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bacterias

Monday 22 November 2004

Bacterial cells are prokaryotes, meaning that they have a cell membrane but lack membrane-bound nuclei and other membrane-enclosed organelle.

Bacteria are bound by a cell wall usually consisting of peptidoglycan, a polymer of mixed sugars and amino acids. There are two forms of cell wall structures: a thick wall surrounding the cell membrane that retains crystal-violet stain (Gram-positive bacteria) or a thin cell wall sandwiched between two phospholipid bilayer membranes (Gram-negative bacteria).

Bacteria are classified by Gram staining (positive or negative), shape (e.g., spherical ones are cocci; rod-shaped ones are bacilli), and form of respiration (aerobic or anaerobic).

Many bacteria have flagella, long helical filaments extending from the cell surface, which enable bacteria to move in their environment. Some bacteria possess pili, another kind of surface projection, which can attach bacteria to host cells. Most bacteria synthesize their own DNA, RNA, and proteins, but they depend on the host for favorable growth conditions.

Normal healthy people can be colonized by as many as 10-12 bacteria on the skin, 10-10 bacteria in the mouth, and 10-14 bacteria in the gastrointestinal tract. Bacteria colonizing the skin include Staphylococcus epidermidis and Propionibacterium acnes, the cause of acne. Aerobic and anaerobic bacteria in the mouth, particularly Streptococcus mutans, contribute to dental plaque, a major cause of tooth decay.

In the colon, 99.9% of bacteria are anaerobic, including Bacteroides species. Many bacteria remain extracellular when they invade the body, while others can survive and replicate either outside or inside of host cells (facultative intracellular bacteria) and some grow only inside host cells (obligate intracellular bacteria).

Types

cocci
bacilles

mycobacteria

bacterias

D. General pathology

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