Bacteria may reproduce and change using the following methods:

• Binary fission: An asexual form of reproduction, in which a cell continues to grow until a new cell wall grows through the center, forming two cells. These separate, making two cells with the same genetic material.
• Transfer of genetic material: Cells acquire new genetic material through processes known as conjugation, transformation, or transduction. These processes can make bacteria stronger and more able to resist threats, such as antibiotic medication.
• Spores: When some types of bacteria are low on resources, they can form spores. Spores hold the organism’s DNA material and contain the enzymes needed for germination. They are very resistant to environmental stresses. The spores can remain inactive for centuries, until the right conditions occur. Then they can reactivate and become bacteria.
• Spores can survive through periods of environmental stress, including ultraviolet (UV) and gamma radiation, desiccation, starvation, chemical exposure, and extremes of temperature.

Some bacteria produce endospores, or internal spores, while others produce exospores, which are released outside. These are known as cysts.

Clostridium is an example of an endospore-forming bacterium. There are about 100 species of Clostridium, including Clostridium botulinim (C. botulinim) or botulism, responsible for a potentially fatal kind of food poisoning, and Clostridium difficile (C. Difficile), which causes colitis and other intestinal problems.

Bacteria can be found in soil, water, plants, animals, radioactive waste, deep in the earth’s crust, arctic ice and glaciers, and hot springs. There are bacteria in the stratosphere, between 6 and 30 miles up in the atmosphere, and in the ocean depths, down to 32,800 feet or 10,000 meters deep.

Aerobes, or aerobic bacteria, can only grow where there is oxygen. Some types can cause problems for the human environment, such as corrosion, fouling, problems with water clarity, and bad smells.

Anaerobes, or anaerobic bacteria, can only grow where there is no oxygen. In humans, this is mostly in the gastrointestinal tract. They can also cause gas, gangrene, tetanus, botulism, and most dental infections.

Facultative anaerobes, or facultative anaerobic bacteria, can live either with or without oxygen, but they prefer environments where there is oxygen. They are mostly found in soil, water, vegetation and some normal flora of humans and animals. Examples include Salmonella.

Mesophiles, or mesophilic bacteria, are the bacteria responsible for most human infections. They thrive in moderate temperatures, around 37°C. This is the temperature of the human body.

Examples include Listeria monocytogenes, Pesudomonas maltophilia, Thiobacillus novellus, Staphylococcus aureus, Streptococcus pyrogenes, Streptococcus pneumoniae, Escherichia coli, and Clostridium kluyveri.

The human intestinal flora, or gut microbiome, contains beneficial mesophilic bacteria, such as dietary Lactobacillus acidophilus.

Extremophiles, or extremophilic bacteria, can withstand conditions considered too extreme for most life forms.

Thermophiles can live in high temperatures, up to 75 to 80°C, and hyperthermophiles can surivive in temperatures up to 113°C.

Deep in the ocean, bacteria live in total darkness by thermal vents, where both temperature and pressure are high. They make their own food by oxidizing sulfur that comes from deep inside the earth.

Other extremophiles include:

• halophiles, found only in a salty environment
• acidophiles, some of which live in environments as acidic as pH 0
• alkaliphiles, living in alkiline environments up to pH 10.5
• psychrophiles, found in cold temperatures, for example, in glaciers

Extremophiles can survive where no other organism can.

The fact that pathogenic bacteria are constantly battling their host’s immune system might account for the bewildering number of different strains, or types, of bacteria that belong to the same species but are distinguishable by serological tests. Microbiologists often identify bacteria by the presence of specific molecules on their cell surfaces, which are detected with specific antibodies. Antibodies are serum proteins that bind very tightly to foreign molecules (antigens) in an immune reaction aimed at removing or destroying the antigens. Antibodies have remarkable specificity, and the substitution of even one amino acid in a protein might prevent that protein from being recognized by an antibody.

For many bacterial species there are thousands of different strains (called serovars, for serological variants), which differ from one another mainly or solely in the antigenic identity of their lipopolysaccharide, flagella, or capsule. Different serovars of enteric bacteria—such as E. coli and Salmonella enterica, for example—are often found to be associated with the ability to inhabit different host animals or to cause different diseases. Formation of these numerous serovars reflects the ability of bacteria to respond effectively to the intense defensive actions of the immune system.

