bacterium: Definition and Much More from Answers.com

n., pl. -te·ri·a (-tîr'ē-ə).

Any of the unicellular prokaryotic microorganisms of the class Schizomycetes, which vary in terms of morphology, oxygen and nutritional requirements, and motility, and may be free-living, saprophytic, or pathogenic in plants or animals.

[New Latin bactērium, from Greek baktērion, diminutive of baktron, rod.]

Pl. bacteria [L.] any prokaryotic microorganism. Bacteria are single-celled microorganisms that differ from all other organisms (the eukaryotes) in lacking a true nucleus and organelles such as mitochondria, chloroplasts and lysosomes. Their genetic material consists of a single double-stranded DNA molecule, whereas the genetic material of eukaryotes consists of multiple chromosomes, which are complex structures of DNA and protein.
Bacteria reproduce by binary fission and generally have a very high rate of population growth and mutation. Genetic material can be transferred between bacteria by three processes: transformation (absorption of naked DNA), transduction (transfer by a temperate bacteriophage), and conjugation (transfer by independently replicating DNA molecules, called plasmids). Some bacteria can also form spores, dehydrated forms that are relatively resistant to heat, cold, lack of water, toxic chemicals and radiation.
Most bacteria have a rigid cell wall outside of the cell membrane primarily composed of a dense layer of peptidoglycan, a network of polysaccharide chains with polypeptide cross-links. Some antimicrobial agents, the penicillins and cephalosporins, act by interfering with peptidoglycan synthesis.
Bacteria can have any of three types of external structures: flagella, which are rotating locomotor organelles; pili or fimbriae, which are minute filamentous appendages; and a capsule, which is a layer of gelatinous material around the cell. Large pili called sex pili are involved in conjugation while other pili are involved in adherence of bacteria to mucosal surfaces. The capsule is associated with virulent strains of bacteria and protects the bacterium from phagocytosis. See also bacteria.

• acid-fast b. — one that, because of wax-like composition of the cell wall, is not readily decolorized by acids after staining, especially Mycobacterium spp.
• coliform b. — particularly found in the gut (colon) of animals. See aerobacter aerogenes, escherichia and paracolobactrum.
• hemophilic b. — microorganisms of the genera Haemophilus and Bordetella, which have a nutritional requirement for fresh blood or whose growth is significantly stimulated on blood-containing media.
• lactic acid b. — bacteria that, in suitable media, ferment carbohydrates to form lactic acid.
• lysogenic b. — any bacterial cell harboring in its genome the genetic material (prophage) of a temperate bacteriophage and thus reproducing the bacteriophage DNA in each cell division; occasionally the prophage becomes nonintegrated (induced), replicates, lyses the bacterial cell, and is free to infect other cells.

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