologic period and system that extends from about 251 to 199 Ma . 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 and microbes such as bacteria can also freely exchange genes by
conjugationBacterial conjugation is the transfer of genetic material between bacteria through direct cell-to-cell contact, or through a bridge-like connection between the two cells...
,
transformationIn molecular biology, transformation is the genetic alteration of a cell resulting from the uptake, genomic incorporation, and expression of foreign genetic material ....
and
transductionTransduction 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...
between widely-divergent species. This
horizontal gene transferHorizontal 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. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g...
, coupled with a high
mutationIn biology, a mutation is a randomly derived change to the nucleotide sequence of the genetic material of an organism.Mutations can be caused by copying errors in the genetic material during cell division, or by exposure to mutagens , or can be induced by the organism itself, by cellular processes...
rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
) 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-bugsAntibiotic resistance is the ability of a microorganism to withstand the effects of antibiotics. It is a specific type of drug resistance. Antibiotic resistance evolves via natural selection acting upon random mutation, but it can also be engineered by applying an evolutionary stress on a population...
' — pathogenic
bacteriaThe bacteria are a large group of unicellular 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
antibioticIn common usage, an antibiotic is a substance or compound that kills or inhibits the growth of bacteria...
s.
Pre-Microbiology
The possibility that microorganisms exist was discussed for many centuries before their actual discovery in the 17th century. The first ideas about microorganisms were those of the
RomanAncient Rome was a civilization that grew out of a small agricultural community founded on the Italian Peninsula as early as the 10th century BC. Located along the Mediterranean Sea, it became one of the largest empires in the ancient world....
scholar
Marcus Terentius VarroMarcus Terentius Varro , also known as Varro Reatinus to distinguish him from his younger contemporary Varro Atacinus, was a Roman scholar and writer.-Biography:...
in a 1st century BC book titled
On Agriculture in which he warns against locating a homestead near swamps:
This passage seems to indicate that the ancients were aware of the possibility that diseases could be spread by yet unseen organisms.
In
The Canon of MedicineThe Canon of Medicine is a 14-volume medical encyclopedia written by Islamic scientist and physician Ibn Sīnā...
(1020), Abū Alī ibn Sīnā (Avicenna) stated that bodily
secretionSecretion is the process of elaborating, releasing, and oozing chemicals from a cell, a secreted chemical substance or amount of substance. In contrast to excretion, the substance may have a certain function, rather than being a waste product....
is contaminated by foul foreign earthly bodies before being infected. He also hypothesized that
tuberculosisTuberculosis is a common and often deadly infectious disease caused by mycobacteria...
and other diseases might be contagious,
i.e. that they were
infectious diseaseAn infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including pathogenic viruses, pathogenic bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions...
s, and used
quarantineQuarantine is voluntary or compulsory isolation, typically to contain the spread of something considered dangerous, often but not always disease...
to limit their spread.
When the
Black DeathThe Black Death was one of the deadliest pandemics in human history, peaking in Europe between 1348 and 1350. It is widely thought to have been an outbreak of bubonic plague caused by the bacterium Yersinia pestis, but this view has recently been challenged...
bubonic plaguePlague is a deadly infectious disease caused by the enterobacteria Yersinia pestis . Plague is a zoonotic, primarily carried by rodents and spread to humans via fleas. Plague is notorious throughout history, due to the unrivaled scale of death and devastation it brought...
reached
AndalusiaAl-Andalus was the Arabic name given to the parts of the Iberian Peninsula and Septimania governed by Arab and North African Muslims , at various times in the period between 711 and 1492....
in Spain, in the 14th century, Ibn Khatima wrote that infectious diseases were caused by contagious "minute bodies" that enter the human body. Later, in 1546,
Girolamo FracastoroGirolamo Fracastoro was an Venetian physician, scholar , poet and atomist....
proposed that
epidemicDefining an epidemic can be subjective, depending in part on what is "expected". An epidemic may be restricted to one locale , more general or even global...
