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Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of Prokaryote . Typically a few in length, bacteria were among the first life forms to appear on Earth, and are present in most of its . Bacteria inhabit soil, water, Hot spring, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of cadaver; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and , extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and 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. Most are in the gut flora, though there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are probiotic, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. are used to treat Infection and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of , the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector (biomining, bioleaching), as well as in biotechnology, and the manufacture of antibiotics and other chemicals.
Once regarded as constituting the class Schizomycetes ("fission fungi"), bacteria are now classified as . Unlike cells of animals and other , bacterial cells do not contain a cell nucleus and rarely harbour cell membrane-bound . 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 evolution from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.
Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiont associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea. This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbiosis to form either mitochondrion or , which are still found in all known Eukarya (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of in algae and plants. This is known as primary endosymbiosis.
| + Extremophile bacteria ! Habitat !! Species !! Reference |
Shape. Most bacterial species are either spherical, called coccus ( singular coccus, from Greek kókkos, grain, seed), or rod-shaped, called bacilli ( sing. bacillus, from Latin baculus, stick). Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called Spirillum, or tightly coiled, called . A small number of other unusual shapes have been described, such as star-shaped bacteria. 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 predation.
Multicellularity. Most bacterial species exist as single cells; others associate in characteristic patterns: Neisseria forms diploids (pairs), Streptococcus form chains, and Staphylococcus group together in "bunch of grapes" clusters. Bacteria can also group to form larger multicellular structures, such as the elongated filaments of Actinomycetota species, the aggregates of Myxobacteria species, and the complex hyphae of Streptomyces species. 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. 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.
Biofilms. Bacteria often attach to surfaces and form dense aggregations called and larger formations known as . 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, and archaea. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolony, through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of 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 , and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.
Many important biochemistry reactions, such as energy generation, occur due to diffusion 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. However, in many photosynthetic bacteria, the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane. These light-gathering complexes may even form lipid-enclosed structures called in green sulfur bacteria.
Bacteria do not have a membrane-bound nucleus, and their material is typically a single circular bacterial chromosome of DNA located in the cytoplasm in an irregularly shaped body called the nucleoid. The nucleoid contains the chromosome with its associated proteins and RNA. Like all other , bacteria contain for the production of proteins, but the structure of the bacterial ribosome is different from that of and archaea.
Some bacteria produce intracellular nutrient storage granules, such as glycogen, polyphosphate, sulfur or polyhydroxyalkanoates. Bacteria such as the Photosynthesis cyanobacteria, produce internal Gas vesicle, which they use to regulate their buoyancy, allowing them to move up or down into water layers with different light intensities and nutrient levels.
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.
Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and . In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second Lipid bilayer containing lipopolysaccharides and . Most bacteria have the Gram-negative cell wall, and only members of the Bacillota group and actinomycetota (previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the 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 , such as Haemophilus influenzae or Pseudomonas aeruginosa. Some bacteria have cell wall structures that are neither classically Gram-positive or Gram-negative. This includes clinically important bacteria such as mycobacteria which have a thick peptidoglycan cell wall like a Gram-positive bacterium, but also a second outer layer of lipids.
In many bacteria, an S-layer of rigidly arrayed protein molecules covers the outside of the cell. This layer provides chemical and physical protection for the cell surface and can act as a macromolecule diffusion barrier. S-layers have diverse functions and are known to act as virulence factors in Campylobacter species and contain surface in Bacillus stearothermophilus.
Flagellum 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 down an electrochemical gradient across the cell membrane. (2024). 9780123646378 ISBN 9780123646378
Fimbriae (sometimes called "attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometres 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. Pilus ( sing. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called conjugation where they are called conjugation pili or sex pili (see bacterial genetics, below). They can also generate movement where they are called type IV pili.
Glycocalyx is produced by many bacteria to surround their cells, 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 (part of the human immune system). They can also act as and be involved in cell recognition, as well as aiding attachment 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.
Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of ultraviolet, gamma ray, , , heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years. Endospores even allow bacteria to survive exposure to the Hard vacuum and radiation of outer space, leading to the possibility that bacteria could be distributed throughout the Universe by space dust, , asteroids, comets, planetoids, or directed panspermia.
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. Clostridioides difficile infection, a common problem in healthcare settings, is caused by spore-forming bacteria.
Phototrophy bacteria derive energy from light using photosynthesis, while chemotrophy bacteria breaking down chemical compounds through oxidation, driving metabolism by transferring electrons from a given electron donor to a terminal electron acceptor in a redox. 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 , while those that use organic compounds are called . Still, more specifically, use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide.
