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    [α]The bacteria ( [bækˈtr]ɪəɪə (help?info); singular: bacterium) are a large group

    of unicellular, prokaryote, microorganisms. Typically a few micrometres in length,

    bacteria have a wide range of shapes, ranging from spheres to rods and

    spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot [2]springs, radioactive waste, water, and deep in the Earth's crust, as well as in organic matter and the live bodies of plants and animals. There are typically 40 million

    bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh 30water; in all, there are approximately five nonillion (5×10) bacteria on Earth,[3][3] forming much of the world's biomass. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms, such as the fixation

    of nitrogen from the atmosphere andputrefaction. However, most bacteria have not been characterized, and only about half of the phyla of bacteria have species that can [4]be grown in the laboratory. The study of bacteria is known as bacteriology, a branch

    of microbiology.

    There are approximately ten times as many bacterial cells in the human flora of

    bacteria as there are human cells in the body, with large numbers of bacteria on [5]the skin and as gut flora. The vast majority of the bacteria in the body are rendered harmless by the protective effects of the immune system, and a few

    are beneficial. However, a few species of bacteria are pathogenic and cause infectious

    diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most

    common fatal bacterial diseases arerespiratory infections, with tuberculosis alone [6]killing about 2 million people a year, mostly in sub-Saharan Africa. In developed

    countries, antibiotics are used to treatbacterial infections and in agriculture,

    so antibiotic resistance is becoming common. In industry, bacteria are important in sewage treatment, the production of cheese andyoghurt through fermentation, as [7]well as in biotechnology, and the manufacture of antibiotics and other chemicals. Once regarded as plants constituting the class Schizomycetes, bacteria are now

    classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells

    do not contain a nucleus and rarely harbour membrane-bound organelles. Although

    the term bacteria traditionally included all prokaryotes, the scientific

    classification changed after the discovery in the 1990s that prokaryotes consist of two

    very different groups of organisms that evolved independently from an ancient [8]common ancestor. Theseevolutionary domains are called Bacteria and Archaea.



     1 History of bacteriology

     2 Origin and early evolution

     3 Morphology

     4 Cellular structure

    o 4.1 Intracellular structures

    o 4.2 Extracellular structures

    o 4.3 Endospores

     5 Metabolism

     6 Growth and reproduction

     7 Genetics

    o 7.1 Bacteriophages

     8 Behavior

    o 8.1 Secretion

    o 8.2 Bioluminescence

    o 8.3 Multicellularity

    o 8.4 Movement

     9 Classification and identification

     10 Interactions with other organisms

    o 10.1 Predators

    o 10.2 Mutualists

    o 10.3 Pathogens

     11 Significance in technology and industry

     12 See also

     13 Notes

     14 References

     15 Further reading

     16 External links

    History of bacteriology

    Further information: Microbiology

    Antonie van Leeuwenhoek, the

    firstmicrobiologist and the first person to observe

    bacteria using a microscope.

    Bacteria were first observed by Antonie van

    Leeuwenhoek in 1676, using a single-[9]lens microscope of his own design. He called

    them "animalcules" and published his

    observations in a series of letters to the Royal [10][11][12]Society. The name bacterium was

    introduced much later, by Christian Gottfried [13]Ehrenberg in 1838.

    Louis Pasteur demonstrated in 1859 that

    the fermentation process is caused by the growth of microorganisms, and that this

    growth is not due to spontaneous generation.(Yeasts and molds, commonly associated with fermentation, are not bacteria, but rather fungi. ) Along with his

    contemporary, Robert Koch, Pasteur was an early advocate of the germ theory of [14]disease. Robert Koch was a pioneer in medical microbiology and worked

    on cholera, anthrax and tuberculosis. In his research into tuberculosis, Koch finally proved the germ theory, for which he was awarded a Nobel Prize in 1905.[15] In Koch's postulates, he set out criteria to test if an organism is the cause of [16]a disease; these postulates are still used today. Though it was known in the nineteenth century that bacteria are the cause of many [17]diseases, no effective antibacterial treatments were available. In 1910, Paul

    Ehrlichdeveloped the first antibiotic, by changing dyes that selectively

    stained Treponema pallidum—the spirochaete that causes syphilis—into compounds [18]that selectively killed the pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work on immunology, and pioneered the use of stains to detect and identify

bacteria, with his work being the basis of the Gram stain and the Ziehl-Neelsen [19]stain.