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
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
.
History of microorganisms' discovery
Anton van LeeuwenhoekAntonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, the Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...
was the one of the first people to observe microorganisms, and used a microscope of his own design, and made one of the most important contributions to
biologyBiology is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy...
.
Robert HookeRobert Hooke, FRS was an English natural philosopher, architect and polymath who played an important role in the scientific revolution, through both experimental and theoretical work....
was the first to use a microscope to observe living things; his 1665 book
MicrographiaMicrographia is a historical book by Robert Hooke, detailing the then twenty-eight year-old Hooke's observations through various lenses. Published in September 1665, it was an immediate best-seller.-Observations:...
contained descriptions of plant cells.
Before Leeuwenhoek's discovery of microorganisms in 1675, it had been a mystery why grapes could be turned into
wineWine is an alcoholic beverage typically made of fermented grape juice. The natural chemical balance of grapes is such that they can ferment without the addition of sugars, acids, enzymes or other nutrients. Wine is produced by fermenting crushed grapes using various types of yeast. Yeast consumes...
,
milkMilk is an opaque white liquid produced by the mammary glands of mammals. It provides the primary source of nutrition for young mammals before they are able to digest other types of food. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce...
into
cheeseCheese is a food consisting of proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. It is produced by coagulation of the milk protein casein. Typically, the milk is acidified and addition of the enzyme rennet causes coagulation. The solids are separated and pressed into...
, 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 SpallanzaniLazzaro Spallanzani was an Italian biologist and physiologist who made important contributions to the experimental study of bodily functions, animal reproduction, and essentially discovered echolocation...
and
Louis PasteurLouis Pasteur was a French chemist and microbiologist born in Dole. He is best known for his remarkable breakthroughs in the causes and preventions of disease. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies. His experiments supported the germ...
, ended the long-held belief that life
spontaneously appearIn the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
ed from non-living substances during the process of spoilage.
Lazzaro Spallanzani found that boiling broth would
steriliseSterilization refers to any process that effectively kills or eliminates transmissible agents from a surface, equipment, article of food or medication, or biological culture medium. Sterilization does not, however, remove prions...
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 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
sporeIn biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions...
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 theoryThe 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 KochHeinrich Hermann Robert Koch was a German physician. He became famous for isolating Bacillus anthracis , the Tuberculosis bacillus and the Vibrio cholera 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
anthraxAnthrax is an acute disease caused by Bacillus anthracis. It affects both humans and animals. Most forms of the disease are highly lethal...
always had large numbers of
Bacillus anthracisBacillus anthracis 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 the only bacterium with a protein capsule , and the only pathogenic bacteria 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 postulatesKoch'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...
. Alhough 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
taxonomicTaxonomy is the practice and science of classification. The word finds its roots in the Greek , taxis and , nomos...
organization of life on the planet.
BacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
are almost always microscopic, while a number of
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s are also microscopic, including most protists, some
fungiA fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants, animals and bacteria...
, as well as some
animalsMicro-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
microbiologyMicrobiology is the study of microorganisms, which are unicellular or cell-cluster microscopic organisms. This includes eukaryotes such as fungi and protists, and prokaryotes. Viruses, though not strictly classed as living organisms, are also studied...
also encompasses the study of viruses.
Prokaryotes
Prokaryotes are organisms that lack a
cell nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", 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...
and the other membrane bound organelles. They are almost always unicellular, although some species such as
myxobacteriaThe 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 cycleA 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
domainsIn 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...
,
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, the prokaryotes are the most diverse and abundant group of
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s on
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and inhabit practically all environments where some liquid water is available and the temperature is below +140 °C. They are found in sea water,
soilSoil 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. It is composed of particles of broken rock that have been altered by chemical and...
,
airThe Earth's atmosphere 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 tractThe human gastrointestinal tract , digestive tract, guts or gut is the system of organs within humans that takes in food, digests it to extract energy and nutrients, and expels the remaining matter...
s,
hot springA hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas.-Definitions:...
s and even deep beneath the Earth's crust in
rockIn geology, rock 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 × 10
30.
