Bacteroides fragilis Subspecies in Clinical Isolates

Abstract

Isolates of B. fragilis from 83 infected patients were collected during 2 yr. Of 103 isolates, 68% were B. fragilis ssp. fragilis. This was the single most common subspecies isolated from blood (12/17), intra-abdominal (11/28), wound (17/20), perirectal (11/16), pelvic (6/10) and other (8/11) sites. Sixteen patients had B. fragilis isolated in pure culture. Of these, 9/12 blood cultures and 4/4 cultures from other sites were B. fragilis ssp. fragilis. The recent observation that B. fragilis ssp. fragilis organisms are encapsulated whereas the other subspecies are generally unencapsulated was supported by indirect fluorescent antibody studies using antibodies prepared to the capsular material of B. fragilis ssp. fragilis. All 33 strains of B. fragilis ssp. fragilis and none of 13 strains of other subspecies were positive. The predominance of B. fragilis ssp. fragilis in clinical isolates compared to its relative infrequency in normal feces and its predominance in pure cultures, especially of blood, indicate that this is the most virulent B. fragilis ssp. This virulence may be related to encapsulation.