Encyclopedia
Bacteria are a major group of living
organisms. The term "bacteria" has variously applied to all prokaryotes or to a major group of them, otherwise called the
eubacteria, depending on ideas about their relationships. Here,
bacteria is used specifically to refer to the eubacteria. Another major group of bacteria are the
Archaea. The study of bacteria is known as
, a subfield of
microbiology.
Bacteria are the most abundant of all organisms. They are ubiquitous in
soil,
water, and as
symbionts of other organisms. Many pathogens are bacteria. Most are minute, usually only 0.5-5.0 µm in their longest dimension, although giant bacteria like
Thiomargarita namibiensis and
Epulopiscium fishelsoni may grow past 0.5 mm in size. They generally have
cell walls, like
plant and
fungal cells, but bacterial cell walls are normally made out of
peptidoglycan instead of
cellulose or
chitin , and are not homologous with
eukaryotic cell walls. Many move around using
flagella, which are different in structure from the flagella of other groups.
History
The first bacteria were observed by
Anton van Leeuwenhoek in 1674 using a single-lens
microscope of his own design. The name
bacterium was introduced much later, by
Ehrenberg in 1828, derived from the Greek word ßa?t????? meaning "small stick". Because of the difficulty in describing individual bacteria and the importance of their discovery to fields such as medicine, biochemistry, and geochemistry, the history of bacteriology is generally described as the history of
microbiology.
Cellular structure
As prokaryotes all bacteria have a relatively simple cell structure lacking a
cell nucleus and
organelles such as
mitochondria and
chloroplasts. Most bacteria are relatively small and possess distinctive cell and colony morphologies as described below.
The most important bacterial structural characteristic is the
cell wall. Bacteria can be divided into two groups based on differences in cell wall structure as revealed by
Gram staining. Gram positive bacteria possess a cell wall containing a thick
peptidoglycan layer and teichoic acids while Gram negative bacteria have an outer, lipopolysaccharide-containing membrane and a thin
peptidoglycan layer located in the periplasm .
Many bacteria contain other extracellular structures such as
flagella and fimbriae which are used for motility , attachment, and conjugation respectively. Some bacteria also contain capsules or slime layers that also facilitate bacterial attachment to surfaces and
biofilm formation. Bacteria contain relatively few intracellular structures compared to
eukaryotes but do contain a tightly supercoiled
chromosome,
ribosomes, and several other species-specific structures such as intracellular membranes, nutrient storage structures, gas vesicles, and magnetosomes.
Some bacteria are capable of forming
endospores which allows them to survive extreme environmental and chemical stresses. This property is restricted to specific Gram positive organisms such as
Bacillus is a genus [i] of rod-shaped, Gram-positive [i] bacteria [i] and a member of the...
and
Clostridium.
Metabolism
Main article: Microbial metabolismIn contrast to higher organisms, bacteria exhibit an extremely wide variety of metabolic types. In fact, it is widely accepted that
eukaryotic metabolism is largely a derivative of bacterial metabolism with
mitochondria having descended from a lineage within the
a-Proteobacteria and
chloroplasts from the Cyanobacteria by ancient endosymbiotic events.
Bacterial metabolism can be divided broadly on the basis of the kind of energy used for growth, electron donors and electron acceptors and by the source of carbon used. Most bacteria are
heterotrophic; using
organic carbon compounds as both carbon and energy sources. In aerobic organisms,
oxygen is used as the terminal electron acceptor. In anaerobic organisms other inorganic compounds, such as
nitrate,
sulfate or
carbon dioxide as terminal electron acceptors leading to the environmentally important processes of denitrification, sulfate reduction and acetogenesis, respectively. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves. As an alternative to
heterotrophy many bacteria are
autotrophic, fixing
carbon dioxide into cell mass.