Bacteria
Bacteria are practically all invisible to the naked eye, with a few extremely rare exceptions, such as
Thiomargarita namibiensisThiomargarita 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, generally wide, but sometimes up to ....
. They are unicellular organisms and lack membrane-bound organelles. Their genome is usually a single loop of
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
, although they can also harbor small pieces of DNA called
plasmidA plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA. In many cases, it is circular and double-stranded...
s. These plasmids can be transferred between cells through
bacterial conjugationBacterial conjugation is the transfer of genetic material between bacteria through direct cell-to-cell contact, or through a bridge-like connection between the two cells...
. Bacteria are surrounded by a
cell wallA cell wall is a tough, flexible and 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 act as a...
, which provides strength and rigidity to their cells. They reproduce by
binary fissionBinary fission, or prokaryotic fission, is the form of asexual reproduction and cell division used by all prokaryotic and some eukaryotic organisms...
or sometimes by
buddingBudding is a form of asexual reproduction.The new organism is naturally genetically identical to the primary one . When yeast buds, one cell becomes two cells.Budding is also seen in hydra . In budding parent organism survives with offsprings unlike in fission. When a sponge buds, a part of the...
, but do not undergo
sexual reproductionSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
. Some species form extraordinarily resilient
sporesAn endospore is a dormant, tough, and non-reproductive structure produced by Gram-positive bacteria from the Firmicute phylum that forms when a bacterium produces a thick internal wall that encloses its DNA and part of its cytoplasm...
, but for
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
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[[br>
While the Monera were briefly understood to be one of [[Kingdom #Five kingdoms|five biological kingdoms]], they were soon understood to comprise two kingdoms: the eubacteria and the archaebacteria . The Monera kingdom included most organisms with a [[prokaryote|prokaryotic]] [[cell |cell]]...
. However, in 1990 the microbiologist
Carl WoeseCarl 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...
proposed the
three-domain systemThe three-domain system is a biological classification introduced by Carl Woese in 1990 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
phosphoglyceridesPhospholipids are a class of lipids and 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
esterEsters 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 lipidEther 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 springA hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas.-Definitions:...
s, but have since been found in all types of habitats. Only now are scientists beginning to appreciate how common archaea are in the environment, with
crenarchaeotaIn taxonomy, the Crenarchaeota are a phylum of the Archaea. Initially, the Crenarchaeota were thought to be extremophiles but recent studies have identified them as the most abundant archaea in the marine environment...
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
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
oxidation.
Eukaryotes
thumb
Most living things which are visible to the naked eye in their adult form are
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
s, including
humanHumans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving...
s. However, a large number of eukaryotes are also microorganisms. Unlike
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, eukaryotes contain
organelleIn 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 membrane....
s such as the
cell nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", 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...
, the
Golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells...
and
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
in their
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, 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
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
that makes up a cell's
genomeIn modern molecular biology the genome refers to all of its hereditary information encoded in DNA .The genome includes both the genes and the non-coding sequences of the DNA. The term was adapted in 1920 by Hans Winkler, Professor of Botany at the University of Hamburg, Germany...
. DNA itself is arranged in complex
chromosomeA chromosome is an organized structure of DNA and protein that is 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...
s.
Mitochondria are organelles vital in
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
as they are the site of the
citric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or more rarely, the Szent-Györgyi-Krebs cycle — is a series of enzyme-catalysed chemical reactions, which is of central importance in all living cells that use oxygen as part of cellular respiration...
and
oxidative phosphorylationOxidative 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 carry out oxidative phosphorylation to produce ATP, the molecule that...
. They evolved from symbiotic bacteria and retain a remnant genome. Like bacteria,
plant cellPlant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s have
cell wallA cell wall is a tough, flexible and 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 act as a...
s, and contain organelles such as
chloroplastChloroplasts 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.The word chloroplast is...
s in addition to the organelles in other eukaryotes. Chloroplasts produce energy from
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
by
photosynthesisPhotosynthesis is a 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...
