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  • Genus: Bacteroides
  • Species: fragilis, thetaiotaomicron, melaninogenicus

  • Bacteroides comprise a major portion of the human normal flora, predominating in the intestinal tract. These organisms are, like other anaerobes, generally opportunistic and can cause a variety of infections throughout the body. The most common infections include pleuropulmonary, intraabdominal and infections of the female urogenital tract. Bacteroides make up about one-third of the total anaerobic isolates obtained from various infections.
  • B. fragilis is particularly important because of 1) the frequency of isolation and 2) its resistance to antibiotics.
  • Bacteroides produce several exoenzymes including collagenase, neuraminidase, DNAse, heparinase and some proteases. These enzymes may play a role in the pathogenesis of the organism, assisting the bacteria in the invasion of host tissues following an initial trauma.

  • Bacteroides are Gram-negative, anaerobic bacilli or cocco-bacilli. Each species is morphologically distinct and most are encapsulated.
  • The activity of the Bacteroides LPS endotoxin is reduced compared to other Gram-negative endotoxins because the lipid A moiety has fewer fatty acids and phosphate groups.
  • Most B. fragilis strains deconjugate bile acids.
  • B. thetaiotaomicron converts lithocholic acid to ethyl ester. This is significant because lithocholic acid may promote the occurrence of tumors; conversion inactivates this carcinogenic potential.

  • Bacteroides are indigenous normal flora that may be opportunistic under predisposing conditions such as immunosuppression, aminoglycoside therapy, tissue damage, malignancy, etc.
  • The production of collagenase seems important in the pathogenesis of B. melaninogenicus while the pathogenesis of B. fragilis is enhanced by production of an anti-phagocytic capsule.

  • Polymorphonuclear leukocytes provide an important first line of phagocytic defense.
  • Anticapsular immunoglobulin plus complement enhance this phagocytosis.

  • These are infections of endogenous origin.
  • The percentage of Bacteroides in the gingival crevice equals about 16-20% of total flora (8-17% in plaque).
  • Bacteroides predominate in the feces, reaching densities of 1011/g feces.


  • Clinical: Clinical diagnosis relies on factors that are general for anaerobic infections including a foul odor of discharge, the localization of the infection, tissue necrosis, production of gas, etc.
  • Laboratory: A Gram stain and isolation of the responsible agent can be routine employing appropriate sampling, transport and growth procedures.


  • Sanitary: Generally, avoiding the introduction of bacteria is most important in preventing disease. Proper wound cleansing and surgical procedures can be highly effective.
  • Immunological: None are currently in practice.
  • Chemotherapeutic: Prophylaxis prior to dental, bowel, gynecological surgery can prevent infection. Active disease can be treated using carbenicillin, cefoxitin or chloramphenicol.

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