Energy metabolism of bacteria is either based on phototrophy or
chemotrophy, i. e. the use of either light or exergonic chemical reactions for fueling life processes. Lithotrophic bacteria use inorganic electron donors for respiration or biosynthesis and
carbon dioxide fixation , opposed by organotrophs which need organic compounds as electron donors for biosynthetic reactions . Common inorganic electron donors are
hydrogen,
carbon monoxide,
ammonia ,
ferrous iron, other reduced metal ions or even elemental iron and several reduced
sulfur compounds. Additionally,
methane metabolism, although formally counted as organotrophic, is actually more related to lithotrophic metabolic pathways. In both aerobic phototrophy and chemolithotrophy
oxygen is used as a terminal electron acceptor, while under anaerobic conditions inorganic compounds are used instead. Most photolithotrophic and chemolithotrophic organisms are
autotrophic, meaning that they obtain cellular carbon by fixation of
carbon dioxide, whereas photoorganotrophic and chemoorganotrophic organisms are
heterotrophic.
In addition to carbon, some organisms also fix
nitrogen gas . This environmentally important trait can be found in bacteria of nearly all the metabolic types listed above but is not universal.
The distribution of metabolic traits within a group of organisms has traditionally been used to define their taxonomy, although these traits often do not correspond with genetic techniques .
Growth and reproduction
All bacteria reproduce through
asexual reproduction binary fission, which results in
cell division. Two identical
clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that facilitate the dispersal of the newly-formed daughter cells. Examples include fruiting body formation by
Myxococcus and arial
hyphae formation by
Streptomyces is a genus of Actinobacteria [i], a group of Gram-positive [i] and generally high GC-content [i] ...
, or budding. Budding is resulted of a 'bud' of a cell growing from another cell, and then finally breaking away.
In the laboratory, bacteria are usually grown using two methods, solid and liquid. Solid growth media such as
agar plates are used to isolate pure cultures of a bacterial strain. When quantitation of growth or large volumes of cells are required liquid growth media are generally used. Growth in liquid media, with stirring, most often occurs as an even cell suspension making the cultures easier to divide and transfer compared to solid media, although the isolation of individual cells from liquid media is extremely difficult. In both liquid and solid media there exist a finite amount of nutrients, which allows for the study of the
bacterial cell cycle. These limitations can be avoided by the use of a
chemostat, which maintains a bacterial culture under steady-state conditions by the continuous addition of nutrients and the removal of waste products and cells. Large
chemostats are often used for industrial-scale microbial processes.
Most techniques commonly used to grow bacteria are designed to optimise the amount of cells produced, the amount of time needed to produce them, and the cost to produce them. In a bacterium's natural environment nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This constant limitation of nutrients has led the evolution of many different growth strategies in different types of organisms . Some possess the ability to grow extremely rapidly when nutrients become available, such as the formation of algal blooms that often occur in lakes during the summer. Other organisms have devised more specialized strategies to make them more successful in a harsh environment, such as the production of antibiotics by
Streptomyces is a genus of Actinobacteria [i], a group of Gram-positive [i] and generally high GC-content [i] ...
; often at the expense of a slower growth rate. In a natural environment, many organisms live in communities which may allow for increased supply of nutrients and protection of environmental stresses. Often these relationships are essential for growth of a particular organism or group of organisms . These evolutionary tactics to overcome nutrient limitation must be accounted for in an industrial/laboratory bacterial growth experiment. For instance bacteria that tend to agglutinate may need more vigorous stirring to break apart any large bacterial masses. The main growth attribute that must be understood for controlled growth is that bacteria have defined growth phases.
A controlled bacterial growth will follow three distinct phases. Nearly all cultures start from taking a relatively old stock of bacteria and diluting them in to fresh media; these cells need to adapt to the nutrient rich environment. The first phase of growth is the lag phase, a period of slow growth most often attributed to the need for cells to adapt to fast growth. The lag phase has high biosynthesis rates; enzymes needed to metabolise a variety of substrates are produced. The second phase of growth is the logarithmic phase , also known as the exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the
growth rate . The time it takes the cells to double during the log phase is known as the
generation time . During the log phase, nutrients are metabolised at maximum speed until they are all gone. The final phase of growth is the stationary phase. This phase of growth is caused by depleted nutrients. The cells begin to shut down their metabolic activity, as well as break-down their own non-essential proteins. The stationary phase is a transition from rapid growth to dormancy. Without positive signals from the environment transcription of many non-essential genes are no longer promoted to conserve ATP.