The cytoplasm of bacteria contains high concentrations of enzymes, metabolites, and salts. In addition, the proteins of the cell are made on ribosomes that are scattered throughout the cytoplasm. Bacterial ribosomes are different from ribosomes in eukaryotic cells in that they are smaller, have fewer constituents (consist of three types of ribosomal RNA [rRNA] and 55 proteins, as opposed to four types of rRNA and 78 proteins in eukaryotes), and are inhibited by different antibiotics than those that act on eukaryotic ribosomes.

There are numerous inclusion bodies, or granules, in the bacterial cytoplasm. These bodies are never enclosed by a membrane and serve as storage vessels. Glycogen, which is a polymer of glucose, is stored as a reserve of carbohydrate and energy. Volutin, or metachromatic granules, contains polymerized phosphate and represents a storage form for inorganic phosphate and energy. Many bacteria possess lipid droplets that contain polymeric esters of poly-β-hydroxybutyric acid or related compounds. This is in contrast to eukaryotes, which use lipid droplets to store triglycerides. In bacteria, storage granules are produced under favourable growth conditions and are consumed after the nutrients have been depleted from the medium. Many aquatic bacteria produce gas vacuoles, which are protein-bound structures that contain air and allow the bacteria to adjust their buoyancy. Bacteria can also have internal membranous structures that form as outgrowths of the cytoplasmic membrane.

Many bacterial cells secrete some extracellular material in the form of a capsule or a slime layer. A slime layer is loosely associated with the bacterium and can be easily washed off, whereas a capsule is attached tightly to the bacterium and has definite boundaries. Capsules can be seen under a light microscope by placing the cells in a suspension of India ink. The capsules exclude the ink and appear as clear halos surrounding the bacterial cells. Capsules are usually polymers of simple sugars (polysaccharides), although the capsule of Bacillus anthracis is made of polyglutamic acid. Most capsules are hydrophilic (“water-loving”) and may help the bacterium avoid desiccation (dehydration) by preventing water loss. Capsules can protect a bacterial cell from ingestion and destruction by white blood cells (phagocytosis). While the exact mechanism for escaping phagocytosis is unclear, it may occur because capsules make bacterial surface components more slippery, helping the bacterium to escape engulfment by phagocytic cells. The presence of a capsule in Streptococcus pneumoniae is the most important factor in its ability to cause pneumonia. Mutant strains of S. pneumoniae that have lost the ability to form a capsule are readily taken up by white blood cells and do not cause disease. The association of virulence and capsule formation is also found in many other species of bacteria.

Acinetobacter calcoaceticus The capsular material surrounding these bacteria (Acinetobacter calcoaceticus) is revealed in a suspension of India ink and viewed through a light microscope (magnified about 2,500×).From W.H. Taylor and E. Juni, “Pathways for Biosynthesis of a Bacterial Capsular Polysaccharide,” Journal of Bacteriology (May 1961)

A capsular layer of extracellular polysaccharide material can enclose many bacteria into a biofilm and serves many functions. Streptococcus mutans, which causes dental caries, splits the sucrose in food and uses one of the sugars to build its capsule, which sticks tightly to the tooth. The bacteria that are trapped in the capsule use the other sugar to fuel their metabolism and produce a strong acid (lactic acid) that attacks the tooth enamel. When Pseudomonas aeruginosa colonizes the lungs of persons with cystic fibrosis, it produces a thick capsular polymer of alginic acid that contributes to the difficulty of eradicating the bacterium. Bacteria of the genus Zoogloea secrete fibres of cellulose that enmesh the bacteria into a floc that floats on the surface of liquid and keeps the bacteria exposed to air, a requirement for the metabolism of this genus. A few rod-shaped bacteria, such as Sphaerotilus, secrete long chemically complex tubular sheaths that enclose substantial numbers of the bacteria. The sheaths of these and many other environmental bacteria can become encrusted with iron or manganese oxides.

The bacterial cell surface (or envelope) can vary considerably in its structure, and it plays a central role in the properties and capabilities of the cell. The one feature present in all cells is the cytoplasmic membrane, which separates the inside of the cell from its external environment, regulates the flow of nutrients, maintains the proper intracellular milieu, and prevents the loss of the cell’s contents. The cytoplasmic membrane carries out many necessary cellular functions, including energy generation, protein secretion, chromosome segregation, and efficient active transport of nutrients. It is a typical unit membrane composed of proteins and lipids, basically similar to the membrane that surrounds all eukaryotic cells. It appears in electron micrographs as a triple-layered structure of lipids and proteins that completely surround the cytoplasm.

peptidoglycan layer of Bacillus coagulans A portion of the gram-positive bacterium Bacillus coagulans showing the cell wall’s thick peptidoglycan layer that surrounds the cell membrane.Visuals Unlimited/© T.J. Beveridge

Lying outside of this membrane is a rigid wall that determines the shape of the bacterial cell. The wall is made of a huge molecule called peptidoglycan (or murein). In gram-positive bacteria the peptidoglycan forms a thick meshlike layer that retains the blue dye of the Gram stain by trapping it in the cell. In contrast, in gram-negative bacteria the peptidoglycan layer is very thin (only one or two molecules deep), and the blue dye is easily washed out of the cell.

