Growth of bacterial cultures is defined as an increase in the number of bacteria in a population rather than in the size of individual cells. The growth of a bacterial population occurs in a geometric or exponential manner: with each division cycle (generation), one cell gives rise to 2 cells, then 4 cells, then 8 cells, then 16, then 32, and so forth. The time required for the formation of a generation, the generation time (G), can be calculated from the following formula:

In the formula, B is the number of bacteria present at the start of the observation, b is the number present after the time period t, and n is the number of generations. The relationship shows that the mean generation time is constant and that the rate at which the number of bacteria increases is proportional to the number of bacteria at any given time. This relationship is valid only during the period when the population is increasing in an exponential manner, called the log phase of growth. For this reason, graphs that show the growth of bacterial cultures are plotted as the logarithm of the number of cells.

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The generation time, which varies among bacteria, is controlled by many environmental conditions and by the nature of the bacterial species. For example, Clostridium perfringens, one of the fastest-growing bacteria, has an optimum generation time of about 10 minutes; Escherichia coli can double every 20 minutes; and the slow-growing Mycobacterium tuberculosis has a generation time in the range of 12 to 16 hours. Some researchers have suggested that certain bacteria populations living deep below Earth’s surface may grow at extremely slow rates, reproducing just once every several thousand years. The composition of the growth medium is a major factor controlling the growth rate. The growth rate increases up to a maximum when the medium provides a better energy source and more of the biosynthetic intermediates that the cell would otherwise have to make for itself.

When bacteria are placed in a medium that provides all of the nutrients that are necessary for their growth, the population exhibits four phases of growth that are representative of a typical bacterial growth curve. Upon inoculation into the new medium, bacteria do not immediately reproduce, and the population size remains constant. During this period, called the lag phase, the cells are metabolically active and increase only in cell size. They are also synthesizing the enzymes and factors needed for cell division and population growth under their new environmental conditions. The population then enters the log phase, in which cell numbers increase in a logarithmic fashion, and each cell generation occurs in the same time interval as the preceding ones, resulting in a balanced increase in the constituents of each cell. The log phase continues until nutrients are depleted or toxic products accumulate, at which time the cell growth rate slows, and some cells may begin to die. Under optimum conditions, the maximum population for some bacterial species at the end of the log phase can reach a density of 10 to 30 billion cells per milliliter.

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The log phase of bacterial growth is followed by the stationary phase, in which the size of a population of bacteria remains constant, even though some cells continue to divide and others begin to die. The stationary phase is followed by the death phase, in which the death of cells in the population exceeds the formation of new cells. The length of time before the onset of the death phase depends on the species and the medium. Bacteria do not necessarily die even when starved of nutrients, and they can remain viable for long periods of time.

Distribution in nature

Prokaryotes are ubiquitous on Earth’s surface. They are found in every accessible environment, from polar ice to bubbling hot springs, from mountaintops to the ocean floor, and from plant and animal bodies to forest soils. Some bacteria can grow in soil or water at temperatures near freezing (0 °C [32 °F]), whereas others thrive in water at temperatures near boiling (100 °C [212 °F]). Each bacterium is adapted to live in a particular environmental niche, be it oceanic surfaces, mud sediments, soil, or the surfaces of another organism. The level of bacteria in the air is low but significant, especially when dust has been suspended. In uncontaminated natural bodies of water, bacterial counts can be in the thousands per milliliter; in fertile soil, bacterial counts can be in the millions per gram; and in feces, bacterial counts can exceed billions per gram.

Prokaryotes are important members of their habitats. Although they are small in size, their sheer numbers mean that their metabolism plays an enormous role—sometimes beneficial, sometimes harmful—in the conversion of elements in their external environment. Probably every naturally occurring substance, and many synthetic ones, can be degraded (metabolized) by some species of bacteria. The largest stomach of the cow, the rumen, is a fermentation chamber in which bacteria digest the cellulose in grasses and feeds, converting them to fatty acids and amino acids, which are the fundamental nutrients used by the cow and the basis for the cow’s production of milk. Organic wastes in sewage or compost piles are converted by bacteria either into suitable nutrients for plant metabolism or into gaseous methane (CH₄) and carbon dioxide. The remains of all organic materials, including plants and animals, are eventually converted to soil and gases through the activities of bacteria and other microorganisms and are thereby made available for further growth.

