Pathogenicity of the Bacteroides fragilis group.

The Bacteroides fragilis group is one of the most important pathogens in polymicrobial infections. The distribution of the different members of the B. fragilis group in clinical infections varies. Bacteroides fragilis accounts for 63 percent of all the group isolates, Bacteroides thetaiotaomicron for 14 percent, Bacteroides vulgatus and Bacteroides ovatus for seven percent each, Bacteroides distasonis for six percent and Bacteroides uniformis for two percent. All members of the group induced bacteremia that was associated with an average mortality of 27 percent. The B. fragilis group resist beta lactam antibiotics by producing the enzyme beta-lactamase. This enzyme can be detected in abscess fluid, and can interfere with the eradication of other bacteria that are susceptible to penicillins and cephalosporins. All members of the B. fragilis group can become encapsulated during an inflammatory process as was demonstrated in a subcutaneous abscess model in mice. Non-encapsulated strains can become encapsulated with the assistance of their aerobic counterparts. These encapsulated strains are more virulent to the host than non-encapsulated strains. This increased virulence can be demonstrated by a higher rate of induction of bacteremia, and a greater enhancement of growth of other bacteria in mixed infection. Antimicrobial therapy directed only at the eradication of the aerobic bacteria did not prevent encapsulation, or reduce the number of Bacteroides species. The virulence of all members of the B. fragilis group highlights the need to direct antimicrobial therapy against all members of this group.