, and were also originally symbiotic
bacteriaThe bacteria are a large group of unicellular 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
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, 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
multicellularMulticellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions in the organism...
eukaryotes consist of a single cell called a
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
at the beginning of their life cycles. Microbial eukaryotes can be either haploid or diploid, and some organisms have multiple
cell nucleiIn cell biology, the nucleus , also sometimes referred to as the "control center", 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...
(see
coenocyteA 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...
). However, not all microorganisms are unicellular as some microscopic eukaryotes are made from multiple cells.
Protists
Of
eukaryoticA eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried...
groups, the protists are most commonly unicellular and microscopic. This is a highly diverse group of organisms that are not easy to classify. Several
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
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
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s include dust mites and
spider miteSpider mites are members of the Acari family Tetranychidae, which includes about 1600 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
crustaceanCrustaceans are a very large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles...
s include
copepodCopepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic , but more 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
cladoceraCladocera or cladocerans are small crustaceans commonly called water fleas, part of the Class Branchiopoda. They form a monophyletic group, which is currently divided into four suborders, 14 families, 87 genera, and about 400 species...
, while many
nematodeThe "roundworms" or "nematodes" are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasitic. It has been estimated that the total number of...
s are too small to be seen with the naked eye. Another particularly common group of microscopic animals are the
rotiferThe 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 cerevisiaeSaccharomyces cerevisiae is a species of budding yeast. It is perhaps the most useful yeast owing to its use since ancient times in baking and brewing. It is believed that it was originally isolated from the skins of grapes...
) and fission yeast (
Schizosaccharomyces pombeSchizosaccharomyces 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 albicansCandida albicans is a diploid fungus and a causal agent of opportunistic oral and genital infections in humans. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients...
, can undergo
phenotypic switchingPhenotypic 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...
and grow as single cells in some environments, and
filamentous hyphaeA hypha is a long, branching filamentous cell 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 more...
in others. Fungi reproduce both asexually, by budding or binary fission, as well by producing spores, which are called
conidiaConidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus; they are also called mitospores due to the way they are generated through the cellular process of mitosis...
when produced asexually, or
basidiosporeA 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...
s when produced sexually.
Plants
The
green algaeThe 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
protistProtists , are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista but this group is no longer recognized in modern taxonomy...
s, others such as
charophytaThe 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
embryophyteThe embryophytes are the most familiar group of plants. They are often 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, flowers, ferns, mosses, and various other green land...
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
flagellateFlagellates are cells with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do...
s, usually but not always with two
flagellaA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
per cell, as well as various colonial, coccoid, and filamentous forms. In the
CharalesCharales 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 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
habitatA habitat is an ecological or environmental area that is inhabited by a particular animal and plant species...
present in nature. Even in hostile environments such as the
polesA 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...
,
desertA desert is a landscape or region that receives almost no precipitation. Deserts are defined as areas with an average annual precipitation of less than per year, or as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen climate classification system,...
s,
geyserA geyser is a hot spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase . The name geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb gjósa, "to gush".The...
s,
rockIn geology, rock 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 seaThe deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms or more. Little or no light penetrates this area of the ocean, and most of its organisms rely on falling organic matter produced in the photic zone for subsistence...
. 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
vacuumIn 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," but in reality, no volume of space can ever be perfectly empty...
, and can be highly resistant to radiation, which may even allow them to survive in space. Many types of microorganisms have intimate
symbioticThe term symbosis commonly describes close and often long-term interactions between different biological species...
relationships with other larger organisms; some of which are mutually beneficial (
mutualismMutualism is a biological interaction between two organisms, where each individual derives a fitness benefit . Similar interactions within a species are known as co-operation...
), while others can be damaging to the
hostIn 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 (
parasitismParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host....