Many bacteria live in streams and other sources of water, and their presence at low population densities in a sample of water does not necessarily indicate that the water is unfit for consumption. However, water that contains bacteria such as E. coli, which are normal inhabitants of the intestinal tract of humans and animals, indicates that sewage or fecal material has recently polluted that water source. Such coliform bacteria may be pathogens (disease-causing organisms) themselves, and their presence signals that other, less easily detected bacterial and viral pathogens may also be present. Procedures used in water purification plants—settling, filtration, and chlorination—are designed to remove these and any other microorganisms and infectious agents that may be present in water that is intended for human consumption. Also, sewage treatment is necessary to prevent the release of pathogenic bacteria and viruses from wastewater into water supplies. Sewage treatment plants also initiate the decay of organic materials (proteins, fats, and carbohydrates) in the wastewater. The breakdown of organic material by microorganisms in the water consumes oxygen (biochemical oxygen demand), causing a decrease in the oxygen level, which can be very harmful to aquatic life in streams and lakes that receive the wastewater. One objective of sewage treatment is to oxidize as much organic material as possible before its discharge into the water system, thereby reducing the biochemical oxygen demand of the wastewater. Sewage digestion tanks and aeration devices specifically exploit the metabolic capacity of bacteria for this purpose. (For more information about the treatment of wastewater, see environmental works: Water-pollution control.)

Soil bacteria are extremely active in effecting biochemical changes by transforming the various substances, humus and minerals, that characterize soil. Elements that are central to life, such as carbon, nitrogen, and sulfur, are converted by bacteria from inorganic gaseous compounds into forms that can be used by plants and animals. Bacteria also convert the end products of plant and animal metabolism into forms that can be used by bacteria and other microorganisms. The nitrogen cycle can illustrate the role of bacteria in effecting various chemical changes. Nitrogen exists in nature in several oxidation states, as nitrate, nitrite, dinitrogen gas, several nitrogen oxides, ammonia, and organic amines (ammonia compounds containing one or more substituted hydrocarbons). Nitrogen fixation is the conversion of dinitrogen gas from the atmosphere into a form that can be used by living organisms. Some nitrogen-fixing bacteria, such as Azotobacter, Clostridium pasteurianum, and Klebsiella pneumoniae, are free-living, whereas species of Rhizobium live in an intimate association with leguminous plants. Rhizobium organisms in the soil recognize and invade the root hairs of their specific plant host, enter the plant tissues, and form a root nodule. This process causes the bacteria to lose many of their free-living characteristics. They become dependent upon the carbon supplied by the plant, and, in exchange for carbon, they convert nitrogen gas to ammonia, which is used by the plant for its protein synthesis and growth. In addition, many bacteria can convert nitrate to amines for purposes of synthesizing cellular materials or to ammonia when nitrate is used as electron acceptor. Denitrifying bacteria convert nitrate to dinitrogen gas. The conversion of ammonia or organic amines to nitrate is accomplished by the combined activities of the aerobic organisms Nitrosomonas and Nitrobacter, which use ammonia as an electron donor.

In the carbon cycle, carbon dioxide is converted into cellular materials by plants and autotrophic prokaryotes, and organic carbon is returned to the atmosphere by heterotrophic life-forms. The major breakdown product of microbial decomposition is carbon dioxide, which is formed by respiring aerobic organisms.

Methane, another gaseous end product of carbon metabolism, is a relatively minor component of the global carbon cycle but of importance in local situations and as a renewable energy source for human use. Methane production is carried out by the highly specialized and obligately anaerobic methanogenic prokaryotes, all of which are archaea. Methanogens use carbon dioxide as their terminal electron acceptor and receive electrons from hydrogen gas (H₂). A few other substances can be converted to methane by these organisms, including methanol, formic acid, acetic acid, and methylamines. Despite the extremely narrow range of substances that can be used by methanogens, methane production is very common during the anaerobic decomposition of many organic materials, including cellulose, starch, proteins, amino acids, fats, alcohols, and most other substrates. Methane formation from these materials requires that other anaerobic bacteria degrade these substances either to acetate or to carbon dioxide and hydrogen gas, which are then used by the methanogens. The methanogens support the growth of the other anaerobic bacteria in the mixture by removing hydrogen gas formed during their metabolic activities for methane production. Consumption of the hydrogen gas stimulates the metabolism of other bacteria.

Despite the fact that methanogens have such a restricted metabolic capability and are quite sensitive to oxygen, they are widespread on Earth. Large amounts of methane are produced in anaerobic environments, such as swamps and marshes, but significant amounts also are produced in soil and by ruminant animals. At least 80 percent of the methane in the atmosphere has been produced by the action of methanogens, the remainder being released from coal deposits or natural gas wells.