Bacteria
Basics

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 (pathogenic ones) are harmful. Bacteria are the chief cause of infectious diseases in humans.

Bacterium, the singular of bacteria, are any of a group of microscopic organisms that are prokaryotic, i.e., that lack a membrane-bound nucleus and organelles (an identifying molecular structure). 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 proven useful in defining these groups.

Bacteria are unicellular (one-celled) and may have spherical (coccus), rod- like (bacillus), or curved (vibrio, spirillum, or spirochete) bodies. Bacteria are on average about one micrometer (0.000039 inch) long and 0.5 micrometer in diameter. A lipid membrane that regulates the flow of materials in and out of the cell surrounds all bacteria. 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 desiccation (drying). Many species of bacteria swim by means of flagella (or hair-like) structures whose whip like lashing provides propulsion. The DNA of most bacteria is found in a single circular chromosome and is distributed throughout the cytoplasm rather than in membrane-bound nucleus. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm. There are a number of other structures distributed about the bacterial cytoplasm, including ribosomes.

When applied to bacteria, the term growth refers to an increase in the size of a population rather than in that 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 hetero-trophs, 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, can not 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. 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. The ability of a bacterium to cause a disease is called virulence. One contributing factor to the degree of virulence is the type of capsule a bacterium has. Some bacteria have specificity for various parts of the body: meningococcal bacteria infect the brain membranes, and tubercule bacteria infect the lungs. Some are slightly more generalized: staphylococcal bacteria can infect the skin, causing boils; the bloodstream, causing blood poisoning; and the bones, causing the condition known as osteomyelitis.

Although human interest in bacteria frequently focuses on their harmful effects, most bacteria are harmless to human beings, and many of them are 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 (including nitrogen, carbon, and phosphorus) would cease to be available 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. In environmental microbiology, bacteria are used to bioremediate wastes in contaminated soil and water. Wastewater-treatment facilities utilize such bacteria to break down the organic matter that is present in sewage. In aquaculture, beneficial bacterial inoculants digest fish and shrimp wastes that can be toxic to animals causing slower growth and even mortality. In agriculture, bacterial inoculants help preserve silage, can biologically digest thatch in turf and can reduce manure odors in hog and cattle operations. Bacteria are nature's waste treatment army and, best of all, they work cheap (for food)!

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