Genetic variation
Bacteria, as asexual organisms, inherit an identical copy of their parent's genes . All bacteria, however, have the ability to evolve and change their genetic material, either through mutation or
genetic recombination. Mutation occurs as a result of errors made during the replication of a gene and is most often gradual. It occurs naturally and as a result of the presence of mutagens. Mutation rates can vary among different species of bacteria, but is usually in the range of 10
-5 to 10
-7 mutations per gene per generation. Some bacteria can increase the rate of mutation during DNA replication as a response to stress.
Asexual reproduction does not afford an organism many opportunities to evolve its genome. Certain types of bacteria are also capable of exchanging genetic information through
bacterial conjugation. The genetic material transferred may be either
chromosomal or from a
plasmid. In conjugation one bacterium, referred to as the F+ type, transfers genetic material to another through a mating bridge. The F factor is the plasmid that contains genes coding for autonomous replication, pilli formation, and conjugal transfer functions. If the plasmid has integrated into the chromosome then it is referred to as a "high frequency recombination" strain . Conjugation increases the genetic variability of bacterial populations and facilitates the emergences of antibiotic resistance. This is often thought of as a primitive form of sexual reproduction; however, since gametes are not uniting to form a zygote , this cannot be considered sexual reproduction. The ability to transfer DNA is not ubiquitous in the bacterial kingdom, so most bacteria also rely on other transfer methods to diversify their DNA. The most frequent genetic changes in bacterial genomes come from random mutation. Bacteria can also undergo genetic recombination. Many bacteria can take-up exogenous environmental DNA from closely related genera in a process called transformation. In the process of transduction, a
virus can alter the
DNA of a bacterium by becoming lysogenic and introducing foreign DNA into the host chromosome, which can then be transcribed and replicated. The generic term for gene acquisition from the environment is horizontal gene transfer.
Because of their ability to quickly grow, and the relative ease with which they can be manipulated, bacteria have historically been the workhorses for the fields of
molecular biology, genetics and
biochemistry. By making mutations in bacteria and examining the resulting phenotypes, scientists have been able to determine the function of many different genes and enzymes. Lessons learned from bacteria can then be applied to more complex organisms which are often more difficult to study.
Movement
Motile bacteria can move about, using
flagella, bacterial gliding, or changes of buoyancy. A unique group of bacteria, the
spirochaetes, have structures similar to flagella, called
axial filaments, between two membranes in the periplasmic space. They have a distinctive
helical body that twists about as it moves.
Bacterial flagella are arranged in many different ways. Bacteria can have a single polar flagellum at one end of a cell, clusters of many flagella at one end or flagella scattered all over the cell, as with
peritrichous. Many bacteria have two distinct modes of movement: forward movement and tumbling. The tumbling allows them to reorient and introduces an important element of randomness in their forward movement.
Motile bacteria are attracted or repelled by certain stimuli, behaviors called
taxes - for instance,
chemotaxis, phototaxis, mechanotaxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies. The myxobacteria move only when on solid surfaces, unlike
