BACTERIUM

plural Bacteria, any of a group of microscopic organisms that are prokaryotic, i.e., that lack a membrane-bound nucleus and organelles. Bacteria are unicellular (one-celled) and may have spherical (coccus), rodlike (bacillus), or curved (vibrio, spirillum, or spirochete) bodies. Different bacteria inhabit virtually all environments, including soil, water, organic matter, and the bodies of eukaryotes (multicellular animals). Some bacteria are known to be beneficial to humans and the higher animals, while many others are harmful; bacteria are the chief cause of infectious diseases in humans. On average, bacteria are about 1 micrometre (0.000039 inch) long and 0.5 micrometre in diameter. All bacteria are surrounded by a lipid membrane that regulates the flow of materials in and out of the cell. A rigid cell wall completely surrounds the bacterium and lies outside the membrane. Gram-positive bacteria are stained blue by the gram stain, because their cell walls have a relatively thick and meshlike structure that traps the dye. In gram-negative bacteria, the cell wall is thin and releases the dye readily when washed with an alcohol solution. Outside the cell wall, some species of bacteria also have a capsule made up of polysaccharides. Such capsules have many functions, including protecting the bacterium from phagocytes and from dessication (drying). Many species of bacteria swim by means of flagellae, i.e., hairlike structures whose whiplike lashing provides propulsion. The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than in a membrane-bound nucleus. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm. A number of other structures are distributed about the bacterial cytoplasm, including ribosomes. When applied to bacteria, the term growth refers to an increase in the number of bacteria in a population rather than in the size of an individual microorganism. Bacteria usually reproduce through binary fission, an asexual process in which the mother cell increases in size until it divides into two identical daughter cells. There are also bacteria that reproduce through budding, through chains of spores, and through the segmentation of elementary units. Bacteria do not reproduce sexually, but there are several mechanisms by which DNA is exchanged in a one-way transfer between them. All bacteria require carbon, nitrogen, sulfur, phosphorus, inorganic salts, and micronutrients. Bacteria that use an organic compound as their source of carbon are called heterotrophs, while those that use an inorganic source are called autotrophs. In addition, some bacteria use photosynthesis to generate energy in the form of the compound ATP; these are called phototrophs. Some species of bacteria are parasitic and can grow only within a living host cell; examples include the genera Rickettsia and Chlamydia, both of which are parasites in eukaryotic cells. Those bacteria that require oxygen, such as Bacillus, are called aerobes; anaerobes, such as Clostridium, cannot survive in the presence of oxygen. Various types of bacteria that are present in water can cause disease in humans, and water-purification plants are designed to destroy these microorganisms. Bacteria from industrial wastes may also act as pathogens, or agents of disease. Conversely, some types of bacteria act as cleansing agents in water, and water-treatment facilities utilize some such bacteria to break down the organic matter that is present in sewage. Various types of bacteria contaminate foods and can cause food poisoning in humans. Pasteurization is routinely used to neutralize bacteria that may be present in milk, for example. Other sterilization techniques include high temperature, radiation, ethylene oxide, and other antiseptics and germicides. A number of bacteria cause diseases in humans. Some, such as meningococcal bacteria which infect the brain membranes, have a specificity for a particular part of the body; others, such as staphylococcal bacteria, can affect various parts of the body. Although human interest in bacteria frequently focuses on their harmful effects, most bacteria are harmless to human beings, and many of them are actually beneficial. Saprophytic bacteria, for example, perform an ecologically indispensable role in the breakdown of dead organisms and organic wastes; without such agents of decomposition, the cycling of various elements vital to living organisms would cease in the biosphere. Bacteria also form highly beneficial associations with animals. For instance, the bacterial inhabitants of the ruminant stomach break down cellulose; this enables cows, sheep, and other ruminants to digest grass. Humans also harbour beneficial bacteria, such as those in the lower intestine that synthesize vitamin K. Bacteria are also used in various industrial processes, especially in the food industry; the production of buttermilk, yogurt, cheeses, pickles, and sauerkraut are all dependent upon bacterial action. The scientific classification of bacteria is in transition, particularly at the higher taxonomic levels. The bacteria form the only prokaryotic kingdom, that of the Monera. Within this kingdom, at least two groups have been distinguished, the eubacteria and archaebacteria. DNA hybridization studies of ribosomal RNA have proved useful in defining these groups.