Bacteria

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For other uses, see Bacteria (disambiguation).

Bacteria Fossil range: Archean or earlier - Recent
Escherichia coli cells magnified 25,000 times
Scientific classification
Domain: Bacteria
Phyla
Acidobacteria Actinobacteria Aquificae Bacteroidetes Chlamydiae Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia

Bacteria (singular: bacterium) are unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods to spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste,Fredrickson J, Zachara J, Balkwill D, et al (2004). "Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the Hanford site, Washington state". Appl Environ Microbiol 70 (7): 4230–41. PMID 15240306.  seawater, and deep in the Earth\'s crust. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5×10³⁰) bacteria on Earth,Whitman W, Coleman D, Wiebe W (1998). "Prokaryotes: the unseen majority". Proc Natl Acad Sci U S A 95 (12): 6578–83. PMID 9618454.  forming much of the world\'s biomass.Whitman W, Coleman D, Wiebe W (1998). "Prokaryotes: the unseen majority". Proc Natl Acad Sci U S A 95 (12): 6578–83. PMID 9618454.  Bacteria are vital in recycling nutrients, and many important steps in nutrient cycles depend on bacteria, such as the fixation of nitrogen from the atmosphere. However, most of these bacteria have not been characterized, and only about half of the phyla of bacteria have species that can be cultured in the laboratory.Rappé MS, Giovannoni SJ (2003). "The uncultured microbial majority". Annu. Rev. Microbiol. 57: 369-94. doi:10.1146/annurev.micro.57.030502.090759. PMID 14527284.  The study of bacteria is known as bacteriology, a branch of microbiology.

There are approximately ten times as many bacterial cells as human cells in the human body, with large numbers of bacteria on the skin and in the digestive tract.Sears CL (2005). "A dynamic partnership: celebrating our gut flora". Anaerobe 11 (5): 247-51. doi:10.1016/j.anaerobe.2005.05.001. PMID 16701579.  Although the vast majority of these bacteria are rendered harmless or beneficial by the protective effects of the immune system, a few are pathogenic bacteria and cause infectious diseases, including cholera, syphilis, anthrax, leprosy and bubonic plague. The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people a year, mostly in sub-Saharan Africa.2002 WHO mortality data. Retrieved on 2007-01-20. In developed countries, antibiotics are used to treat bacterial infections and in various agricultural processes, so antibiotic resistance is becoming common. In industry, bacteria are important in processes such as sewage treatment, the production of cheese and yoghurt, and the manufacture of antibiotics and other chemicals.Ishige T, Honda K, Shimizu S (2005). "Whole organism biocatalysis". Curr Opin Chem Biol 9 (2): 174–80. PMID 15811802. 

Bacteria are 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 prokaryotic life consists of two very different groups of organisms that evolved independently from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci U S A 87 (12): 4576–9. PMID 2112744. 

Contents 1 History of bacteriology 2 Origin and early evolution 3 Morphology 4 Cellular structure 4.1 Intracellular structures 4.2 Extracellular structures 4.3 Endospores 5 Metabolism 6 Growth and reproduction 7 Genetics 8 Movement 9 Classification and identification 10 Interactions with other organisms 10.1 Mutualists 10.2 Pathogens 11 Significance in technology and industry 12 See also 13 References 14 Further reading 15 External links

History of bacteriology

Further information: Microbiology

Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.

Bacteria were first observed by Antonie van Leeuwenhoek in 1676, using a single-lens microscope of his own design.Porter JR (1976). "Antony van Leeuwenhoek: Tercentenary of his discovery of bacteria". Bacteriological reviews 40 (2): 260-9. PMID 786250. Retrieved on 2007-08-19.  He called them "animalcules" and published his observations in a series of letters to the Royal Society.van Leeuwenhoek A (1684). "An abstract of a letter from Mr. Anthony Leevvenhoek at Delft, dated Sep. 17, 1683, Containing Some Microscopical Observations, about Animals in the Scurf of the Teeth, the Substance Call\'d Worms in the Nose, the Cuticula Consisting of Scales". Philosophical Transactions (1683–1775) 14: 568-74. Retrieved on 2007-08-19. van Leeuwenhoek A (1700). "Part of a Letter from Mr Antony van Leeuwenhoek, concerning the Worms in Sheeps Livers, Gnats, and Animalcula in the Excrements of Frogs". Philosophical Transactions (1683–1775) 22: 509–18. Retrieved on 2007-08-19. van Leeuwenhoek A (1702). "Part of a Letter from Mr Antony van Leeuwenhoek, F. R. S. concerning Green Weeds Growing in Water, and Some Animalcula Found about Them". Philosophical Transactions (1683–1775) 23: 1304–11. Retrieved on 2007-08-19.  The name bacterium was introduced much later, by Christian Gottfried Ehrenberg in 1828, and is derived from the Greek word βακτήριον -α , bacterion -a , meaning "small staff".Etymology of the word "bacteria". Online Etymology dictionary. Retrieved on 2006-11-23.

