Encapsulated strains of Bacteroides fragilis in clinical specimens.

Studies were made of the presence of capsules in 294 strains of the Bacteroides fragilis group of bacteria isolated from clinical infections and in 30 strains from the fecal flora using the Indian ink and ruthenium red staining methods. A total of 77% of the B. fragilis strains were encapsulated; higher figures were observed in isolates from blood and wound infections. Of the B. ovatus, B. distasonis, B. vulgatus, B. uniformis, and B. thetaiotaomicron isolates from clinical infections, 17% were encapsulated. The percentage of encapsulated Bacteroides strains isolated from feces was approximately the same as in the strains isolated from clinical infections.