Many bacteria, called , derive their carbon from other organic compound. Others, such as cyanobacteria and some purple bacteria, are , meaning they obtain cellular carbon by Carbon fixation carbon dioxide. In unusual circumstances, the gas methane can be used by bacteria as both a source of and a substrate for carbon anabolism.
| + Nutritional types in bacterial metabolism | |||
| Sunlight | Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs) | Cyanobacteria, Green sulfur bacteria, Chloroflexota, or Purple bacteria | |
| Inorganic compounds | Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs) | Thermodesulfobacteriota, Hydrogenophilaceae, or Nitrospirota | |
| Organic compounds | Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs) | Bacillus, Clostridium, or Enterobacteriaceae |
In many ways, bacterial metabolism provides traits that are useful for ecological stability and for human society. For example, have the ability to fix nitrogen gas using the enzyme nitrogenase. This trait, which can be found in bacteria of most metabolic types listed above, leads to the ecologically important processes of denitrification, sulfate reduction, and acetogenesis, respectively. Bacterial metabolic processes are important drivers in biological responses to pollution; for example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms of mercury (methylmercury and dimethylmercury) in the environment. Nonrespiratory 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.
In the laboratory, bacteria are usually grown using solid or liquid media. Solid Growth medium, such as , 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.
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 bloom and cyanobacterial blooms that often occur in lakes during the summer. Other organisms have adaptations to harsh environments, such as the production of multiple by Streptomyces that inhibit the growth of competing microorganisms. In nature, many organisms live in communities (e.g., ) that may allow for increased supply of nutrients and protection from environmental stresses. These relationships can be essential for growth of a particular organism or group of organisms (syntrophy).
Bacterial growth follows four 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 growth are produced. The second phase of growth is the logarithmic phase, also known as the exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the growth rate ( 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 third 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 gene expression involved in DNA repair, antioxidant and active transport. (2024). 9780120277445 ISBN 9780120277445 The final phase is the death phase where the bacteria run out of nutrients and die.
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 do exist in bacteria, these are much rarer than in eukaryotes.
Bacteria, as asexual organisms, inherit an identical copy of the parent's genome and are Clonal colony. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or . Mutations arise from errors made during the replication of DNA or from exposure to . Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. 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.
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. 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. 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.Bernstein H, Bernstein C, Michod RE (2012). "DNA repair as the primary adaptive function of sex in bacteria and eukaryotes". Chapter 1: pp. 1–49 in: DNA Repair: New Research, Sakura Kimura and Sora Shimizu (eds.). Nova Sci. Publ., Hauppauge, NY . Second, can integrate into the bacterial chromosome, introducing foreign DNA in a process known as transduction. Many types of bacteriophage exist; some infect and lytic cycle their host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA 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. Third, bacteria can transfer genetic material through direct cell contact via conjugation.
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. In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.
Bacteria can use flagella in different ways to generate different kinds of movement. Many bacteria (such as Escherichia coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-Dimension random walk. 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 , are found between two membranes in the periplasmic space. They have a distinctive helix body that twists about as it moves.
Two other types of bacterial motion are called twitching motility that relies on a structure called the type IV pilus, and 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.
Motile bacteria are attracted or repelled by certain stimuli in behaviours called taxis: these include chemotaxis, phototaxis, taxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores. The myxobacteria move only when on solid surfaces, unlike E. coli, which is motile in liquid or solid media.
Several Listeria and Shigella species move inside host cells by usurping the cytoskeleton, which is normally used to move inside the cell. By promoting actin biopolymer at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.
Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for Cell signaling and engaging in coordinated multicellular behaviour.
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. For example, bacteria in biofilms can have more than five hundred times increased resistance to Bactericide agents than individual "planktonic" bacteria of the same species.
One type of intercellular communication by a cell signal is called quorum sensing, which serves the purpose of determining whether the local population density is sufficient to support investment in processes that are only successful if large numbers of similar organisms behave similarly, such as excreting or emitting light. (2024). 9783030285234 ISBN 9783030285234 Quorum sensing enables bacteria to coordinate gene expression and to produce, release, and detect or pheromones that accumulate with the growth in cell population.
Historically, bacteria were considered a part of the plants, the Plant kingdom, and were called "Schizomycetes" (fission-fungi)." Schizomycetes". Merriam-Webster Medical Dictionary. Accessed 3 August 2021. For this reason, collective bacteria and other microorganisms in a host are often called "flora". 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. 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. 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. For example, Cavalier-Smith argued that the Archaea and Eukaryotes evolved from Gram-positive bacteria.
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.
The Gram stain, developed in 1884 by Hans Christian Gram, characterises bacteria based on the structural characteristics of their cell walls. The thick layers of peptidoglycan in the "Gram-positive" cell wall stain purple, while the thin "Gram-negative" cell wall appears pink. By combining morphology and Gram-staining, most bacteria can be classified as belonging to one of four groups (Gram-positive cocci, Gram-positive bacilli, Gram-negative cocci and Gram-negative bacilli). Some organisms are best identified by stains other than the Gram stain, particularly mycobacteria or Nocardia, which show acid-fastness on Ziehl–Neelsen or similar stains. Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology.
