Bacteria

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This article is about the microorganisms. For the genus, see Bacterium (genus). For other uses, see Bacteria (disambiguation).

Bacteria
Temporal range: Archean–Present 3500–0 Ma [1]
Scanning electron micrograph of Escherichia coli rods
Scientific classification Edit this classification
Domain: Bacteria
Woese et al. 1990
Phyla

See § Phyla

Synonyms
  • "Bacteria" (
    Cavalier-Smith
    1983
  • "Bacteria" Haeckel 1894
  • "Bacteria" Cavalier-Smith 2002
  • "Bacteriaceae" Cohn 1872a
  • "Bacteriobionta" Möhn 1984
  • "Bacteriophyta" Schussnig 1925
  • "Eubacteria" Woese and Fox 1977
  • "Neobacteria" Möhn 1984
  • "Schizomycetaceae" de Toni and Trevisan 1889
  • "Schizomycetes" Nägeli 1857

Bacteria (

parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology
.

Like all animals, humans carry vast numbers (approximately 1013 to 1014) of bacteria.

fermentation, the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology
, and the manufacture of antibiotics and other chemicals.

Once regarded as

scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.[4]

Etymology

Rod-shaped Bacillus subtilis

The word bacteria is the plural of the

Latinisation of the Ancient Greek βακτήριον (baktḗrion),[5] the diminutive of βακτηρία (baktēría), meaning "staff, cane",[6] because the first ones to be discovered were rod-shaped.[7][8]

Origin and early evolution

Main article: Evolution of bacteria
Further information:
Timeline of evolution
Phylogenetic tree of Bacteria, Archaea and Eucarya, with the last universal common ancestor (LUCA) at the root.[9]

The ancestors of bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago.[10] For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[11][12][13] Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology 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 lineage.[14] The most recent common ancestor (MRCA) of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion–3.2 billion years ago.[15][16][17] The earliest life on land may have been bacteria some 3.22 billion years ago.[18]

Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes.

primary endosymbiosis.[23]

Habitat

Bacteria are ubiquitous, living in every possible habitat on the planet including soil, underwater, deep in Earth's crust and even such extreme environments as acidic hot springs and radioactive waste.

hydrogen sulphide and methane, to energy.[29] They live on and in plants and animals. Most do not cause diseases, are beneficial to their environments, and are essential for life.[3][30] The soil is a rich source of bacteria and a few grams contain around a thousand million of them. They are all essential to soil ecology, breaking down toxic waste and recycling nutrients. They are even found in the atmosphere and one cubic metre of air holds around one hundred million bacterial cells. The oceans and seas harbour around 3 x 1026 bacteria which provide up to 50% of the oxygen humans breathe.[31] Only around 2% of bacterial species have been fully studied.[32]

Extremophile bacteria
Habitat Species Reference
Cold (minus 15 °C Antarctica)
Cryptoendoliths
 [33]
Hot (70–100 °C geysers) Thermus aquaticus [32]
Radiation, 5M
Rad
 Deinococcus radiodurans [33]
Saline, 47% salt (Dead Sea, Great Salt Lake) several species [32][33]
Acid pH 3 several species [24]
Alkaline pH 12.8 betaproteobacteria [33]
Space (6 years on a NASA satellite) Bacillus subtilis [33]
3.2 km underground several species [33]
High pressure (Mariana Trench – 1200 atm) Moritella, Shewanella and others [33]

Morphology

Further information: Bacterial cell structure § Cell morphology
a diagram showing bacteria morphology
Bacteria display many cell morphologies and arrangements[8]

Size. Bacteria display a wide diversity of shapes and sizes. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 

Epulopiscium fishelsoni reaches 0.7 mm,[35] and Thiomargarita magnifica can reach even 2 cm in length, which is 50 times larger than other known bacteria.[36][37] Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.3 micrometres, as small as the largest viruses.[38] Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied.[39]

Shape. Most bacterial species are either spherical, called cocci (singular coccus, from Greek kókkos, grain, seed), or rod-shaped, called bacilli (sing. bacillus, from Latin baculus, stick).[40] Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of other unusual shapes have been described, such as star-shaped bacteria.[41] 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.[42][43]

The range of sizes shown by prokaryotes (Bacteria), relative to those of other organisms and biomolecules.[44]

Multicellularity. Most bacterial species exist as single cells; others associate in characteristic patterns:

filaments of Actinomycetota species, the aggregates of Myxobacteria species, and the complex hyphae of Streptomyces species.[45] These multicellular structures are often only seen in certain conditions. 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.[46] In these fruiting bodies, the bacteria perform separate tasks; for example, about one in ten cells migrate to the top of a fruiting body and differentiate into a specialised dormant state called a myxospore, which is more resistant to drying and other adverse environmental conditions.[47]

Biofilms. Bacteria often attach to surfaces and form dense aggregations called biofilms[48] and larger formations known as microbial mats.[49] These biofilms and mats can range from a few micrometres in thickness to up to half a metre 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.[50][51] In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms.[52] 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 biofilms are much harder to kill than individual isolated bacteria.[53]

Cellular structure

Further information: Bacterial cell structure
Gram-positive
bacterial cell (seen by the fact that only one cell membrane is present).

