Isolation and identification of encapsulated strains of Bacteroides fragilis

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.