Single-celled microorganisms that are invisible to the naked eye. The singular form of the term is bacterium. Abundant in the air, soil, and water, most bacteria are harmless to humans. Some bacteria, such as those that live in the intestine, are beneficial and help to break down food for digestion. Bacteria that cause disease are known as pathogens and are classified by shape into 3 main groups: cocci (spherical); bacilli (rod-shaped); and spirochaetes or spirilla (spiral-shaped). Many bacteria have whip-like threads called flagella, which enable them to move in fluids, and pili, which anchor them to other cells. Aerobic bacteria require oxygen to grow and multiply; in the body, these are most commonly found on the skin or in the respiratory system. Anaerobic bacteria thrive where there is no oxygen, deep within tissue or wounds. They reproduce by simple division, which can take place every 20 minutes. Some bacteria also produce spores that can survive high temperatures, dry conditions, and lack of nourishment; and some produce poisons (either endotoxins or exotoxins) that are harmful to human cells. The body’s immune system attacks invading bacteria, but in some cases treatment with antibiotic drugs is necessary and will speed recovery. Superficial inflammation and infected wounds may be treated with antiseptics. Immunity to invading bacterial diseases, such as some types of meningitis, can be acquired by active immunization. (See also infectious disease.)


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