Culture techniques are designed to promote the growth and identify particular bacteria while restricting the growth of the other bacteria in the sample. Often these techniques are designed for specific specimens; for example, a sputum sample will be treated to identify organisms that cause pneumonia, while feces specimens are cultured on 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. Once a pathogenic organism has been isolated, it can be further characterised by its morphology, growth patterns (such as aerobic organism or anaerobic growth), patterns of hemolysis, and staining.
As with bacterial classification, identification of bacteria is increasingly using molecular methods, and mass spectroscopy. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods. These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing. 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; 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.
In soil, microorganisms that reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compounds. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the can contribute to gut immunity, synthesise , such as folic acid, vitamin K and biotin, convert Milk protein to lactic acid (see Lactobacillus), as well as fermenting complex undigestible . The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and these beneficial bacteria are consequently sold as probiotic dietary supplements.
Nearly all Life is dependent on bacteria for survival as only bacteria and some archaea possess the genes and enzymes necessary to synthesise vitamin B12, also known as cobalamin, and provide it through the food chain. Vitamin B12 is a water-soluble vitamin that is involved in the metabolism of every cell of the human body. It is a cofactor in DNA replication and in both fatty acid and amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the myelinogenesis.
If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus (caused by Clostridium tetani), typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy (caused by Mycobacterium leprae) and tuberculosis (caused by Mycobacterium tuberculosis). A pathogenic cause for a known medical disease may only be discovered many years later, as was the case with Helicobacter pylori and peptic ulcer disease. Bacterial diseases are also important in agriculture, and bacteria cause leaf spot, fire blight and Wilting in plants, as well as Johne's disease, mastitis, salmonellosis and anthrax in farm animals. Each species of pathogen has a characteristic spectrum of interactions with its human hosts. Some organisms, such as Staphylococcus or Streptococcus, can cause skin infections, pneumonia, meningitis and sepsis, a systemic Inflammation producing shock, massive vasodilator and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as Rickettsia, which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of Rickettsia causes typhus, while another causes Rocky Mountain spotted fever. Chlamydia, another phylum of obligate intracellular parasites, contains species that can cause pneumonia or urinary tract infection and may be involved in coronary heart disease. Some species, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Mycobacterium avium, are opportunistic pathogens and cause disease mainly in people who are immunosuppressed or have cystic fibrosis. Some bacteria produce Microbial toxin, which cause diseases. These are , which come from broken bacterial cells, and , which are produced by bacteria and released into the environment. The bacterium Clostridium botulinum for example, produces a powerful exotoxin that cause respiratory paralysis, and Salmonellae produce an endotoxin that causes gastroenteritis. Some exotoxins can be converted to toxoids, which are used as vaccines to prevent the disease.
Bacterial infections may be treated with , which are classified as Bactericide if they kill bacteria or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics, and each class enzyme inhibitor a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial ribosome, but not the structurally different eukaryotic ribosome. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of antibiotic resistance in bacterial populations. 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. such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.
The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and bioremediation. Bacteria capable of digesting the in petroleum are often used to clean up . Fertiliser was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial . In the chemical industry, bacteria are most important in the production of pure chemicals for use as pharmaceuticals or .
Bacteria can also be used in place of 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 -specific under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, , and most other beneficial insects.
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 molecular biology, genetics, and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes, , and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetics and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of Escherichia coli metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineering bacteria for the production of therapeutic proteins, such as insulin, , or antibody.
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.
Christian Gottfried Ehrenberg introduced the word "bacterium" in 1828. In fact, his Bacterium was a genus that contained non-spore-forming rod-shaped bacteria, as opposed to Bacillus, a genus of spore-forming rod-shaped bacteria defined by Ehrenberg in 1835.
Louis Pasteur demonstrated in 1859 that the growth of microorganisms causes the fermentation process and that this growth is not due to spontaneous generation ( and molds, commonly associated with fermentation, are not bacteria, but rather fungus). Along with his contemporary Robert Koch, Pasteur was an early advocate of the germ theory of disease. Before them, Ignaz Semmelweis and Joseph Lister had realised the importance of sanitised 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 sanitising their hands in the 1870s.
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. 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.
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.
Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antiseptic treatments were available. 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. 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.
A major step forward in the study of bacteria came in 1977 when Carl Woese recognised that archaea have a separate line of evolutionary descent from bacteria. This new phylogenetic taxonomy depended on the sequencing of 16S ribosomal RNA and divided prokaryotes into two evolutionary domains, as part of the three-domain system.
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