). If microorganisms can cause
diseaseA disease or medical condition isan abnormal condition of an organism that impairs bodily functions, associated with specific symptoms and signs...
in a host they are known as
pathogenA pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its 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
In physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
: as high as , as low as
- Acid
An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...
ity/alkalinityAlkalinity or AT is a measure of the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. Alkalinity is closely related to the acid neutralizing capacity of a solution and ANC is often incorrectly used to refer to alkalinity.The alkalinity is...
: less than pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
0, up to pH 11.5
- 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 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 atmThe standard atmosphere is an international reference pressure defined as 101,325 Pa and formerly used as unit of pressure . For practical purposes it has been replaced by the bar which is 100,000 Pa...
, down to 0 atm (e.g. vacuumIn 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," but in reality, no volume of space can ever be perfectly empty...
of spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
)
- Radiation
Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number...
: up to 5kGyThe gray is the SI unit of absorbed radiation dose due to ionizing radiation .-Definition:One gray is the absorption of one joule of energy, in the form of ionizing radiation, by one kilogram of matter....
Extremophiles are significant in different ways. They extend terrestrial life into much of the Earth's
hydrosphereA hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....
,
crustIn geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in...
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 lifeExtraterrestrial life is defined as life which does not originate from planet Earth. The existence of life outside the planet is theoretical and all assertions of such life remain disputed....
.
Soil microbes
The
nitrogen cycleNitrogen is the most abundant element in the atmosphere and is essential to all life. The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a cycle that includes gaseous components.Earth's atmosphere is...
in soils depends on the
fixation of atmospheric nitrogenNitrogen fixation usually refers to the biological process by which nitrogen in the atmosphere is converted into ammonia. This process is essential for life because fixed nitrogen is required to biosynthesize a basic building block of life, e.g. nucleotides for DNA and amino acids for proteins...
. One way this can occur is in the nodules in the roots of legumes that contain symbiotic bacteria of the genera
Rhizobium,
Mesorhizobium,
SinorhizobiumSinorhizobium/Ensifer is a genus of nitrogen-fixing bacteria , two of which have been sequenced.-Etymology:...
,
BradyrhizobiumBradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen....
, and
Azorhizobium.
Symbiotic microbes
Symbiotic microbes such as fungi and algae form an association in
lichenLichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
. Certain fungi form mycorhizzal 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 cycleThe carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
and
nitrogen cycleNitrogen is the most abundant element in the atmosphere and is essential to all life. The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is a cycle that includes gaseous components.Earth's atmosphere is...
, as well as fulfilling other vital roles in virtually all
ecosystemAn ecosystem is a system of interdependent organisms which share the same habitat, in an area functioning together with all of the physical factors of the environment. Ecosystems can be permanent or temporary. Ecosystems usually form a number of food webs...
s, such as recycling other organisms' dead remains and waste products through
decompositionDecomposition is the process by which tissues of a dead organism break down into simpler forms of matter. The process is essential for new growth and development of living organisms because it recycles the finite matter that occupies physical space in the biome. Bodies of living organisms begin to...
. Microbes also have an important place in most higher-order multicellular organisms as symbionts. Many blame the failure of
Biosphere 2Biosphere 2 is a structure originally built to be a man-made, 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
brewingBrewing is the production of alcoholic beverages and alcohol fuel through fermentation. The term is used for the production of beer, although the word "brewing" is also used to describe the fermentation process used to create wine and mead. It can also refer to the process of producing sake and soy...
,
winemakingWinemaking, 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...
,
bakingBaking 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, and cookies. Such items are...
,
picklingPickling, 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
foodFood is any substance, usually composed of carbohydrates, fats, proteins and water, that can be eaten or drunk by an animal, including humans, for nutrition or pleasure. Items considered food may be sourced from plants, animals or other categories such as fungus or fermented products like alcohol...
-making processes.