Peptidoglycan occurs only in the Bacteria (except for those without a cell wall, such as Mycoplasma). Peptidoglycan is a long-chain polymer of two repeating sugars (N-acetylglucosamine and N-acetyl muramic acid), in which adjacent sugar chains are linked to one another by peptide bridges that confer rigid stability. The nature of the peptide bridges differs considerably between species of bacteria but in general consists of four amino acids: L-alanine linked to D-glutamic acid, linked to either diaminopimelic acid in gram-negative bacteria or L-lysine, L-ornithine, or diaminopimelic acid in gram-positive bacteria, which is finally linked to D-alanine. In gram-negative bacteria the peptide bridges connect the D-alanine on one chain to the diaminopimelic acid on another chain. In gram-positive bacteria there can be an additional peptide chain that extends the reach of the cross-link; for example, there is an additional bridge of five glycines in Staphylococcus aureus.

Peptidoglycan synthesis is the target of many useful antimicrobial agents, including the β-lactam antibiotics (e.g., penicillin) that block the cross-linking of the peptide bridges. Some of the proteins that animals synthesize as natural antibacterial defense factors attack the cell walls of bacteria. For example, an enzyme called lysozyme splits the sugar chains that are the backbone of peptidoglycan molecules. The action of any of these agents weakens the cell wall and disrupts the bacterium.

In gram-positive bacteria the cell wall is composed mainly of a thick peptidoglycan meshwork interwoven with other polymers called teichoic acids (from the Greek word teichos, meaning “wall”) and some proteins or lipids. In contrast, gram-negative bacteria have a complex cell wall that is composed of multiple layers in which an outer membrane layer lies on top of a thin peptidoglycan layer. This outer membrane is composed of phospholipids, which are complex lipids that contain molecules of phosphate, and lipopolysaccharides, which are complex lipids that are anchored in the outer membrane of cells by their lipid end and have a long chain of sugars extending away from the cell into the medium. Lipopolysaccharides, often called endotoxins, are toxic to animals and humans; their presence in the bloodstream can cause fever, shock, and even death. For most gram-negative bacteria, the outer membrane forms a barrier to the passage of many chemicals that would be harmful to the bacterium, such as dyes and detergents that normally dissolve cellular membranes. Impermeability to oil-soluble compounds is not seen in other biological membranes and results from the presence of lipopolysaccharides in the membrane and from the unusual character of the outer membrane proteins. As evidence of the ability of the outer membrane to confer resistance to harsh environmental conditions, some gram-negative bacteria grow well in oil slicks, jet fuel tanks, acid mine drainage, and even bottles of disinfectants.

The Archaea have markedly different surface structures from the Bacteria. They do not have peptidoglycan; instead, their membrane lipids are made up of branched isoprenoids linked to glycerol by ether bonds. Some archaea have a wall material that is similar to peptidoglycan, except that the specific sugar linked to the amino acid bridges is not muramic acid but talosaminuronic acid. Many other archaeal species use proteins as the basic constituent of their walls, and some lack a rigid wall.

The Gram stain

Bacteria are so small that their presence was only first recognized in 1677, when the Dutch naturalist Antonie van Leeuwenhoek saw microscopic organisms in a variety of substances with the aid of primitive microscopes (more similar in design to modern magnifying glasses than modern microscopes), some of which were capable of more than 200-fold magnification. Now bacteria are usually examined under light microscopes capable of more than 1,000-fold magnification; however, details of their internal structure can be observed only with the aid of much more powerful transmission electron microscopes. Unless special phase-contrast microscopes are used, bacteria have to be stained with a coloured dye so that they will stand out from their background.

Klebsiella pneumoniae in pneumonia Gram-negative bacilli, Klebsiella pneumoniae, isolated from a lung abscess in a patient with pneumonia.A.W. Rakosy/Encyclopædia Britannica, Inc.

