bacteria [pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera Monera, taxonomic kingdom that comprises the prokaryotes ( bacteria and cyanobacteria ). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell , in biology).
..... Click the link for more information. . Bacteria fall into one of two groups, Archaebacteria Archaebacteria (är'kēbăktĭr`ēə)
..... Click the link for more information. (ancient forms thought to have evolved separately from other bacteria) and Eubacteria. A recently proposed system classifies the Archaebacteria, or archaea, and the Eubacteria as major groupings (sometimes called domains) above the kingdom level.

Bacteria were the only form of life on earth for 2 billion years. They were first observed by Antony van Leeuwenhoek Leeuwenhoek, Antony van (än`tōnē vän lā`vənh
..... Click the link for more information. in the 17th cent.; bacteriology as an applied science began to develop in the late 19th cent. as a result of research in medicine and in fermentation processes, especially by Louis Pasteur Pasteur, Louis (păstŭr`, Fr. lwē pästör`), 1822–95, French chemist.
..... Click the link for more information. and Robert Koch Koch, Robert (rō`bĕrt kôkh), 1843–1910, German bacteriologist. He studied at Göttingen under Jacob Henle .
..... Click the link for more information. .

Bacteria are remarkably adaptable to diverse environmental conditions: they are found in the bodies of all living organisms and on all parts of the earth—in land terrains and ocean depths, in arctic ice and glaciers, in hot springs, and even in the stratosphere. Our understanding of bacteria and their metabolic processes has been expanded by the discovery of species that can live only deep below the earth's surface and by species that thrive without sunlight in the high temperature and pressure near hydrothermal vents hydrothermal vent, crack along a rift or ridge in the deep ocean floor that spews out water heated to high temperatures by the magma under the earth's crust.
..... Click the link for more information. on the ocean floor. There are more bacteria, as separate individuals, than any other type of organism; there can be as many as 2.5 billion bacteria in one gram of fertile soil.

Characteristics

Bacteria are grouped in a number of different ways. Most bacteria are of one of three typical shapes—rod-shaped (bacillus), round (coccus, e.g., streptococcus), and spiral (spirillum). An additional group, vibrios, appear as incomplete spirals. The cytoplasm and plasma membrane of most bacterial cells are surrounded by a cell wall; further classification of bacteria is based on cell wall characteristics (see Gram's stain Gram's stain, laboratory staining technique that distinguishes between two groups of bacteria by the identification of differences in the structure of their cell walls.
..... Click the link for more information. ). They can also be characterized by their patterns of growth, such as the chains formed by streptococci. Many bacteria, chiefly the bacillus and spirillum forms, are motile, swimming about by whiplike movements of flagella; other bacteria have rigid rodlike protuberances called pili that serve as tethers.

Some bacteria (those known as aerobic forms) can function metabolically only in the presence of free or atmospheric oxygen; others (anaerobic bacteria) cannot grow in the presence of free oxygen but obtain oxygen from compounds. Facultative anaerobes can grow with or without free oxygen; obligate anaerobes are poisoned by oxygen.

Reproduction

In bacteria the genetic material is organized in a continuous strand of DNA. This circle of DNA is localized in an area called the nucleoid, but there is no membrane surrounding a defined nucleus as there is in the eukaryotic cells of protists, fungi, plants, and animals (see eukaryote eukaryote (y
..... Click the link for more information. ). In addition to the nucleoid, the bacterial cell may include one or more plasmids, separate circular strands of DNA that can replicate independently, and that are not responsible for the reproduction of the organism. Drug resistance is often conveyed via plasmid genes.

Reproduction is chiefly by binary fission, cell division yielding identical daughter cells. Some bacteria reproduce by budding or fragmentation. Despite the fact that these processes should produce identical generations, the rapid rate of mutation possible in bacteria makes them very adaptable. Some bacteria are capable of specialized types of genetic recombination recombination, process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction , the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents.
..... Click the link for more information. , which involves the transfer of nucleic acid by individual contact (conjugation), by exposure to nucleic acid remnants of dead bacteria (transformation), by exchange of plasmid genes, or by a viral agent, the bacteriophage bacteriophage (băktēr`ēəfāj')
..... Click the link for more information. (transduction). Under unfavorable conditions some bacteria form highly resistant spores with thickened coverings, within which the living material remains dormant in altered form until conditions improve. Others, such as the radioactivity-resistant Deinococcus radiodurans, can withstand serious damage by repairing their own DNA.

Nutrition

Most bacteria are heterotrophic, living off other organisms. Most of these are saprobes, bacteria that live off dead organic matter. The bacteria that cause disease are heterotrophic parasites. There are also many non-disease-causing bacterial parasites, many of which are helpful to their hosts. These include the "normal flora" of the human body.

Autotrophic bacteria manufacture their own food by the processes of photosynthesis photosynthesis (fō'tōsĭn`thəsĭs)
..... Click the link for more information. and chemosynthesis chemosynthesis, process in which carbohydrates are manufactured from carbon dioxide and water using chemical nutrients as the energy source, rather than the sunlight used for energy in photosynthesis . Most life on earth is fueled directly or indirectly by sunlight.
..... Click the link for more information. (see autotroph autotroph (ôt`ətrōf'), in biology, an organism capable of synthesizing its own organic substances from inorganic compounds.
..... Click the link for more information. ). The photosynthetic bacteria include the green and purple bacteria and the cyanobacteria cyanobacteria (sī'ənōbăktĭr`ēə, sī-ăn'ō–) or blue-green algae,
..... Click the link for more information. . Many of the thermophilic archaebacteria are chemosynthetic autotrophs.