Intracellular structures

The bacterial cell is surrounded by a cell membrane, which is made primarily of phospholipids. This membrane 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.[54] Unlike eukaryotic cells, bacteria usually lack large membrane-bound structures in their cytoplasm such as a nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells.[55] However, some bacteria have protein-bound organelles in the cytoplasm which compartmentalize aspects of bacterial metabolism,[56][57] such as the carboxysome.[58] Additionally, bacteria have a multi-component cytoskeleton to control the localisation of proteins and nucleic acids within the cell, and to manage the process of cell division.[59][60][61]

Many important biochemical reactions, such as energy generation, occur due to concentration gradients across membranes, creating a potential difference analogous to a battery. The general lack of internal membranes in bacteria means these reactions, such as electron transport, occur across the cell membrane between the cytoplasm and the outside of the cell or periplasm.[62] However, in many photosynthetic bacteria, the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane.[63] These light-gathering complexes may even form lipid-enclosed structures called chlorosomes in green sulfur bacteria.[64]

Halothiobacillus neapolitanus cells with carboxysomes
inside, with arrows highlighting visible carboxysomes. Scale bars indicate 100 nm.

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

circular bacterial chromosome of DNA located in the cytoplasm in an irregularly shaped body called the nucleoid.[65] The nucleoid contains the chromosome with its associated proteins and RNA. Like all other organisms, bacteria contain ribosomes for the production of proteins, but the structure of the bacterial ribosome is different from that of eukaryotes and archaea.[66]

Some bacteria produce intracellular nutrient storage granules, such as glycogen,[67] polyphosphate,[68] sulfur[69] or polyhydroxyalkanoates.[70] Bacteria such as the photosynthetic cyanobacteria, produce internal gas vacuoles, which they use to regulate their buoyancy, allowing them to move up or down into water layers with different light intensities and nutrient levels.[71]

Extracellular structures

Further information: Cell envelope

Around the outside of the cell membrane is the cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by peptides containing D-amino acids.[72] Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively.[73] The cell wall of bacteria is also distinct from that of achaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin (produced by a fungus called Penicillium) is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.[73]

There are broadly speaking two different types of cell wall in bacteria, that classify bacteria into Gram-positive bacteria and Gram-negative bacteria. The names originate from the reaction of cells to the Gram stain, a long-standing test for the classification of bacterial species.[74]

Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and

mycobacteria which have a thick peptidoglycan cell wall like a Gram-positive bacterium, but also a second outer layer of lipids.[77]

In many bacteria, an

Bacillus stearothermophilus.[79][80]

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface
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.[81]

genetic material between bacterial cells in a process called conjugation where they are called conjugation pili or sex pili (see bacterial genetics, below).[84] They can also generate movement where they are called type IV pili.[85]

Glycocalyx is produced by many bacteria to surround their cells,[86] and varies in structural complexity: ranging from a disorganised slime layer of extracellular polymeric substances to a highly structured capsule. These structures can protect cells from engulfment by eukaryotic cells such as macrophages (part of the human immune system).[87] They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms.[88]

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.[88]

Endospores

Further information: Endospore
Anthrax stained purple
Bacillus anthracis (stained purple) growing in cerebrospinal fluid[89]

Some genera of Gram-positive bacteria, such as Bacillus, Clostridium, Sporohalobacter, Anaerobacter, and Heliobacterium, can form highly resistant, dormant structures called endospores.[90] Endospores develop within the cytoplasm of the cell; generally, a single endospore develops in each cell.[91] Each endospore contains a core of DNA and ribosomes surrounded by a cortex layer and protected by a multilayer rigid coat composed of peptidoglycan and a variety of proteins.[91]

Endospores show no detectable

comets, planetoids, or directed panspermia.[96][97]

Endospore-forming bacteria can 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, which, like botulism, is caused by a toxin released by the bacteria that grow from the spores.[98] Clostridioides difficile infection, a common problem in healthcare settings, is caused by spore-forming bacteria.[99]