One of the most useful staining reactions for bacteria is called the Gram stain, developed in 1884 by the Danish physician Hans Christian Gram. Bacteria in suspension are fixed to a glass slide by brief heating and then exposed to two dyes that combine to form a large blue dye complex within each cell. When the slide is flushed with an alcohol solution, gram-positive bacteria retain the blue colour and gram-negative bacteria lose the blue colour. The slide is then stained with a weaker pink dye that causes the gram-negative bacteria to become pink, whereas the gram-positive bacteria remain blue. The Gram stain reacts to differences in the structure of the bacterial cell surface, differences that are apparent when the cells are viewed under an electron microscope.

Although bacterial cells are much smaller and simpler in structure than eukaryotic cells, the bacteria are an exceedingly diverse group of organisms that differ in size, shape, habitat, and metabolism. Much of the knowledge about bacteria has come from studies of disease-causing bacteria, which are more readily isolated in pure culture and more easily investigated than are many of the free-living species of bacteria. It must be noted that many free-living bacteria are quite different from the bacteria that are adapted to live as animal parasites or symbionts. Thus, there are no absolute rules about bacterial composition or structure, and there are many exceptions to any general statement.

Individual bacteria can assume one of three basic shapes: spherical (coccus), rodlike (bacillus), or curved (vibrio, spirillum, or spirochete). Considerable variation is seen in the actual shapes of bacteria, and cells can be stretched or compressed in one dimension. Bacteria that do not separate from one another after cell division form characteristic clusters that are helpful in their identification. For example, some cocci are found mainly in pairs, including Streptococcus pneumoniae, a pneumococcus that causes bacterial lobar pneumonia, and Neisseria gonorrhoeae, a gonococcus that causes the sexually transmitted disease gonorrhea. Most streptococci resemble a long strand of beads, whereas the staphylococci form random clumps (the name “staphylococci” is derived from the Greek word staphyle, meaning “cluster of grapes”). In addition, some coccal bacteria occur as square or cubical packets. The rod-shaped bacilli usually occur singly, but some strains form long chains, such as rods of the corynebacteria, normal inhabitants of the mouth that are frequently attached to one another at random angles. Some bacilli have pointed ends, whereas others have squared ends, and some rods are bent into a comma shape. These bent rods are often called vibrios and include Vibrio cholerae, which causes cholera. Other shapes of bacteria include the spirilla, which are bent and rebent, and the spirochetes, which form a helix similar to a corkscrew, in which the cell body is wrapped around a central fibre called the axial filament.

Streptococcus mutans The bacterium Streptococcus mutans is an example of a spherical (coccus) bacterium. This species of bacteria commonly aggregates into pairs and short chains.David M. Phillips/Visuals Unlimited

Bacteria are the smallest living entities. An average-size bacterium—such as the rod-shaped Escherichia coli, a normal inhabitant of the intestinal tract of humans and animals—is about 2 micrometres (μm; millionths of a metre) long and 0.5 μm in diameter, and the spherical cells of Staphylococcus aureus are up to 1 μm in diameter. A few bacterial types are even smaller, such as Mycoplasma pneumoniae, which is one of the smallest bacteria, ranging from about 0.1 to 0.25 μm in width and roughly 1 to 1.5 μm in length; the rod-shaped Bordetella pertussis, which is the causative agent of whooping cough, ranging from 0.2 to 0.5 μm in diameter and 0.5 to 1 μm in length; and the corkscrew-shaped Treponema pallidum, which is the causative agent of syphilis, averaging only 0.1 to 0.2 μm in diameter but 6 to 15 μm in length. The cyanobacterium Synechococcus averages about 0.5 to 1.6 μm in diameter. Some bacteria are relatively large, such as Azotobacter, which has diameters of 2 to 5 μm or more; and Achromatium, which has a minimum width of 5 μm and a maximum length of 100 μm, depending on the species. Giant bacteria can be visible with the unaided eye, such as Thiomargarita namibiensis, which averages 750 μm in diameter, and the rod-shaped Epulopiscium fishelsoni, which ranges from 30 to more than 600 μm in length.

Bacteria are unicellular microorganisms and thus are generally not organized into tissues. Each bacterium grows and divides independently of any other bacterium, although aggregates of bacteria, sometimes containing members of different species, are frequently found. Many bacteria can form aggregated structures called biofilms. Organisms in biofilms often display substantially different properties from the same organism in the individual state or the planktonic state. Bacteria that have aggregated into biofilms can communicate information about population size and metabolic state. This type of communication is called quorum sensing and operates by the production of small molecules called autoinducers or pheromones. The concentration of quorum-sensing molecules—most commonly peptides or acylated homoserine lactones (AHLs; special signaling chemicals)—is related to the number of bacteria of the same or different species that are in the biofilm and helps coordinate the behaviour of the biofilm.

