Introduction to Bacteria

Bacteria, (singular: Bacterium), microscopic, one-celled organisms. Most bacteria range in size from 0.15 to 4 micrometers. (A micrometer is 0.001 millimeter [about 1/25,000 of an inch].) The largest known bacterium, Thiomargarita namibiensis, is more than 750 micrometers long. Bacteria are found in soil, in water, and in the air. They are also found on plants, on the surfaces of decayed or decaying matter, and in the intestinal and reproductive tracts of humans and other animals.

Although many infectious diseases are caused by bacteria, there are relatively few disease-producing species compared to the large number of harmless and useful ones.


A bacterial cell consists of a tiny mass of cytoplasm surrounded by three distinct layers: the cytoplasmic membrane, the cell wall, and the capsule. Within the cytoplasm is DNA, material containing genetic information. Unlike that of higher organisms, the DNA of bacteria is not enclosed in a membrane-bound structure, or nucleus. Some strains of bacteria have whiplike appendages, called flagella, that enable them to move with a swimming motion. Some bacteria also have pili, short, hairlike appendages that help them adhere to certain surfaces. Some bacteria contain a special substance that allows them to emit light by a process called bioluminescence. )

Life and Growth of Bacteria

Though bacteria occur in a wide range of environments, they are most abundant in moist places of moderate temperature. Many bacteria die if exposed to drought or extreme temperatures. Some bacteria, however, are able to withstand these conditions—certain types survive by changing into endospores. An endospore is an inactive cell, containing a bacterium's DNA, surrounded by a thick, protective coating. Some endospores can survive for centuries. Endospores change back into active bacteria when environmental conditions improve.

Most bacteria are heterotrophic; that is, they obtain food from other organisms. Others are autotrophic; that is, they manufacture their own food. Heterotrophic bacteria usually feed on dead organic matter, although some species are parasitic. Most autotrophic bacteria use energy from sunlight to manufacture food by photosynthesis. Heterotrophic and autotrophic bacteria can be either aerobic or anaerobic. Aerobic bacteria live in environments in which free (molecular) oxygen is present; anaerobic bacteria live in environments in which free oxygen is absent.

Bacteria generally reproduce by simple, or binary, fission (splitting). Under favorable conditions, an adult organism splits apart to form two new organisms. Bacteria multiply rapidly, often doubling their population every 20 minutes. Some bacteria reproduce by budding, a process in which a small outgrowth from a single parent develops into a new, identical organism. A few bacteria can reproduce sexually under certain conditions. In this case two bacteria unite to form a zygote, which then splits into new cells. Some bacteria form colonies that may become large enough to be seen with the unaided eye.

Useful Bacteria

Some bacteria are not only useful, but are essential to life. Organic fertilizers are broken down by bacteria into materials that can be used by plants. Decomposed vegetable and animal matter is thus transformed into nutrients that are absorbed through the roots of other plants. Many species of soil bacteria fix nitrogen; that is, they convert atmospheric nitrogen into nitrates, compounds that are readily absorbed by plants. Some of these bacteria live in colonies in the roots of legumes, such as peas, clover, and alfalfa.

Various commercial processes depend on bacteria. Anaerobic bacteria that ferment certain substances are used in the production of vinegar and some drugs, and in the aging process of cheeses. Some bacteria produce waste products that are useful to humans. One such waste product is lactic acid, which is produced by intestinal bacteria and promotes digestion in humans. It is grown commercially and is added to certain foods such as yogurt.

Bacteria are also used to chemically break down the tough, woody tissues of flax, jute, hemp, and coconut. Modern methods of sewage disposal often make use of bacteria to decompose organic wastes. In a process known as bioremediation, bacteria are added to water or soil to convert toxic pollutants, such as pesticides and oil, into harmless substances. Through genetic engineering, bacteria have been developed for bioremediation and also to produce drugs and other chemicals.

Disease-producing Bacteria

Disease-producing, or pathogenic, bacteria are parasites of living organisms. Bacteria enter humans in a variety of ways. Airborne bacteria enter through abrasions in the skin or by passing directly into the nose, throat, or lungs. They cause such diseases as diphtheria, pertussis (whooping cough), staph infections, strep throat, tetanus, and tuberculosis. Other bacteria enter the body through contaminated food or water. They cause such diseases as typhoid fever, dysentery, cholera, and botulism. Some strains of bacteria secrete toxins (poisons) into the bloodstream, which carries them to other parts of the body. Some bacterial diseases can be prevented by routine immunizations during childhood. If the diseases are contracted, they are usually treated with antibiotics, such as penicillin, and with sulfa drugs. Persons who are especially susceptible to bacterial infections, such as those recovering from surgery or those undergoing chemotherapy, are routinely given antibiotics as a preventive measure.

Bacteria also cause disease in plants and in other animals. In plants, they infect the roots, leaves, or stems, causing such diseases as rots, blights, or wilts. Bacterial diseases of livestock include anthrax and Bang's disease.

Classification of Bacteria

Bacteria and blue-green algae are prokaryotes—that is, they are organisms that lack membranes surrounding their genetic material. Today most scientists place prokaryotes in the kingdom Monera. Formerly, bacteria and blue-green algae were considered to be primitive plants and were classified in the plant kingdom.

Bacteria are generally classified by two methods. The simplest and oldest method is by shape. The three principal categories are:


(singular: Bacillus), rod-shaped bacteria; the most numerous of all types. They include coccobacilli and streptobacilli.


(singular: Coccus), spherical bacteria. This group is divided into bacteria that occur in pairs, such as the diplococci; in clusters, such as the staphylococci; and in chains, such as the streptococci.


(singular: Spirillum), spiral-shaped bacteria; the least numerous type.

Some scientists add a fourth category, the vibrios (S-shaped or comma-shaped bacteria) to this list.

The second method of classification is based on other characteristics, such as size, color, and chemical composition. An abbreviated and simplified version of this system follows.

Order Actinomycetales,

branching, filamentous bacteria that have by fungus-like growth. They are important in soil fertilization and as a source of antibiotics.

Order Beggiatoales,

filamentous bacteria that move along a surface in a creeping or gliding motion. They use photosynthesis to manufacture food. Many oxidize sulphur compounds.

Order Chlamydobacteriales,

bacteria encased in a protective covering called a sheath. They grow in algae-like clusters and use iron compounds for growth.

Order Eubacteriales,

rigid-cell bacteria that are rod-shaped or coccus-shaped. Most are immobile.

Order Hyphomicrobiales,

rigid-cell bacteria that reproduce by budding.

Order Myxobacteriales,

flexible, rod-shaped bacteria. They excrete slime and some produce appendages that are visible to the unaided eye.

Order Rickettsiales,

small, rod-shaped or coccus-shaped bacteria that are internal parasites in humans and other animals. Most are pathogenic.

Order Spirochaetales,

the spirochetes—long, thin, spiral-shaped bacteria. Most are pathogenic.

Most scientists classify bacteria in the phylum Schizophyta of the kingdom Monera.