Metabolism

Further information: Microbial metabolism

Bacteria exhibit an extremely wide variety of

energy, the electron donors used, and the source of carbon used for growth.[102]

terminal electron acceptor in a redox reaction. Chemotrophs are further divided by the types of compounds they use to transfer electrons. Bacteria that derive electrons from inorganic compounds such as hydrogen, carbon monoxide, or ammonia are called lithotrophs, while those that use organic compounds are called organotrophs.[103] Still, more specifically, aerobic organisms use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide.[103]

Many bacteria, called

fixing carbon dioxide.[104] In unusual circumstances, the gas methane can be used by methanotrophic bacteria as both a source of electrons and a substrate for carbon anabolism.[105]

Nutritional types in bacterial metabolism
Nutritional type Source of energy Source of carbon Examples
 Phototrophs  Sunlight  Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs)  Cyanobacteria, Green sulfur bacteria, Chloroflexota, or Purple bacteria 
 Lithotrophs Inorganic compounds  Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs)  Thermodesulfobacteriota, Hydrogenophilaceae, or Nitrospirota 
 Organotrophs Organic compounds  Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs)  Bacillus, Clostridium, or Enterobacteriaceae 

In many ways, bacterial metabolism provides traits that are useful for

terminal electron acceptors depending on the environmental conditions in which they find themselves.[110]

Growth and reproduction

Further information: Bacterial growth
binary fission, which is compared to mitosis and meiosis
in this image.
A culture of Salmonella
E. coli colony
A colony of Escherichia coli[111]

Unlike in multicellular organisms, increases in cell size (

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 hyphae formation by Streptomyces species, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell.[114]

In the laboratory, bacteria are usually grown using solid or liquid media.[115] Solid growth media, such as agar plates, are used to isolate pure cultures of a bacterial strain. However, liquid growth media are used when the 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 organisms.[116]

Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly.

cyanobacterial blooms that often occur in lakes during the summer.[117] Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by Streptomyces that inhibit the growth of competing microorganisms.[118] In nature, many organisms live in communities (e.g., biofilms) that may allow for increased supply of nutrients and protection from environmental stresses.[52] These relationships can be essential for growth of a particular organism or group of organisms (syntrophy).[119]

death phase where the bacteria run out of nutrients and die.[123]

Genetics

Main article: Bacterial genetics
T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The bacterial cells are ~ 0.5 µm wide.[124]

Most bacteria have a single circular

antibiotic resistance, metabolic capabilities, or various virulence factors.[130]

Bacteria genomes usually encode a few hundred to a few thousand genes. The genes in bacterial genomes are usually a single continuous stretch of DNA. Although several different types of introns do exist in bacteria, these are much rarer than in eukaryotes.[131]

Bacteria, as asexual organisms, inherit an identical copy of the parent's genome and are clonal. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations arise from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria.[132] Genetic changes in bacterial genomes emerge from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate.[133]

Some bacteria transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment in a process called transformation.[134] Many bacteria can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA.[135] The development of competence in nature is usually associated with stressful environmental conditions and seems to be an adaptation for facilitating repair of DNA damage in recipient cells.[136] Second, bacteriophages can integrate into the bacterial chromosome, introducing foreign DNA in a process known as transduction. Many types of bacteriophage exist; some infect and lyse their host bacteria, while others insert into the bacterial chromosome.[137] Bacteria resist phage infection through restriction modification systems that degrade foreign DNA[138] and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference.[139][140] Third, bacteria can transfer genetic material through direct cell contact via conjugation.[141]

In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species, and this may have significant consequences, such as the transfer of antibiotic resistance.[142][143] In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.[144]

Behaviour

Movement

Main article: Bacterial motility
Desulfovibrio vulgaris
showing a single flagellum at one end of the cell. Scale bar is 0.5 micrometers long.

Many bacteria are motile (able to move themselves) and do so using a variety of mechanisms. The best studied of these are flagella, long filaments that are turned by a motor at the base to generate propeller-like movement.[145] The bacterial flagellum is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly.[145] The flagellum is a rotating structure driven by a reversible motor at the base that uses the electrochemical gradient across the membrane for power.[146]

The different arrangements of bacterial flagella: A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous

Bacteria can use flagella in different ways to generate different kinds of movement. Many bacteria (such as E. coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-dimensional random walk.[147] Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum (monotrichous), a flagellum at each end (amphitrichous), clusters of flagella at the poles of the cell (lophotrichous), while others have flagella distributed over the entire surface of the cell (peritrichous). The flagella of a group of bacteria, the spirochaetes, are found between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves.[145]

