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pl. bacteria [L. bacterium, fr Gr. baktērion, a small staff]
A one-celled organism without a true nucleus or cell organelles, belonging to the kingdom Procaryotae (Monera). The cytoplasm is surrounded by a rigid cell wall composed of carbohydrates and other chemicals that provide the basis for the Gram stain. Some bacteria produce a polysaccharide or polypeptide capsule, which inhibits phagocytosis by white blood cells. Bacteria synthesize DNA, RNA, and proteins, and they can reproduce independently but may need a host to provide food and a favorable environment. Millions of nonpathogenic bacteria live on human skin and mucous membranes; these are called normal flora. Bacteria that cause disease are called pathogens.
bacterial (-ăl), adj. SEE TABLE: Common Bacterial Infections

Shape: There are three principal forms of bacteria. Spherical or ovoid bacteria occur as single cells (micrococci) or in pairs (diplococci), clusters (staphylococci), chains (streptococci), or cubical groups (sarcinae). Rod-shaped bacteria are called bacilli, more oval ones are called coccobacilli, and those forming a chain are called streptobacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios).


Size: On average, bacilli measure about 1 μm in diameter by 4 μm in length. They range in size from less than 0.5 to 1.0 μm in diameter to 10 to 20 μm in length for some of the spirilla.

Reproduction: Binary fission is the usual method of reproduction, but some bacteria exchange genetic material with members of the same species or different species. Reproductive rate is affected by changes in temperature, nutrition, and pH. If the environment becomes unfavorable, some bacilli form spores, in which their genetic material is condensed and surrounded by a thick wall. Spores are highly resistant to heat, drying, and disinfectants. When the environment again becomes favorable, the spores germinate.

Mutation: Bacteria, like all living things, undergo mutations, and the environment determines which mutations are beneficial and have survival value. Certainly beneficial to bacteria, though not at all to humans, are the mutations that provide resistance to the potentially lethal effects of antibiotics.

Motility: None of the cocci are capable of moving, but most bacilli and spiral forms can move independently. Locomotion depends on the possession of one or more flagella, slender whiplike appendages that work like propellers.

Food and oxygen requirements: Most bacteria are heterotrophic (require organic material as food). If they feed on living organisms, they are called parasites; if they feed on nonliving organic material, they are called saprophytes. Bacteria that obtain their energy from inorganic substances, including many of the soil bacteria, are called autotrophic (self-nourishing). Bacteria that require oxygen are called aerobes; those that grow only in the absence of oxygen are called anaerobes. Bacteria that grow both with and without oxygen are facultative anaerobes. Most bacteria in the human intestines are anaerobic.
SEE: infection, opportunistic

Temperature requirements: Although some bacteria live at very low or very high temperatures, the optimum temperature for most human pathogens is 97° to 99°F (36° to 38°C).

Enzyme production: Bacteria produce enzymes that act on complex food molecules, breaking them down into simpler materials; they are the principal agents of decay and putrefaction. Putrefaction, the decomposition of nitrogenous and other organic materials in the absence of air, produces foul odors. Decay is the gradual decomposition of organic matter exposed to air by bacteria and fungi.

Toxin production: Cell wall molecules called adhesins bind bacteria to the host cells. Once attached, the bacteria may produce poisonous substances called toxins. There are two types: exotoxins, enzymes that are released by bacteria into their host, and endotoxins, which are parts of the cell walls of gram-negative bacteria and are toxic even after the death of the cell. Exotoxins include hemolysins, leukocidins, coagulases, and fibrinolysins. Endotoxins stimulate production of cytokines that can produce widespread vasodilation and shock.
SEE: endotoxin; SEE: sepsis

Miscellaneous: Some bacteria produce pigments; some produce light. Soil bacteria are essential for the nitrogen cycle in the processes of nitrogen fixation, nitrification, and denitrification.

Several methods are used to identify bacteria in the laboratory:

Culture: Bacteria are grown on various culture media; a visible colony containing millions of cells may be visible within several hours. A colony is usually composed of the descendants of a single cell. Each species of bacteria grows in colonies with a characteristic color, shape, size, texture, type of margin or edge, and particular chemical features. Groups of cells can then be examined under a microscope, usually with Gram's stain. In addition, colonies can be separated and antibiotics applied to assess their sensitivity to different drugs.

Hanging drop: Unstained bacteria in a drop of liquid are examined under ordinary or dark-field illumination.

Gram's stain: Gram-positive bacteria retain dye, turning purple; gram-negative bacteria can be decolorized by alcohol and colored red by a second dye; acid-fast bacteria retain the dye even when treated with an acid alcohol decolorizer. Bacteria are often described by a combination of their response to Gram's stain and their appearance. For example, “gram-positive staphylococcus” indicates a cluster of spheres that stain purple, whereas gram-negative bacilli are rod-shaped and pink.

Immunofluorescence: Bacteria stained with fluorescein and examined under a microscope equipped with fluorescent light appearing yellow-green.

Common Bacterial Infections
OrganismType and/or Site of Infection
Gram-Positive Bacteria
Clostridium difficilePseudomembranous colitis
Staphylococcus aureusPneumonia, cellulitis, boils, impetigo, toxic shock, postoperative bone/joints, eyes, peritonitis
Staphylococcus epidermidisPostoperative bone/joints, IV line-related phlebitis
Streptococcus pneumoniae (pneumococcus)Pneumonia, meningitis, otitis media, sinusitis, septicemia
Streptococcus pyogenesScarlet fever, pharyngitis, impetigo, rheumatic fever, erysipelas
Viridans group StreptococciEndocarditis
Gram-Negative Bacteria
Campylobacter jejuniDiarrhea (most common worldwide cause)
Escherichia coliUrinary tract, pyelonephritis, septicemia, gastroenteritis, peritonitis
Haemophilus influenzaePneumonia, meningitis, otitis media, epiglottitis
Klebsiella pneumoniaePneumonia, wounds
Legionella pneumophiliaPneumonia
Neisseria gonorrhoeaeGonorrhea
Neisseria meningitidis (meningococcus)Meningitis
Pseudomonas aeruginosaWounds, urinary tract, pneumonia, IV lines
Salmonella enteritidisGastroenteritis, food poisoning
Salmonella typhiTyphoid fever
Shigella dysenteriaeDysentery
Vibrio choleraeCholera

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