They are also used to control the
fermentationFermentation 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. A more restricted definition of fermentation is the chemical conversion of sugars into ethanol...
process in the production of cultured
dairy productDairy products are generally defined as foodstuffs produced from 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 generally comes from cows, but occasionally from other mammals such as...
s such as yogurt and
cheeseCheese is a food consisting of proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. It is produced by coagulation of the milk protein casein. Typically, the milk is acidified and addition of the enzyme rennet causes coagulation. The solids are separated and pressed into...
. 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
bioaugmentationBioaugmentation 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
biogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel....
reactors to produce
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
. Scientists are researching the use of
algae to produce liquid fuelsAlgae fuel, also called algal fuel, oilgae, algaeoleum or third-generation biofuel, is a biofuel from algae.High oil prices, competing demands between foods and other biofuel sources and the world food crisis have ignited interest in algaculture for making vegetable oil, biodiesel, bioethanol,...
, and bacteria to convert various forms of agricultural and urban waste into
usable fuelsCellulosic 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
biotechnologyBiotechnology is technology based on biology, agriculture, food science, and medicine. Modern use of the term usually refers to genetic engineering as well as cell- and tissue culture technologies...
,
biochemistryBiochemistry is the study of the chemical processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
,
geneticsGenetics, , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding...
, and
molecular biologyMolecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
. The yeasts (
Saccharomyces cerevisiaeSaccharomyces cerevisiae is a species of budding yeast. It is perhaps the most useful yeast owing to its use since ancient times in baking and brewing. It is believed that it was originally isolated from the skins of grapes...
) and fission yeast (
Schizosaccharomyces pombeSchizosaccharomyces 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 organismA model organism is a 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...
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
geneticsGenetics, , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding...
,
genomicsGenomics is 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, epistasis, pleiotropy and other...
and
proteomicsProteomics 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 AgesThe Middle Ages of European history is a period of European history covering roughly a millennium in the 5th century through 16th centuries. More specific starting and ending points are sometimes adopted by scholars to suit their respective specializations or current focus...
, diseased corpses were thrown into castles during
siegeA 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"....
s using catapults or other
siege engineA 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
endosymbioticAn 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
vitaminA vitamin is an organic compound required as a nutrient in tiny amounts by an organism. The term 'vitamin' first became popular in the early 1800's as a contraction of the words 'vital' and 'mineral', though the actual meaning of the word has developed somewhat since that time...
s such as
folic acidFolic acid and folate , as well as pteroyl-L-glutamic acid and pteroyl-L-glutamate, are forms of the water-soluble vitamin B
9...
and
biotinBiotin, also known as vitamin H or B7, is a water-soluble B-complex vitamin which is composed of an 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
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
s.
Diseases and immunology
Microorganisms are the cause of many infectious diseases. The organisms involved include
pathogenic bacteriaPathogenic bacteria are bacteria that cause infectious diseases. 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
plaguePlague is a deadly infectious disease caused by the enterobacteria Yersinia pestis . Plague is a zoonotic, primarily carried by rodents and spread to humans via fleas. Plague is notorious throughout history, due to the unrivaled scale of death and devastation it brought...
,
tuberculosisTuberculosis is a common and often deadly infectious disease caused by mycobacteria...
and
anthraxAnthrax is an acute disease caused by Bacillus anthracis. It affects both humans and animals. Most forms of the disease are highly lethal...
; protozoa, causing diseases such as
malariaMalaria is a vector-borne infectious disease caused by a eukaryotic protist of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, there are approximately 350–500 million cases of malaria, killing between one and...
, sleeping sickness and
toxoplasmosisToxoplasmosis 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 faeces of a cat that has itself...
; and also fungi causing diseases such as
ringwormRingworm is a fungal infection of the skin in humans and domestic animals such as sheep and cattle. Fungi are organisms that survive by eating plant or animal material. Those that cause parasitic infection feed on keratin, the material found in the outer layer of skin, hair, and nails...