Two other types of bacterial motion are called

gliding motility, that uses other mechanisms. In twitching motility, the rod-like pilus extends out from the cell, binds some substrate, and then retracts, pulling the cell forward.[149]

Motile bacteria are attracted or repelled by certain stimuli in behaviours called taxes: these include chemotaxis, phototaxis, energy taxis, and magnetotaxis.[150][151][152] In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores.[47] The myxobacteria move only when on solid surfaces, unlike E. coli, which is motile in liquid or solid media.[153]

Several Listeria and Shigella species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell. By promoting actin polymerisation at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.[154]

Communication

See also: Prokaryote § Sociality

A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals.[155]

Bacteria often function as multicellular aggregates known as

biofilms, exchanging a variety of molecular signals for intercell communication and engaging in coordinated multicellular behaviour.[156][157]

The communal benefits of multicellular cooperation include a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types.[156] For example, bacteria in biofilms can have more than five hundred times increased resistance to antibacterial agents than individual "planktonic" bacteria of the same species.[157]

One type of intercellular communication by a

pheromones that accumulate with the growth in cell population.[160]

Classification and identification

Main article: Bacterial taxonomy
Further information:
Scientific classification, Systematics, Bacterial phyla, and Clinical pathology
blue stain of Streptococcus mutans
Streptococcus mutans visualised with a Gram stain.
CPR ultramicrobacterias, Terrabacteria and Gracilicutes according to recent genomic analyzes (2019).[161]

International Code of Nomenclature of Bacteria.[166]

Historically, bacteria were considered a part of the

Plantae, the Plant kingdom, and were called "Schizomycetes" (fission-fungi).[167] For this reason, collective bacteria and other microorganisms in a host are often called "flora".[168]
The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes. However, molecular systematics showed prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor.[4] The archaea and eukaryotes are more closely related to each other than either is to the bacteria. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in microbiology.[169] However, due to the relatively recent introduction of molecular systematics and a rapid increase in the number of genome sequences that are available, bacterial classification remains a changing and expanding field.[170][171] For example, Cavalier-Smith argued that the Archaea and Eukaryotes evolved from Gram-positive bacteria.[172]

The identification of bacteria in the laboratory is particularly relevant in medicine, where the correct treatment is determined by the bacterial species causing an infection. Consequently, the need to identify human pathogens was a major impetus for the development of techniques to identify bacteria.[173]

The

Ziehl–Neelsen or similar stains.[175] Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology.[176]

selective media to identify organisms that cause diarrhea while preventing growth of non-pathogenic bacteria. Specimens that are normally sterile, such as blood, urine or spinal fluid, are cultured under conditions designed to grow all possible organisms.[116][178] Once a pathogenic organism has been isolated, it can be further characterised by its morphology, growth patterns (such as aerobic or anaerobic growth), patterns of hemolysis, and staining.[179]

As with bacterial classification, identification of bacteria is increasingly using molecular methods,[180] and mass spectroscopy.[181] Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory.[182] Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods.[183] These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing.[184] However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty. Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea;[185] but attempts to estimate the true number of bacterial diversity have ranged from 107 to 109 total species—and even these diverse estimates may be off by many orders of magnitude.[186][187]

Phyla

Main article: Bacterial phyla

The following phyla have been validly published according to the

Bacteriological Code:[188]

Interactions with other organisms

Further information:
Microbes in human culture
chart showing bacterial infections upon various parts of human body
Overview of bacterial infections and main species involved.[189]

Despite their apparent simplicity, bacteria can form complex associations with other organisms. These symbiotic associations can be divided into parasitism, mutualism and commensalism.[190]

Commensals

The word "commensalism" is derived from the word "commensal", meaning "eating at the same table"[191] and all plants and animals are colonised by commensal bacteria. In humans and other animals, millions of them live on the skin, the airways, the gut and other orifices.[192][193] Referred to as "normal flora",[194] or "commensals",[195] these bacteria usually cause no harm but may occasionally invade other sites of the body and cause infection. Escherichia coli is a commensal in the human gut but can cause urinary tract infections.[196] Similarly, streptococci, which are part of the normal flora of the human mouth, can cause heart disease.[197]

Predators

Some species of bacteria kill and then consume other microorganisms; these species are called predatory bacteria.[198] These include organisms such as Myxococcus xanthus, which forms swarms of cells that kill and digest any bacteria they encounter.[199] Other bacterial predators either attach to their prey in order to digest them and absorb nutrients or invade another cell and multiply inside the cytosol.[200] These predatory bacteria are thought to have evolved from saprophages that consumed dead microorganisms, through adaptations that allowed them to entrap and kill other organisms.[201]

