One hundred three clinical isolates of Bacteroides fragilis were identified during a two-year period. Most of these isolates were strains of B. fragilis subspecies fragilis, which constitutes a minor component of the fecal flora in comparison with the other subspecies of B. fragilis. By use of several techniques for demonstration of capsules, it was found that only B. fragilis strains classified as subspecies fragilis were encapsulated. An indirect immunofluorescence assay was developed for identification of clinical isolates possessing capsular material that was immunologically similar to that found in the reference strain of B. fragilis subspecies fragilis. All strains examined that were classified as subspecies fragilis were positive in this assay for the capsular material, whereas strains of the other subspecies were negative. This tests represents a rapid and sensitive means of identifying the most prevalent anaerobic gram-negative bacillus involved in human infections. The capsular polysaccharide of B. fragilis subspecies fragilis is a unique factor associated with the predominant subspecies of B. fragilis isolated from clinical material.
The antibody response to the capsular polysaccharide of Bacteroides fragilis subspecies fragilis in women with acute pelvic inflammatory disease was quantitated using a sensitive radioactive antigen-binding assay. Antibody detected by this technique correlated highly with antibody measured by quantitative precipitin analysis (r = 0.99). Paired acute- and convalescent-phase sera obtained from women from whom B. fragilis subspecies fragilis was isolated by culdocentesis showed significantly greater changes in antibody concentration than corresponding sera obtained from women with gonococcal or nongonococcal pelvic inflammatory disease when B. fragilis subspecies fragilis was not isolated from the cul-de-sac. These data indicate that the steady state of immunity to this capsular antigen is perturbed in patients with acute nongonococcal pelvic inflammatory disease from whom this organism can be cultured. This finding may indicate that B. fragilis can play a significant role in acute pelvic inflammatory disease and can be involved early in the infectious process.
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