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 disease.Pasteur\'s Papers on the Germ Theory. LSU Law Center\'s Medical and Public Health Law Site, Historic Public Health Articles. Retrieved on 2006-11-23. 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.The Nobel Prize in Physiology or Medicine 1905. Nobelprize.org. Retrieved on 2006-11-22. In Koch\'s postulates, he set out criteria to test if an organism is the cause of a disease; these postulates are still used today.O\'Brien S, Goedert J (1996). "HIV causes AIDS: Koch\'s postulates fulfilled". Curr Opin Immunol 8 (5): 613–18. PMID 8902385. 

Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antibacterial treatments were available.Thurston A (2000). "Of blood, inflammation and gunshot wounds: the history of the control of sepsis". Aust N Z J Surg 70 (12): 855-61. PMID 11167573.  In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes that selectively stained Treponema pallidum—the spirochaete that causes syphilis—into compounds that selectively killed the pathogen.Schwartz R (2004). "Paul Ehrlich\'s magic bullets". N Engl J Med 350 (11): 1079–80. PMID 15014180.  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 stain.Biography of Paul Ehrlich. Nobelprize.org. Retrieved on 2006-11-26.

A major step forward in the study of bacteria was the recognition in 1977 by Carl Woese that archaea have a separate line of evolutionary descent from bacteria.Woese C, Fox G (1977). "Phylogenetic structure of the prokaryotic domain: the primary kingdoms". Proc Natl Acad Sci U S A 74 (11): 5088–90. PMID 270744.  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 system.Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci U S A 87 (12): 4576–79. PMID 2112744. 

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 dominant forms of life.Schopf J (1994). "Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic". Proc Natl Acad Sci U S A 91 (15): 6735–42. PMID 8041691. DeLong E, Pace N (2001). "Environmental diversity of bacteria and archaea". Syst Biol 50 (4): 470–78. PMID 12116647.  Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the past 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 lineage.Brown JR, Doolittle WF (1997). "Archaea and the prokaryote-to-eukaryote transition". Microbiol. Mol. Biol. Rev. 61 (4): 456-502. PMID 9409149.  The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion–3.2 billion years ago.Di Giulio M (2003). "The universal ancestor and the ancestor of bacteria were hyperthermophiles". J Mol Evol 57 (6): 721–30. PMID 14745541. Battistuzzi F, Feijao A, Hedges S. "A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land.". BMC Evol Biol 4: 44. PMID 15535883. 

Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from ancient bacteria entering into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.Poole A, Penny D (2007). "Evaluating hypotheses for the origin of eukaryotes". Bioessays 29 (1): 74–84. PMID 17187354. Dyall S, Brown M, Johnson P (2004). "Ancient invasions: from endosymbionts to organelles". Science 304 (5668): 253–7. PMID 15073369.  This involved the engulfment by proto-eukaryotic cells of alpha-proteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still being found in all known Eukarya (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later on, an independent second engulfment by some mitochondria-containing eukaryotes of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants. There are even some algal groups known that clearly originated from subsequent events of endosymbiosis by heterotrophic eukaryotic hosts engulfing a eukaryotic algae that developed into "second-generation" plastids.Lang B, Gray M, Burger G. "Mitochondrial genome evolution and the origin of eukaryotes". Annu Rev Genet 33: 351-97. PMID 10690412. McFadden G (1999). "Endosymbiosis and evolution of the plant cell". Curr Opin Plant Biol 2 (6): 513-9. PMID 10607659. 

Morphology

Bacteria display a large diversity of cell morphologies and arrangements

Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial cells are about 10 times smaller than eukaryotic cells and are typically 0.5–5.0 micrometres in length. However, a few species–for example Thiomargarita namibiensis and Epulopiscium fishelsoni–are up to half a millimetre long and are visible to the unaided eye.Schulz H, Jorgensen B. "Big bacteria". Annu Rev Microbiol 55: 105–37. PMID 11544351.  Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.3 micrometres, as small as the largest viruses.Robertson J, Gomersall M, Gill P. (1975). "Mycoplasma hominis: growth, reproduction, and isolation of small viable cells". J Bacteriol. 124 (2): 1007–18. PMID 1102522. 