,
candidiasisCandidiasis or thrush is a fungal infection of any of the Candida species, of which Candida albicans is the most common. Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases...
or
histoplasmosisHistoplasmosis, also known as Darling's disease,is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease primarily affects the lungs. Occasionally, other organs are affected; this is called disseminated histoplasmosis, and it can be fatal if...
. However, other diseases such as
influenzaInfluenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals. The name influenza is Italian and means "influence"...
,
yellow feverYellow fever is an acute viral disease. The virus, a 40 to 50 nm enveloped RNA virus of the family of Flaviviridae is transmitted by the bite of mosquitoes...
or
AIDSAcquired 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. As of 2007, no clear examples of archaean pathogens are known, although a relationship has been proposed between the presence of some methanogens and human
periodontal diseasePeriodontitis refers to a number of inflammatory diseases affecting the periodontium — that is, 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...
.
Importance in ecology
Microbes are critical to the processes of
decompositionDecomposition is the process by which tissues of a dead organism break down into simpler forms of matter. The process is essential for new growth and development of living organisms because it recycles the finite matter that occupies physical space in the biome. Bodies of living organisms begin to...
required to cycle nitrogen and other elements back to the natural world.
Hygiene
Hygiene is the avoidance of
infectionAn infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply, usually at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning of the...
or
foodFood is any substance, usually composed of carbohydrates, fats, proteins and water, that can be eaten or drunk by an animal, including humans, for nutrition or pleasure. Items considered food may be sourced from plants, animals or other categories such as fungus or fermented products like alcohol...
spoiling by eliminating microorganisms from the surroundings. As microorganisms, particularly
bacteriaThe bacteria are a large group of unicellular 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
virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es. A good example of this is a
hypodermic needleA hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body or extract liquids from the body...
.
In food preparation microorganisms are reduced by preservation methods (such as the addition of
vinegarVinegar is an acidic liquid processed from the fermentation of ethanol in a process that yields its key ingredient, acetic acid. It also may come in a diluted form. The acetic acid concentration typically ranges from 4 to 8 percent by volume for table vinegar and higher concentrations for pickling...
), clean utensils used in preparation, short storage periods or by cool temperatures. If complete sterility is needed, the two most common methods are
irradiationIrradiation is the process by which an item is exposed to radiation. The exposure can be intentional, sometimes to serve a specific purpose, or it can be accidental...
and the use of an
autoclaveAn autoclave is a device to sterilize equipment and supplies by subjecting them to high pressure steam at 121° C or more. It was invented by Charles Chamberland in 1879, although a precursor known as the steam digester was created by Denis Papin in 1679....
, 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"
bacteriaThe bacteria are a large group of unicellular 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),
moldMolds include all species of microscopic fungi that grow in the form of multicellular filaments, called hyphae. In contrast, microscopic fungi that grow as single cells are called yeasts...
s (conditions where the growth of
bacteriaThe bacteria are a large group of unicellular 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.
antibioticIn common usage, an antibiotic is a substance or compound that kills or inhibits the growth of bacteria...
s) and
coliformThe 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...
bacteriaThe bacteria are a large group of unicellular 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
- Biological warfare
Biological warfare , also known as germ warfare, is the use of pathogens such as viruses, bacteria, other disease-causing biological agents, or the toxins produced by them as biological weapons ....
- Biology
Biology is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy...
- Culture collection
- 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
Nanobacteria is the 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
A petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...
- Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles...
- Soil contamination
Soil pollution is caused by the presence of man-made chemicals or other alteration in the natural soil environment. This type of contamination typically arises from the rupture of underground storage tanks, application of pesticides, percolation of contaminated surface water to subsurface strata,...
- Staining
Staining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image.In biochemistry it involves adding a class-specific dye to a substrate to qualify or quantify the presence of a specific compound...
- Virus
A virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
- Bacterium
- Protozoa
Protozoa or Cornelius protozoans Protozoa or Cornelius protozoans Protozoa or Cornelius protozoans (from Greek πρῶτον proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes....
- Fungi
External links