    A major step forward in the study of bacteria was the recognition in 1977 by Carl [20]Woese that archaea have a separate line of evolutionary descent from bacteria. This

    new phylogenetic taxonomy was based on the sequencing of 16S ribosomal RNA, and

    divided prokaryotes into two evolutionary domains, as part of the three-domain [21]system.

    Origin and early evolution

    Further information: Timeline of evolution

    The ancestors of modern bacteria were single-celled microorganisms that were the first forms of life to develop on earth, about 4 billion years ago. For about 3 billion years, all organisms were microscopic, and bacteria and archaea were the [22][23]dominant forms of life. Although bacterial fossils exist, such as stromatolites,

    their lack of distinctivemorphology prevents them from being used to examine the

    history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny,

    and these studies indicate that bacteria diverged first from the archaeal/eukaryotic [24]lineage. The most recent common ancestor of bacteria and archaea was probably [25][26]a hyperthermophile that lived about 2.5 billion–3.2 billion years ago.

    Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from ancient bacteria entering intoendosymbiotic associations with the ancestors of eukaryotic cells, which were [27][28]themselves possibly related to the Archaea. This involved the engulfment by

    proto-eukaryotic cells of alpha-proteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya

    (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation of chloroplasts in algae and

    plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-[29][30]generation" plastid. This is known as secondary endosymbiosis.


    Further information: Bacterial cellular morphologies

    Bacteria display many cell morphologies and arrangements

    Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial

    cells are about one tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometresin length. However, a few species–for example Thiomargarita

    namibiensis and Epulopiscium fishelsoni–are up to half a millimetre long and are [31]visible to the unaided eye. Among the smallest bacteria are members of the

    genus Mycoplasma, which measure only 0.3 micrometres, as small as the [32]largest viruses. Some bacteria may be even smaller, but [33]these ultramicrobacteria are not well-studied.

    Most bacterial species are either spherical, called cocci (sing. coccus,

    from Greek kókkos, grain, seed) or rod-shaped, called bacilli (sing. bacillus, [34]from Latin baculus, stick).Elongation is associated with swimming. Some rod-

    shaped bacteria, called vibrio, are slightly curved or comma-shaped; others, can be

spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of [35]species even have tetrahedral or cuboidal shapes. More recently, bacteria were

    discovered deep under the Earth's crust that grow as long rods with a star-shaped cross-section. The large surface area to volume ratio of this morphology may give [36]these bacteria an advantage in nutrient-poor environments. This wide variety of

    shapes is determined by the bacterial cell wall and cytoskeleton, and is important

    because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, [37][38]swim through liquids and escape predators.

    Many bacterial species exist simply as single cells, others associate in characteristic patterns: Neisseria form diploids (pairs), Streptococcus form chains,

    and Staphylococcusgroup together in "bunch of grapes" clusters. Bacteria can also be elongated to form filaments, for example the Actinobacteria. Filamentous bacteria are

    often surrounded by a sheath that contains many individual cells. Certain types, such as species of the genus Nocardia, even form complex, branched filaments, similar in [39]appearance to fungalmycelia.

    The range of sizes shown by prokaryotes, relative to those of other organisms


    Bacteria often attach to surfaces and form dense aggregations

    called biofilms or bacterial mats. These films can range from a few micrometers in

    thickness to up to half a meter in depth, and may contain multiple species of bacteria, protists and archaea. Bacteria living in biofilms display a complex

    arrangement of cells and extracellular components, forming secondary structures such as microcolonies, through which there are networks of channels to enable better [40][41]diffusion of nutrients. In natural environments, such as soil or the surfaces of [42]plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also

    important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted medical devices, and bacteria protected within [43]biofilms are much harder to kill than individual isolated bacteria. Even more complex morphological changes are sometimes possible. For example, when starved of amino acids, Myxobacteria detect surrounding cells in a process

    known asquorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial [44]cells. In these fruiting bodies, the bacteria perform separate tasks; this type of cooperation is a simple type of multicellular organisation. For example, about one in

    10 cells migrate to the top of these fruiting bodies and differentiate into a specialised

    dormant state called myxospores, which are more resistant to drying and other [45]adverse environmental conditions than are ordinary cells.