Most bacterial species are either spherical, called cocci (sing. coccus, from Greek kókkos, grain, seed) or rod-shaped, called bacilli (sing. bacillus, from Latin baculus, stick). 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 species even have tetrahedral or cuboidal shapes.Fritz I, Strömpl C, Abraham W (2004). "Phylogenetic relationships of the genera Stella, Labrys and Angulomicrobium within the \'Alphaproteobacteria\' and description of Angulomicrobium amanitiforme sp. nov". Int J Syst Evol Microbiol 54 (Pt 3): 651-7. PMID 15143003.  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, swim through liquids and escape predators.Cabeen M, Jacobs-Wagner C (2005). "Bacterial cell shape". Nat Rev Microbiol 3 (8): 601–10. PMID 16012516. Young K (2006). "The selective value of bacterial shape". Microbiol Mol Biol Rev 70 (3): 660–703. PMID 16959965. 

Many bacterial species exist simply as single cells, others associate in characteristic patterns: Neisseria form diploids (pairs), Streptococcus form chains, and Staphylococcus group 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 appearance to fungal mycelia.Douwes K, Schmalzbauer E, Linde H, Reisberger E, Fleischer K, Lehn N, Landthaler M, Vogt T (2003). "Branched filaments no fungus, ovoid bodies no bacteria: Two unusual cases of mycetoma". J Am Acad Dermatol 49 (2 Suppl Case Reports): S170–3. PMID 12894113. 

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

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 diffusion of nutrients.Donlan R (2002). "Biofilms: microbial life on surfaces". Emerg Infect Dis 8 (9): 881–90. PMID 12194761. Branda S, Vik S, Friedman L, Kolter R (2005). "Biofilms: the matrix revisited". Trends Microbiol 13 (1): 20–26. PMID 15639628.  In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms.Davey M, O\'toole G (2000). "Microbial biofilms: from ecology to molecular genetics". Microbiol Mol Biol Rev 64 (4): 847–67. PMID 11104821.  Biofilms are also important for chronic bacterial infections and infections of implanted medical devices, as bacteria protected within these structures are much harder to kill than individual bacteria.Donlan RM, Costerton JW (2002). "Biofilms: survival mechanisms of clinically relevant microorganisms". Clin Microbiol Rev 15 (2): 167–93. PMID 11932229. 

Even more complex morphological changes are sometimes possible. For example, when starved of amino acids, Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells.Shimkets L. "Intercellular signaling during fruiting-body development of Myxococcus xanthus.". Annu Rev Microbiol 53: 525–49. PMID 10547700.  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 desiccation and other adverse environmental conditions than are ordinary cells.Kaiser D. "Signaling in myxobacteria". Annu Rev Microbiol 58: 75–98. PMID 15487930. 

Cellular structure

Further information: Bacterial cell structure

Diagram of the cellular structure of a typical bacterial cell

Intracellular structures

The bacterial cell is surrounded by a lipid membrane, or cell membrane, which encompasses 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 intracellular structures. They consequently lack a nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells, such as the Golgi apparatus and endoplasmic reticulum.Berg JM, Tymoczko JL Stryer L (2002). Molecular Cell Biology, 5th ed., WH Freeman. ISBN 0-7167-4955-6.  However, recent research is identifying increasing amounts of structural complexity in bacteria, such as the discovery of the prokaryotic cytoskeleton.Gitai Z (2005). "The new bacterial cell biology: moving parts and subcellular architecture". Cell 120 (5): 577–86. PMID 15766522. Shih YL, Rothfield L (2006). "The bacterial cytoskeleton". Microbiol. Mol. Biol. Rev. 70 (3): 729–54. PMID 16959967. 

Many important biochemical reactions, such as energy generation, occur due to concentration gradients across membranes, creating a potential difference analogous to a battery. The absence of internal membranes in bacteria means these reactions, such as electron transport, occur across the cell membrane, between the cytoplasm and the periplasmic space.Harold F (1972). "Conservation and transformation of energy by bacterial membranes". Bacteriol Rev 36 (2): 172–230. PMID 4261111.  Additionally, while some transporter proteins consume chemical energy, others harness concentration gradients to import nutrients across the cell membrane or to expel undesired molecules from the cytoplasm.

