The one-celled organisms called bacteria live on, in, and around most living and nonliving things. Most bacteria can be seen only with the aid of a microscope, and millions of them would fit on the head of a pin.
Although some bacteria are harmful, many bacterial species are beneficial. Bacteria that live in the intestines of humans are essential in digesting food. Other species play a role in fermentation, a process that produces foods such as yogurt and cheese. Bacteria themselves are vulnerable to infection by viruses called bacteriophages.
The study of bacteria is called bacteriology. It often is integrated with studies of other microorganisms, or microbes, a diverse group that includes fungi, algae, and protozoa. These combined studies are called microbiology. (See also microbiolo
Respiration
Some bacteria species cannot tolerate exposure to oxygen. These bacteria are called anaerobes; they occupy a variety of habitats, such as soil and hot springs. Many are part of the normal flora (microbes) living inside the gastrointestinal tract and mouth. Treponema denticola, which lives in dental plaque, is a good example. Bacteria that require oxygen are called aerobes. Bordetella pertussis, which causes whooping cough, is in this group. Bacteria that can live with or without air are called facultative anaerobes. Some, such as Escherichia coli, are part of the normal body flora. Other facultative anaerobes, such as Salmonella and Shigella, are pathogenic.

Locomotion
Many bacteria are able to move through liquids by means of taillike appendages called flagella, or tiny hairlike structures called cilia. Other species cannot move on their own, but are carried about on animals or insects, or through the air on dust.

Spore Formation
Several groups of bacteria can form structures called endospores, or spores. The spore is a resting stage that enables the organism to endure adverse conditions. When conditions improve, the spores transform into active bacteria. Some spores, such as those that cause the diseases anthrax, botulism, and tetanus, can withstand extremely harsh conditions, including boiling water, extreme cold, and exposure to many chemical disinfectants, over long periods of time.

Reproduction
Most bacteria reproduce asexually by dividing in the middle to form two cells, a process called binary fission. These two new cells grow and then each divides to form two new cells. Thus four cells with identical DNA will have resulted from a single parent cell. Some species divide only every 16 hours or more. In the fastest growing bacteria, however, fission may occur as often as every 15 minutes, producing billions of bacteria with identical DNA within 24 hours.
Some bacteria exchange genetic material before undergoing fission. In these species, a tubelike structure extends between two bacterial cells. The donor transfers portions of its DNA to the recipient. This allows bacteria to transmit certain genetic traits, such as drug resistance, to other bacteria in their population. (See also antibiotic; reproductive system.)

Significance in Nature
The decomposition of organic (substances that contain carbon) material in nature is brought about chiefly by vast numbers of saprophytic bacteria, though certain fungi contribute to the process. If there were no decay, the remains of dead organisms and the waste of cities would accumulate so fast that they would soon interfere with everyday life. As saprophytes break down organic matter, such as rotting leaves and dead insects, they enrich the soil by returning minerals and nutrients to it. Carbon dioxide, a by-product of decomposition, is released into the air for plants to use in photosynthesis. (See also photosynthesis.)
Although plants need nitrogen to grow, and nitrogen is abundant in the air, green plants cannot use it in its gaseous form. Several kinds of bacteria can take nitrogen from the atmosphere and convert it into a form that is usable by plants, a process called nitrogen fixation. Some nitrogen-fixing bacteria, such as Rhizobium, live in a mutualistic relationship with plants such as legumes (peas and beans) by forming nodules on the plant's roots (see symbiosis). Free-living nitrogen-fixing soil bacteria, such as Azotobacter, release fixed nitrogen into the soil, where it is taken up by the plant's roots.gy.)