    Cellular structure

    Further information: Bacterial cell structure

Structure and contents of a typical Gra

    m positive bacterial cell

    Intracellular structures

    The bacterial cell is surrounded by a lipid membrane, or cell membrane, which

    encloses the contents of the cell and acts as a barrier to hold nutrients, proteins and

    other essential components of the cytoplasm within the cell. As they are prokaryotes,

    bacteria do not tend to have membrane-bound organelles in their cytoplasm and thus

    contain few large intracellular structures. They consequently lack a nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic [46]cells, such as the Golgi apparatus and endoplasmic reticulum. Bacteria were once [47]seen as simple bags of cytoplasm, but elements such as prokaryotic cytoskeleton,[48][49] and the localization of proteins to specific locations within the cytoplasm have

    been found to show levels of complexity. These subcellular compartments have been [50]called "bacterial hyperstructures".[51]Micro-compartments such as carboxysome provides a further level of organization, which are compartments within bacteria that are surrounded by polyhedral protein [52]shells, rather than by lipid membranes. These "polyhedral organelles" localize and compartmentalize bacterial metabolism, a function performed by the membrane-[53][54]bound organelles in eukaryotes.

    Many important biochemical reactions, such as energy generation, occur

    by concentration gradients across membranes, a potential difference also found in a battery. The general lack of internal membranes in bacteria means reactions such as electron transport occur across the cell membrane between the cytoplasm and [55]the periplasmic space. However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering [56]membrane. These light-gathering complexs may even form lipid-enclosed [57]structures called chlorosomes in green sulfur bacteria. Other proteins import

    nutrients across the cell membrane, or to expel undesired molecules from the cytoplasm.

    Carboxysomes are protein-enclosed bacterial organelles. Top left is anelectron

    microscope image of carboxysomes in Halothiobacillus neapolitanus, below is an

image of purified carboxysomes. On the right is a model of their structure. Scale bars [58]are 100 nm.

    Bacteria do not have a membrane-bound nucleus, and their genetic material is

    typically a single circular chromosome located in the cytoplasm in an irregularly [59]shaped body called the nucleoid. The nucleoid contains the chromosome with

    associated proteins and RNA. The order Planctomycetes are an exception to the

    general absence of internal membranes in bacteria, because they have a membrane [60]around their nucleoid and contain other membrane-bound cellular structures. Like

    all living organisms, bacteria contain ribosomes for the production of proteins, but the structure of the bacterial ribosome is different from those [61]of eukaryotes and Archaea.

    Some bacteria produce intracellular nutrient storage granules, such as glycogen,[62][63][64][65] polyphosphate, sulfur or polyhydroxyalkanoates. These granules enable

    bacteria to store compounds for later use. Certain bacterial species, such as the photosynthetic Cyanobacteria, produce internal gas vesicles, which they use to regulate their buoyancy - allowing them to move up or down into water layers with [66]different light intensities and nutrient levels.

    Extracellular structures

    Further information: Cell envelope

    Around the outside of the cell membrane is the bacterial cell wall. Bacterial cell walls

    are made of peptidoglycan (called murein in older sources), which is made frompolysaccharide chains cross-linked by unusual peptides containing D-amino [67]acids. Bacterial cell walls are different from the cell walls of plants and fungi, [68]which are made of cellulose and chitin, respectively. The cell wall of bacteria is

    also distinct from that of Archaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin is able to kill [68]bacteria by inhibiting a step in the synthesis of peptidoglycan. There are broadly speaking two different types of cell wall in bacteria, called Gram-

    positive and Gram-negative. The names originate from the reaction of cells to [69]the Gram stain, a test long-employed for the classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid

    membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the

    Gram-negative cell wall, and only the Firmicutes and Actinobacteria (previously

    known as the low G+C and high G+C Gram-positive bacteria, respectively) have the [70]alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility; for instance, vancomycin can kill only Gram-

    positive bacteria and is ineffective against Gram-negative pathogens, such [71]as Haemophilus influenzae or Pseudomonas aeruginosa.

    In many bacteria an S-layer of rigidly arrayed protein molecules covers the outside of [72]the cell. This layer provides chemical and physical protection for the cell surface and can act as a macromolecular diffusion barrier. S-layers have diverse but mostly

    poorly understood functions, but are known to act as virulence factors [73]in Campylobacter and contain surface enzymes in Bacillus stearothermophilus.

    Flagella are rigid protein structures, about 20 nanometres in diameter and up to

    20 micrometres in length, that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell [74]membrane.

    Fimbriae are fine filaments of protein, just 2–10 nanometres in diameter and up to several micrometers in length. They are distributed over the surface of the cell, and

    resemble fine hairs when seen under the electron microscope. Fimbriae are believed

    to be involved in attachment to solid surfaces or to other cells and are essential for the [75]virulence of some bacterial pathogens. Pili (sing. pilus) are cellular appendages,

    slightly larger than fimbriae, that can transfer genetic material between bacterial cells [76]in a process called conjugation (see bacterial genetics, below).