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 shaped body called the nucleoid.Thanbichler M, Wang S, Shapiro L (2005). "The bacterial nucleoid: a highly organized and dynamic structure". J Cell Biochem 96 (3): 506–21. PMID 15988757.  The nucleoid contains the chromosome with associated proteins and RNA. Like all living organisms, bacteria contain ribosomes for the production of proteins, but the structure of the bacterial ribosome is different from those of eukaryotes and Archaea.Poehlsgaard J, Douthwaite S (2005). "The bacterial ribosome as a target for antibiotics". Nat Rev Microbiol 3 (11): 870–81. PMID 16261170.  The order Planctomycetes are an exception to the general absence of internal membranes in bacteria, because they have a membrane around their nucleoid and contain other membrane-bound cellular structures.Fuerst J (2005). "Intracellular compartmentation in planctomycetes". Annu Rev Microbiol 59: 299–328. PMID 15910279. 

Some bacteria produce intracellular nutrient storage granules, such as glycogen,Yeo M, Chater K (2005). "The interplay of glycogen metabolism and differentiation provides an insight into the developmental biology of Streptomyces coelicolor". Microbiology 151 (Pt 3): 855–61. PMID 15758231.  polyphosphate,Shiba T, Tsutsumi K, Ishige K, Noguchi T (2000). "Inorganic polyphosphate and polyphosphate kinase: their novel biological functions and applications". Biochemistry (Mosc) 65 (3): 315–23. PMID 10739474.  sulfurBrune DC. (1995). "Isolation and characterization of sulfur globule proteins from Chromatium vinosum and Thiocapsa roseopersicina". Arch Microbiol 163 (6): 391–99. PMID 7575095.  or polyhydroxyalkanoates.Kadouri D, Jurkevitch E, Okon Y, Castro-Sowinski S. (2005). "Ecological and agricultural significance of bacterial polyhydroxyalkanoates". Crit Rev Microbiol 31 (2): 55–67. PMID 15986831.  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 different light intensities and nutrient levels.Walsby A (1994). "Gas vesicles". Microbiol Rev 58 (1): 94–144. PMID 8177173. 

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 from polysaccharide chains cross-linked by unusual peptides containing D-amino acids.van Heijenoort J (2001). "Formation of the glycan chains in the synthesis of bacterial peptidoglycan". Glycobiology 11 (3): 25R–36R. PMID 11320055.  Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively.Koch A (2003). "Bacterial wall as target for attack: past, present, and future research". Clin Microbiol Rev 16 (4): 673–87. PMID 14557293.  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 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 the Gram stain, a test long-employed for the classification of bacterial species.Gram, HC (1884). "Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten". Fortschr. Med. 2: 185–189. 

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 alternative Gram-positive arrangement.Hugenholtz P (2002). "Exploring prokaryotic diversity in the genomic era". Genome Biol 3 (2): REVIEWS0003. PMID 11864374.  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 as Haemophilus influenzae or Pseudomonas aeruginosa.Walsh F, Amyes S (2004). "Microbiology and drug resistance mechanisms of fully resistant pathogens.". Curr Opin Microbiol 7 (5): 439-44. PMID 15451497. 

In many bacteria an S-layer of rigidly arrayed protein molecules covers the outside of the cell.Engelhardt H, Peters J (1998). "Structural research on surface layers: a focus on stability, surface layer homology domains, and surface layer-cell wall interactions". J Struct Biol 124 (2–3): 276–302. PMID 10049812.  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 in Campylobacter and contain surface enzymes in Bacillus stearothermophilus.Beveridge T, Pouwels P, Sára M, Kotiranta A, Lounatmaa K, Kari K, Kerosuo E, Haapasalo M, Egelseer E, Schocher I, Sleytr U, Morelli L, Callegari M, Nomellini J, Bingle W, Smit J, Leibovitz E, Lemaire M, Miras I, Salamitou S, Béguin P, Ohayon H, Gounon P, Matuschek M, Koval S (1997). "Functions of S-layers". FEMS Microbiol Rev 20 (1–2): 99–149. PMID 9276929. 

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface

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 membrane.Kojima S, Blair D. "The bacterial flagellar motor: structure and function of a complex molecular machine". Int Rev Cytol 233: 93–134. PMID 15037363. 

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 virulence of some bacterial pathogens.Beachey E (1981). "Bacterial adherence: adhesin-receptor interactions mediating the attachment of bacteria to mucosal surface". J Infect Dis 143 (3): 325–45. PMID 7014727.  Pili (sing. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called conjugation (see bacterial genetics, below).Silverman P (1997). "Towards a structural biology of bacterial conjugation". Mol Microbiol 23 (3): 423–9. PMID 9044277. 