Beneficial Bacteria

Harmless and beneficial bacteria far outnumber harmful varieties. Because they are capable of producing so many enzymes necessary for the building up and breaking down of organic compounds, bacteria are employed extensively by humans—for soil enrichment with leguminous crops (see nitrogen cycle nitrogen cycle, the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification.
..... Click the link for more information. ), for preservation by pickling, for fermentation (as in the manufacture of alcoholic beverages, vinegar, and certain cheeses), for decomposition of organic wastes (in septic tanks, in some sewage disposal plants, and in agriculture for soil enrichment) and toxic wastes, and for curing tobacco, retting flax, and many other specialized processes. Bacteria frequently make good objects for genetic study: large populations grown in a short period of time facilitate detection of mutations mutation, in biology, a sudden, random change in a gene , or unit of hereditary material, that can alter an inheritable characteristic. Most mutations are not beneficial, since any change in the delicate balance of an organism having a high level of adaptation to its
..... Click the link for more information. , or rare variations.

Pathogenic Bacteria

Bacterial parasites that cause disease are called pathogens. Among bacterial plant diseases are leaf spot, fire blight blight, general term for any sudden and severe plant disease or for the agent that causes it. The term is now applied chiefly to diseases caused by bacteria (e.g., bean blights and fire blight of fruit trees), viruses (e.g., soybean bud blight), fungi (e.g.
..... Click the link for more information. , and wilts; animal diseases caused by bacteria include tuberculosis tuberculosis (TB), contagious, wasting disease caused by any of several mycobacteria. The most common form of the disease is tuberculosis of the lungs (pulmonary consumption, or phthisis), but the intestines, bones and joints, the skin, and the genitourinary,
..... Click the link for more information. , cholera cholera (kŏl`ərə) or Asiatic cholera, acute infectious disease caused by strains of the bacterium
..... Click the link for more information. , syphilis syphilis (sĭf`əlĭs), contagious sexually transmitted disease caused by the spirochete Treponema pallidum
..... Click the link for more information. , typhoid fever typhoid fever acute, generalized infection caused by Salmonella typhi. The main sources of infection are contaminated water or milk and, especially in urban communities, food handlers who are carriers.
..... Click the link for more information. , and tetanus tetanus (tĕt`nəs, –ənəs) or lockjaw,
..... Click the link for more information. . Some bacteria attack the tissues directly; others produce poisonous substances called toxins. Natural defense against harmful bacteria is provided by antibodies (see immunity immunity, ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
..... Click the link for more information. ). Certain bacterial diseases, e.g., tetanus, can be prevented by injection of antitoxin antitoxin, any of a group of antibodies formed in the body as a response to the introduction of poisonous products, or toxins . By introducing small amounts of a specific toxin into the healthy body, it is possible to stimulate the production of antitoxin so that the
..... Click the link for more information. or of serum containing antibodies against specific bacterial antigens; immunity to some can be induced by vaccination vaccination, means of producing immunity against pathogens, such as viruses and bacteria, by the introduction of live, killed, or altered antigens that stimulate the body to produce antibodies against more dangerous forms.
..... Click the link for more information. ; and certain specific bacterial parasites are killed by antibiotics antibiotic, any of a variety of substances, usually obtained from microorganisms, that inhibit the growth of or destroy certain other microorganisms.

Types of Antibiotics

..... Click the link for more information. .

New strains of more virulent bacterial pathogens, many of them resistant to antibiotics, have emerged in recent years. Many believe this to be due to the overuse of antibiotics, both in prescriptions for minor, self-limiting ailments and as growth enhancers in livestock; such overuse increases the likelihood of bacterial mutations. For example, a variant of the normally harmless Escherichia coli Escherichia coli (ĕsh'ərĭk`ēə kō`lī)
..... Click the link for more information. has caused serious illness and death in victims of food poisoning food poisoning, acute illness following the eating of foods contaminated by bacteria, bacterial toxins, natural poisons, or harmful chemical substances. It was once customary to classify all such illnesses as "ptomaine poisoning," but it was later discovered that
..... Click the link for more information. . See also drug resistance drug resistance, condition in which infecting bacteria can resist the destructive effects of drugs such as antibiotics and sulfa drugs . Drug resistance has become a serious public health problem, since many disease-causing bacteria are no longer susceptible to
..... Click the link for more information. .

Bibliography

See P. Singleton, Introduction to Bacteria (1992); W. Biddle, A Field Guide to Germs (1995).

bacteria

Group of microscopic, single-celled organisms that inhabit virtually all environments, including soil, water, organic matter, and the bodies of multicellular animals. Bacteria are distinguished in part by their morphological and genetic features; for instance, they may have spherical, rodlike, or spiral shapes. They also can be divided into two main groups, gram-positive or gram-negative, based on the structure of their cell wall and their reaction to the gram stain. Many bacteria swim by means of flagella (see flagellum). The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than contained within a membrane-enclosed nucleus. Though some bacteria can cause food poisoning and infectious diseases in humans, most are harmless and many are beneficial. They are used in various industrial processes, especially in the food industry (e.g., the production of yogurt, cheeses, and pickles). Bacteria are genetically distinct from the archaea. As prokaryotic organisms (having no membrane-bound nucleus), they are also distinct from eukaryotes. See also budding bacteria, coliform bacteria, cyanobacteria, denitrifying bacteria, nitrifying bacteria, sheathed bacteria, sulfur bacteria.