    Capsules or slime layers are produced by many bacteria to surround their cells, and vary in structural complexity: ranging from a disorganised slime layer of extra-

    cellularpolymer, to a highly structured capsule or glycocalyx. These structures can [77]protect cells from engulfment by eukaryotic cells, such as macrophages. They can

    also act as antigens and be involved in cell recognition, as well as aiding attachment [78]to surfaces and the formation of biofilms.

    The assembly of these extracellular structures is dependent on bacterial secretion

    systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens, so are intensively studied.[79]


    Further information: Endospores

    Certain genera of Gram-positive bacteria, such

    as Bacillus, Clostridium, Sporohalobacter, Anaerobacter and Heliobacterium, can [80]form highly resistant, dormant structures called endospores. In almost all cases,

    one endospore is formed and this is not a reproductive process, although Anaerobacter can make up to seven endospores in a single [81]cell. Endospores have a central core

    of cytoplasm containing DNA and ribosomes surrounded by a cortex layer and

    protected by an impermeable and rigid coat.

    Endospores show no detectable metabolism and can survive extreme physical and

    chemical stresses, such as high levels of UV light, gamma [82]radiation, detergents,disinfectants, heat, pressure and desiccation. In this dormant [83][84]state, these organisms may remain viable for millions of years, and endospores

    even allow bacteria to survive exposure to the vacuum and radiation in [85]space. Endospore-forming bacteria can also cause disease: for example, anthrax can be contracted by the inhalation of Bacillus

    anthracis endospores, and contamination of deep puncture wounds with Clostridium [86]tetani endospores causes tetanus.


    Further information: Microbial metabolism[87]Bacteria exhibit an extremely wide variety of metabolic types. The distribution of

    metabolic traits within a group of bacteria has traditionally been used to define theirtaxonomy, but these traits often do not correspond with modern genetic [88]classifications. Bacterial metabolism is classified into nutritional groups on the

    basis of three major criteria: the kind of energy used for growth, the source of carbon,

    and the electron donors used for growth. An additional criterion of respiratory [89]microorganisms are theelectron acceptors used for aerobic or anaerobic respiration.

    Nutritional types in bacterial metabolism

    Nutritional Source of Source of carbonExamplestypeenergy

    Sunlight Phototrophs Organic Cyanobacteria, Green sulfur


    (photoheterotrophs) bacteria, Chloroflexi, or Purple bacteria or carbon fixation



    Inorganic compounds Thermodesulfobacteria, Hydrogenophilacompound Lithotrophs(lithoheterotrophs) ceae, or Nitrospirae sor carbon fixation



    Organic compounds Organotroph Bacillus, Clostridium or Enterobacteriacompound(chemoheterotrophssceae s) or carbon fixation


    Carbon metabolism in bacteria is either heterotrophic, where organic

    carbon compounds are used as carbon sources, or autotrophic, meaning that cellular

    carbon is obtained byfixing carbon dioxide. Heterotrophic bacteria include parasitic types. Typical autotrophic bacteria are phototrophic cyanobacteria, green sulfur-

    bacteria and some purple bacteria, but also many chemolithotrophic species, such as [90]nitrifying or sulfur-oxidising bacteria. Energy metabolism of bacteria is either

    based on phototrophy, the use of light through photosynthesis, or on chemotrophy, the

    use of chemical substances for energy, which are mostly oxidised at the expense of oxygen or alternative electron acceptors (aerobic/anaerobic respiration).Finally, bacteria are further divided into lithotrophs that use inorganic electron donors

    and organotrophs that use organic compounds as electron donors. Chemotrophic organisms use the respective electron donors for energy conservation (by aerobic/anaerobic respiration or fermentation) and biosynthetic reactions (e.g. carbon dioxide fixation), whereas phototrophic organisms use them only for biosynthetic purposes. Respiratory organisms use chemical compounds as a source of energy by

    taking electrons from thereduced substrate and transferring them to a terminal

    electron acceptor in a redox reaction. This reaction releases energy that can be used to synthesise ATP and drive metabolism. In aerobic organisms, oxygen is used as the

    electron acceptor. In anaerobic organisms other inorganic compounds, such

    as nitrate, sulfate or carbon dioxide are used as electron acceptors. This leads to the ecologically important processes of denitrification, sulfate reduction and acetogenesis,


    Another way of life of chemotrophs in the absence of possible electron acceptors is fermentation, where the electrons taken from the reduced substrates are transferred to oxidised intermediates to generate reduced fermentation products (e.g. lactate, ethanol, hydrogen, butyric acid). Fermentation is possible, because the energy content of the substrates is higher than that of the products, which allows the [91][92]organisms to synthesise ATP and drive their metabolism.