E. coli which is motile in liquid or solid media.
Groups and identification
Historically, bacteria as originally studied by
botanists were classified in the same way as plants, that is, mainly by shape. Bacteria come in a variety of different cell morphologies , including bacillus ,
coccus , spirillum , and vibrio . However, because of their small size bacteria are relatively uniform in shape and therefore classification based on morphology was unsuccessful. The first formal classification scheme was developed following the development of the
Gram stain by
Hans Christian Gram which separates bacteria based on the structural characteristics of their cell walls. This scheme included:
- Gracilicutes - Gram negative staining bacteria with a second cell membrane
- Firmicutes - Gram positive staining bacteria with a thick peptidoglycan wall
- Mollicutes - Gram negative staining bacteria with no cell wall or second membrane
- Mendosicutes - atypically staining strains now known to belong to the Archaea
Further developments based on this scheme included comparisons of bacteria based on differences in cellular metabolism as determined by a wide variety of specific tests. Bacteria were also classified based on differences in cellular chemical compounds such as
fatty acids, pigments, and
quinones for example. While these schemes allowed for the differentiation between bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. It was not until the utilization of genome-based techniques such as %
guanine+
cytosine ratio determination, genome-genome hybridization and gene sequencing that microbial taxonomy developed into a stable, accurate classification system. It should be noted, however, that due to the existence numerous historical classification schemes and our current poor understanding of microbial diversity, bacterial taxonomy remains a changing and expanding field.
Benefits and dangers
Bacteria are both harmful and useful to the
environment and
animals, including
humans. The role of bacteria in disease and infection is important. Some bacteria act as pathogens and cause tetanus, typhoid fever,
pneumonia,
syphilis,
cholera,
food-borne illness, leprosy, and
tuberculosis. Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as
Streptococcus is a genus [i] of spherical [i], Gram-positive [i] bacteria [i] of th...
,
Staphylococcus is a genus of gram-positive [i] bacteria. ...
, or many gram-negative bacteria. Some bacterial infections can spread throughout the host's body and become
systemic. In
plants, bacteria cause leaf spot, fireblight, and
wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. The hosts infected with the pathogens may be treated with antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively.
Antiseptic measures may be taken to prevent infection by bacteria, for example, by swabbing skin with alcohol prior to piercing the skin with the needle of a syringe. Sterilization of surgical and dental instruments is done to make them
sterile or pathogen-free to prevent contamination and infection by bacteria.
Sanitizers and disinfectants are used to kill bacteria or other pathogens to prevent contamination and risk of infection.
In soil, microorganisms which reside in the rhizosphere help in the transformation of molecular dinitrogen gas as their source of nitrogen, converting it to nitrogenous compounds in a process known as
nitrogen fixation. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as
symbionts in humans and other organisms. For example, the presence of the gut flora in the large intestine can help prevent the growth of potentially harmful microbes.
The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds. For example, the decomposition of
cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus
Cytophaga. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the
hydrocarbons in
petroleum are often used to clean up
oil spills. Some beaches in
Prince William Sound were fertilized in an attempt to facilitate the growth of such bacteria after the infamous 1989
Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil.
Bacteria, often in combination with
yeasts and
molds, are used in the preparation of fermented foods such as
cheese, pickles,
soy sauce,
sauerkraut,
vinegar,
wine, and
yogurt. Using
biotechnology techniques, bacteria can be
bioengineered for the production of therapeutic drugs, such as
insulin, or for the bioremediation of toxic wastes.
"Friendly bacteria" is a term used to refer to those bacteria that offer some benefit to human hosts, such as
Lactobacillus species, which convert milk protein to lactic acid in the gut. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria . Other bacteria that are helpful inside the body are many strains of
E. coli, which are harmless in healthy individuals and provide Vitamin K.
See also
Sources
- Some text in this entry was merged with the Nupedia was a Web-based [i] encyclopedia [i] whose articles were written by experts a ...
article entitled Bacteria, written by Nagina Parmar; reviewed and approved by the Biology group
Further reading
- Alcamo, I. Edward. Fundamentals of Microbiology. 5th ed. Menlo Park, California: Benjamin Cumming, 1997.
- Atlas, Ronald M. Principles of Microbiology. St. Louis, Missouri: Mosby, 1995.
- Holt, John.G. Bergey's Manual of Determinative Bacteriology. 9th ed. Baltimore, Maryland: Williams and Wilkins, 1994.
- Stanier, R.Y., J. L. Ingraham, M. L. Wheelis, and P. R. Painter. General Microbiology. 5th ed. Upper Saddle River, New Jersey: Prentice Hall, 1986.
External links
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