Mutualists

Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of

anaerobic bacteria that consume organic acids, such as butyric acid or propionic acid, and produce hydrogen, and methanogenic archaea that consume hydrogen.[202] The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their surroundings. Only the intimate association with the hydrogen-consuming archaea keeps the hydrogen concentration low enough to allow the bacteria to grow.[203]

In soil, microorganisms that reside in the

competitive exclusion) and these beneficial bacteria are consequently sold as probiotic dietary supplements.[208]

Nearly all

amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin.[209]

Pathogens

Main article: Pathogenic bacteria
Neisseria gonorrhoeae and pus cells from a penile discharge (Gram stain)
Salmonella typhimurium
 (red) invading cultured human cells

The body is continually exposed to many species of bacteria, including beneficial commensals, which grow on the skin and

microorganisms.[210] Unlike some viruses, bacteria evolve relatively slowly so many bacterial diseases also occur in other animals.[211]

If bacteria form a parasitic association with other organisms, they are classed as pathogens.

Johne's disease, mastitis, salmonella and anthrax in farm animals.[215]

Gram-stained micrograph of bacteria from the vagina
In bacterial vaginosis, beneficial bacteria in the vagina (top) are displaced by pathogens (bottom). Gram stain.

Each species of pathogen has a characteristic spectrum of interactions with its human

toxoids, which are used as vaccines to prevent the disease.[222]

Bacterial infections may be treated with

antibiotic resistance in bacterial populations.[224] Infections can be prevented by antiseptic measures such as sterilising the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilised to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.[225]

Significance in technology and industry

Bacteria, often

fermented foods, such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt.[226][227]

The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and

agrichemicals.[230]

Bacteria can also be used in place of pesticides in biological pest control. This commonly involves Bacillus thuringiensis (also called BT), a Gram-positive, soil-dwelling bacterium. Subspecies of this bacteria are used as Lepidopteran-specific insecticides under trade names such as Dipel and Thuricide.[231] Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects.[232][233]

Because of their ability to quickly grow and the relative ease with which they can be manipulated, bacteria are the workhorses for the fields of

bioengineer bacteria for the production of therapeutic proteins, such as insulin, growth factors, or antibodies.[237][238]

Because of their importance for research in general, samples of bacterial strains are isolated and preserved in

Biological Resource Centers. This ensures the availability of the strain to scientists worldwide.[239]

History of bacteriology

For the history of microbiology, see Microbiology. For the history of bacterial classification, see Bacterial taxonomy. For the natural history of Bacteria, see Last universal common ancestor.
painting of Antonie van Leeuwenhoek, in robe and frilled shirt, with ink pen and paper
Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.

Bacteria were first observed by the Dutch microscopist

Royal Society of London.[240] Bacteria were Leeuwenhoek's most remarkable microscopic discovery. Their size was just at the limit of what his simple lenses could resolve, and, in one of the most striking hiatuses in the history of science, no one else would see them again for over a century.[241] His observations also included protozoans which he called animalcules, and his findings were looked at again in the light of the more recent findings of cell theory.[242]

Christian Gottfried Ehrenberg introduced the word "bacterium" in 1828.[243] In fact, his Bacterium was a genus that contained non-spore-forming rod-shaped bacteria,[244] as opposed to Bacillus, a genus of spore-forming rod-shaped bacteria defined by Ehrenberg in 1835.[245]

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.[246] Before them, Ignaz Semmelweis and Joseph Lister had realised the importance of sanitized hands in medical work. Semmelweis, who in the 1840s formulated his rules for handwashing in the hospital, prior to the advent of germ theory, attributed disease to "decomposing animal organic matter." His ideas were rejected and his book on the topic condemned by the medical community. After Lister, however, doctors started sanitizing their hands in the 1870s.[247]

Robert Koch, a pioneer in medical microbiology, worked on cholera, anthrax and tuberculosis. In his research into tuberculosis, Koch finally proved the germ theory, for which he received a Nobel Prize in 1905.[248] In Koch's postulates, he set out criteria to test if an organism is the cause of a disease, and these postulates are still used today.[249]

Ferdinand Cohn is said to be a founder of bacteriology, studying bacteria from 1870. Cohn was the first to classify bacteria based on their morphology.[250][251]

Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antibacterial treatments were available.[252] 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.[253] Ehrlich, who had been awarded a 1908 Nobel Prize for his work on immunology, 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.[254]

A major step forward in the study of bacteria came in 1977 when

phylogenetic taxonomy depended on the sequencing of 16S ribosomal RNA and divided prokaryotes into two evolutionary domains, as part of the three-domain system.[4]

See also

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Bibliography

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Bacteria&oldid=1218483011"