Bacteria as prokaryotes

All living organisms on Earth are made up of one of two basic types of cells: eukaryotic cells, in which the genetic material is enclosed within a nuclear membrane, or prokaryotic cells, in which the genetic material is not separated from the rest of the cell. Traditionally, all prokaryotic cells were called bacteria and were classified in the prokaryotic kingdom Monera. However, their classification as Monera, equivalent in taxonomy to the other kingdoms—Plantae, Animalia, Fungi, and Protista—understated the remarkable genetic and metabolic diversity exhibited by prokaryotic cells relative to eukaryotic cells. In the late 1970s American microbiologist Carl Woese pioneered a major change in classification by placing all organisms into three domains—Eukarya, Bacteria (originally called Eubacteria), and Archaea (originally called Archaebacteria)—to reflect the three ancient lines of evolution. The prokaryotic organisms that were formerly known as bacteria were then divided into two of these domains, Bacteria and Archaea. Bacteria and Archaea are superficially similar; for example, they do not have intracellular organelles, and they have circular DNA. However, they are fundamentally distinct, and their separation is based on the genetic evidence for their ancient and separate evolutionary lineages, as well as fundamental differences in their chemistry and physiology. Members of these two prokaryotic domains are as different from one another as they are from eukaryotic cells.

bacterial, animal, and plant cells compared Bacterial cells differ from animal cells and plant cells in several ways. One fundamental difference is that bacterial cells lack intracellular organelles, such as mitochondria, chloroplasts, and a nucleus, which are present in both animal cells and plant cells.Encyclopædia Britannica, Inc.

Prokaryotic cells (i.e., Bacteria and Archaea) are fundamentally different from the eukaryotic cells that constitute other forms of life. Prokaryotic cells are defined by a much simpler design than is found in eukaryotic cells. The most-apparent simplification is the lack of intracellular organelles, which are features characteristic of eukaryotic cells. Organelles are discrete membrane-enclosed structures that are contained in the cytoplasm and include the nucleus, where genetic information is retained, copied, and expressed; the mitochondria and chloroplasts, where chemical or light energy is converted into metabolic energy; the lysosome, where ingested proteins are digested and other nutrients are made available; and the endoplasmic reticulum and the Golgi apparatus, where the proteins that are synthesized by and released from the cell are assembled, modified, and exported. All of the activities performed by organelles also take place in bacteria, but they are not carried out by specialized structures. In addition, prokaryotic cells are usually much smaller than eukaryotic cells. The small size, simple design, and broad metabolic capabilities of bacteria allow them to grow and divide very rapidly and to inhabit and flourish in almost any environment.

bacillus-type bacterial cell Schematic drawing of the structure of a typical bacterial cell of the bacillus type.Encyclopædia Britannica, Inc.

Prokaryotic and eukaryotic cells differ in many other ways, including lipid composition, structure of key metabolic enzymes, responses to antibiotics and toxins, and the mechanism of expression of genetic information. Eukaryotic organisms contain multiple linear chromosomes with genes that are much larger than they need to be to encode the synthesis of proteins. Substantial portions of the ribonucleic acid (RNA) copy of the genetic information (deoxyribonucleic acid, or DNA) are discarded, and the remaining messenger RNA (mRNA) is substantially modified before it is translated into protein. In contrast, bacteria have one circular chromosome that contains all of their genetic information, and their mRNAs are exact copies of their genes and are not modified.

bacteria, singular bacterium, any of a group of microscopic single-celled organisms that live in enormous numbers in almost every environment on Earth, from deep-sea vents to deep below Earth’s surface to the digestive tracts of humans.

Bacteria lack a membrane-bound nucleus and other internal structures and are therefore ranked among the unicellular life-forms called prokaryotes. Prokaryotes are the dominant living creatures on Earth, having been present for perhaps three-quarters of Earth history and having adapted to almost all available ecological habitats. As a group, they display exceedingly diverse metabolic capabilities and can use almost any organic compound, and some inorganic compounds, as a food source. Some bacteria can cause diseases in humans, animals, or plants, but most are harmless and are beneficial ecological agents whose metabolic activities sustain higher life-forms. Other bacteria are symbionts of plants and invertebrates, where they carry out important functions for the host, such as nitrogen fixation and cellulose degradation. Without prokaryotes, soil would not be fertile, and dead organic material would decay much more slowly. Some bacteria are widely used in the preparation of foods, chemicals, and antibiotics. Studies of the relationships between different groups of bacteria continue to yield new insights into the origin of life on Earth and mechanisms of evolution.