Bacteroides

ORGANISM:

GENERAL CONCEPTS:

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.

DISTINCTIVE PROPERTIES:

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.

PATHOGENESIS:

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.

HOST DEFENSES:

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

EPIDEMIOLOGY:

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.

DIAGNOSIS:

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.

CONTROL:

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.