Bacteria

I

Introduction

Bacteria (Greek bakterion, “little staff”), large group of mostly microscopic, unicellular organisms that lack a distinct nucleus and that usually reproduce by cell division.

Bacteria are tiny, most ranging from 1 to 10 micrometres (1 micrometre equals 1/25,000 in), and are extremely variable in the ways they obtain energy and nourishment. They can be found in nearly all environments—from air, soil, water, and ice to hot springs; even the hydrothermal vents on the deep ocean floor are the home of sulphur-metabolizing bacteria. Certain types are found in nearly all food products, and bacteria also occur in various forms of symbiosis with most plants and animals and other kinds of life. See also Nanobacteria.

II

Classification

In the currently used five-kingdom scheme of classification, bacteria constitute the kingdom Prokaryota, formerly known as Monera—organisms in whose cells the nucleus is not enclosed by a membrane. More than 4,000 species are known (the total may be anywhere between 20,000 and over 5,000,000). Generally, bacteria are classified into species on the basis of characteristics such as shape—cocci (spheres), bacilli (rods), spirochaetes (spirals); cell-wall structure; differential staining (see Gram's Stain); ability to grow in the presence or absence of air (aerobes and anaerobes, respectively); metabolic or fermentative capabilities; ability to form dormant spores under adverse conditions; serologic (serum) identification of surface components; and nucleic-acid relatedness.

The most widely used reference for taxonomic classification of bacteria divides them into four major groups based on cell-wall characteristics. The division Gracilicutes encompasses bacteria with thin, gram-negative-type cell walls; the Firmicutes have thick, gram-positive cell walls; the Tenericutes lack cell walls; and the Mendosicutes have unusual cell walls made of material other than typical bacterial peptidoglycan. Among the Mendosicutes are the archaebacteria, a group of unusual organisms that includes methanogens, strict anaerobes that produce methane from carbon dioxide and hydrogen; halobacteria, which grow at high salt concentrations; and thermoacidophiles, which are sulphur-dependent extreme thermophiles. It has been argued that the archaebacteria should be classified into a separate kingdom because recent biochemical studies have shown that they are as different from other bacteria as they are from eukaryotes (the nucleii of which are membrane-bound). The four major bacterial divisions are further subdivided into about 30 numbered sections, some of which are divided into orders, families, and genera. Section 1, for example, is made up of spirochaetes—long, corkscrew-shaped bacteria with gram-negative cell walls and internal (between the cell wall and cell membrane) filamentous flagella that provide the organisms with motility (ability to move). Treponema pallidum, causing syphilis, is a spirochaete, a member of the order Spirochaetales, and the family Spirochaetaceae.

Not all bacteria can move, but the mobile ones are generally propelled by screw-like appendages—flagella—that may project from all over the cell or from one or both ends, singly or in tufts. Depending on the direction in which the flagella rotate, the bacteria either move forward or tumble in place. The duration of runs versus tumbling is linked to receptors in the bacterial membrane; variations enable the bacteria to move towards attractants such as food sources and away from unfavourable environmental conditions. In some aquatic bacteria that contain iron-rich particles, locomotion has been found to be oriented to the Earth's magnetic field.

III

Genetics

The genetic material of the bacterial cell is in the form of a circular double strand of DNA (see Nucleic Acids). Many bacteria also carry smaller circular DNAs called plasmids, which encode genetic information but are generally not essential for reproduction. Many of these plasmids can be transferred to other bacteria by conjugation, a mechanism of genetic exchange. Other mechanisms whereby bacteria can exchange genetic information include transduction, in which bacterial viruses (see Bacteriophage) transfer DNA, and transformation, in which DNA is taken into the bacterial cell directly from the environment. Bacterial cells multiply by binary fission; the genetic material is duplicated and the bacterium elongates, constricts near the middle, and then undergoes complete division, forming two daughter cells essentially identical to the parent cell. Thus, as with higher organisms, a given species of bacteria reproduces only cells of the same species. Some bacteria divide every 20 to 40 minutes. Under favourable conditions, with one division every 30 minutes, after 15 hours a single cell will have produced roughly 1 billion progeny. This mass, called a colony, may be seen with the naked eye. Under adverse conditions some bacteria may undergo a modified division process to produce spores, dormant forms of the cell that can withstand extremes of temperature and humidity until more favourable conditions return.

IV

Work of Bacteria

Two main groups of bacteria exist: the saprophytes, which live on dead animal and vegetable matter; and the symbionts, which live on or in living animal or vegetable matter. Saprophytes are important because they decompose dead animals and plants into their constituent elements, making them available as food for plants. Symbiotic bacteria are a normal part of many human tissues, including the alimentary canal and the skin, where they may be indispensable to physiological processes. Such a relationship is called mutualistic. Other symbionts gain nutrients from their living host without causing serious damage; this is commensalism. The third type, parasites, can destroy the plants and animals on which they live.

Bacteria are involved in the spoilage of meat, wine, vegetables, and milk and other dairy products. Bacterial action may render such foods unpalatable by changing their composition. Bacterial growth in foods can also lead to food poisoning such as that caused by Staphylococcus aureus or by Clostridium botulinum (see Botulism). On the other hand, bacteria are of great importance in many industries. The fermentative capabilities of various species are manipulated for the production of cheese, yoghurt, pickles, and sauerkraut. Bacteria are also important in the production of tanned leather, tobacco, ensilage, textiles, pharmaceuticals and various enzymes, polysaccharides, and detergents.

Bacteria are found in virtually all environments, where they contribute to various biological processes. For example, they may produce light, such as the phosphorescence of dead fish (see Bioluminescence); and they may produce enough heat to induce spontaneous combustion in haystacks or in hop granaries. By decomposing cellulose (the main constituent of plant cell walls), certain anaerobic forms produce marsh gas in stagnant pools; by oxidizing processes, other bacteria assist in forming deposits of bog iron ore, ochre, and manganese ore.

Bacteria have an immense influence on the nature and composition of the soil. One result of their activities is the complete disintegration of organic remains of plants and animals and of inorganic rock particles. This action produces in the aggregate vast quantities of plant food. In addition, the leguminous plants that enrich soil by increasing its nitrogen content do so with the help of Rhizobium radicicola and other bacteria that infect the roots of the plants and cause nitrogen-fixing nodules to grow (see Nitrogen Fixation). The photosynthetic process on which plant life itself is based was almost certainly first established in bacteria; the recent discovery of an unusual photosynthesizing bacterium called Heliobacterium chlorum may help in understanding this fundamental development in the history of life.