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-cellular polymer, to a highly structured capsule or glycocalyx. These structures can protect cells from engulfment by eukaryotic cells, such as macrophages.Stokes R, Norris-Jones R, Brooks D, Beveridge T, Doxsee D, Thorson L (2004). "The glycan-rich outer layer of the cell wall of Mycobacterium tuberculosis acts as an antiphagocytic capsule limiting the association of the bacterium with macrophages". Infect Immun 72 (10): 5676–86. PMID 15385466.  They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms.Daffé M, Etienne G (1999). "The capsule of Mycobacterium tuberculosis and its implications for pathogenicity". Tuber Lung Dis 79 (3): 153–69. PMID 10656114. 

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.Finlay B, Falkow S (1997). "Common themes in microbial pathogenicity revisited". Microbiol Mol Biol Rev 61 (2): 136–69. PMID 9184008. 

Endospores

Further information: Endospores

Bacillus anthracis (stained purple) growing in cerebrospinal fluid

Certain genera of Gram-positive bacteria, such as Bacillus, Clostridium, Sporohalobacter, Anaerobacter and Heliobacterium, can form highly resistant, dormant structures called endospores.Nicholson W, Munakata N, Horneck G, Melosh H, Setlow P (2000). "Resistance of Bacillus endospores to extreme terrestrial and extraterrestrial environments". Microbiol Mol Biol Rev 64 (3): 548–72. PMID 10974126.  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 cell.Siunov A, Nikitin D, Suzina N, Dmitriev V, Kuzmin N, Duda V. "Phylogenetic status of Anaerobacter polyendosporus, an anaerobic, polysporogenic bacterium". Int J Syst Bacteriol 49 Pt 3: 1119–24. PMID 10425769.  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 radiation, detergents, disinfectants, heat, pressure and desiccation.Nicholson W, Fajardo-Cavazos P, Rebeil R, Slieman T, Riesenman P, Law J, Xue Y (2002). "Bacterial endospores and their significance in stress resistance". Antonie Van Leeuwenhoek 81 (1–4): 27–32. PMID 12448702.  In this dormant state, these organisms may remain viable for millions of years,Vreeland R, Rosenzweig W, Powers D (2000). "Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal". Nature 407 (6806): 897–900. PMID 11057666. Cano R, Borucki M (1995). "Revival and identification of bacterial spores in 25- to 40-million-year-old Dominican amber". Science 268 (5213): 1060–4. PMID 7538699.  and endospores even allow bacteria to survive exposure to the vacuum and radiation in space.Nicholson W, Schuerger A, Setlow P (2005). "The solar UV environment and bacterial spore UV resistance: considerations for Earth-to-Mars transport by natural processes and human spaceflight". Mutat Res 571 (1–2): 249–64. PMID 15748651.  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 tetani endospores causes tetanus.Hatheway C (1990). "Toxigenic clostridia". Clin Microbiol Rev 3 (1): 66–98. PMID 2404569. 

Metabolism

Further information: Microbial metabolism

Filaments of photosynthetic cyanobacteria

In contrast to higher organisms, bacteria exhibit an extremely wide variety of metabolic types.Nealson K (1999). "Post-Viking microbiology: new approaches, new data, new insights". Orig Life Evol Biosph 29 (1): 73–93. PMID 11536899.  The distribution of metabolic traits within a group of bacteria has traditionally been used to define their taxonomy, but these traits often do not correspond with modern genetic classifications.Xu J (2006). "Microbial ecology in the age of genomics and metagenomics: concepts, tools, and recent advances". Mol Ecol 15 (7): 1713–31. PMID 16689892.  Bacterial metabolism is classified 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 microorganisms are the electron acceptors used for aerobic or anaerobic respiration.Zillig W (1991). "Comparative biochemistry of Archaea and Bacteria". Curr Opin Genet Dev 1 (4): 544-51. PMID 1822288. 

Carbon metabolism in bacteria is either heterotrophic, where organic carbon compounds are used as carbon sources, or autotrophic, meaning that cellular carbon is obtained by fixing carbon dioxide. Typical autotrophic bacteria are phototrophic cyanobacteria, green sulfur-bacteria and some purple bacteria, but also many chemolithotrophic species, such as nitrifying or sulfur-oxidising bacteria.Hellingwerf K, Crielaard W, Hoff W, Matthijs H, Mur L, van Rotterdam B (1994). "Photobiology of bacteria". Antonie Van Leeuwenhoek 65 (4): 331–47. PMID 7832590.