    These processes are also important in biological responses to pollution; for

    example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms ofmercury (methyl- and dimethylmercury) in the [93]environment. Non-respiratory anaerobes use fermentation to generate energy and

    reducing power, secreting metabolic by-products (such as ethanol in brewing) as

    waste. Facultative anaerobes can switch between fermentation and different terminal

    electron acceptors depending on the environmental conditions in which they find themselves.

    Lithotrophic bacteria can use inorganic compounds as a source of energy. Common inorganic electron donors are hydrogen, carbon monoxide, ammonia (leading

    to nitrification),ferrous iron and other reduced metal ions, and several

    reduced sulfur compounds. Unusually, the gas methane can be used

    by methanotrophic bacteria as both a source ofelectrons and a substrate for [94] In both aerobic phototrophy and chemolithotrophy, oxygen is carbon anabolism.

    used as a terminal electron acceptor, while under anaerobic conditions inorganic compounds are used instead. Most lithotrophic organisms are autotrophic, whereas organotrophic organisms are heterotrophic.

    In addition to fixing carbon dioxide in photosynthesis, some bacteria also fix nitrogen gas (nitrogen fixation) using the enzyme nitrogenase. This

    environmentally important trait can be found in bacteria of nearly all the metabolic [95]types listed above, but is not universal.

    Growth and reproduction

    Many bacteria reproduce through binary fission

    Further information: Bacterial growth

    Unlike multicellular organisms, increases in the size of bacteria (cell growth) and their

    reproduction by cell division are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual [96]reproduction. Under optimal conditions, bacteria can grow and divide extremely [97]rapidly, and bacterial populations can double as quickly as every 9.8 minutes. In

    cell division, two identical clone daughter cells are produced. Some bacteria, while

    still reproducing asexually, form more complex reproductive structures that help disperse the newly formed daughter cells. Examples include fruiting body formation by Myxobacteria and aerial hyphaeformation by Streptomyces, or budding. Budding

    involves a cell forming a protrusion that breaks away and produces a daughter cell.[98]A growing colony of Escherichia coli cells

    In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth

    media such as agar plates are used to isolate pure cultures of a bacterial

    strain. However, liquid growth media are used when measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific [99]organisms.

    Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely

    rapidly when nutrients become available, such as the formation of algal (and [100]cyanobacterial) blooms that often occur in lakes during the summer. Other

    organisms have adaptations to harsh environments, such as the production of multiple antibiotics by Streptomyces that inhibit the growth of competing [101]microorganisms. In nature, many organisms live in communities (e.g. biofilms)

    which may allow for increased supply of nutrients and protection from environmental [42]stresses. These relationships can be essential for growth of a particular organism or [102]group of organisms (syntrophy).

    Bacterial growth follows three phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new

environment. The first phase of growth is the lag phase, a period of slow growth when

    the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid [103]growth are produced. The second phase of growth is the logarithmic phase (log

    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 (k), and the time it takes the cells to double is known as the generation time (g). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The final phase

    of growth is the stationary phase and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased

    expression of genes involved in DNA repair, antioxidant metabolism and nutrient [104]transport.


    Further information: Plasmid, Genome

     Most bacteria have a single

    circular chromosome that can range in size from

    only 160,000 base pairs in

    the endosymbiotic bacteria Candidatus [105]Carsonella ruddii, to 12,200,000 base pairs in

    the soil-dwelling bacteria Sorangium [106]cellulosum. Spirochaetes of

    the genus Borrelia are a notable exception to this

    arrangement, with bacteria such as Borrelia

    burgdorferi, the cause ofLyme disease, [107]containing a single linear chromosome. The

    genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much more [108]rare than in eukaryotes.

    Bacteria may also

    contain plasmids, which are small

    extra-chromosomal DNAs that

    may contain genes for antibiotic

    resistance or virulence factors.

    Bacteria, as asexual organisms,

    inherit identical copies of their

    parent's genes (i.e., they

    areclonal). However, all bacteria

    can evolve by selection on

    changes to their genetic

    material DNAcaused by genetic

    recombination or mutations. Muta

    tions come from errors made

    during the replication of DNA or

    from exposure

    to mutagens. Mutation rates vary widely among different species of bacteria and even [109]among different clones of a single species of bacteria. Genetic changes in bacterial

    genomes come from either random mutation during replication or "stress-directed

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