The isolation and identification of Bacteroides spp. from the normal human faecal flora

Gram-negative anaerobic bacilli of the Bacteroides group were isolated on an enriched selective medium from specimens of faeces from 20 normal healthy adults and identified by conventional bacteriological methods. A heavy growth of Bacteroides spp. was obtained from all specimens and 10 representative colonies from each subject were identified. Most isolates (84%) belonged to the B. fragilis group. The commonest species identified in this group were B vulgatus and B. thetaiotaomicron (22% each), B. distasonis (18%) and the B. eggerthii/variabilis complex (14%), and only 9% were B. fragilis. B. vulgatus was isolated from 80% of subjects, B. thetaiotaomicron and B. distasonis from 70% each and the B. eggerthii/variabilis complex from 65%, but B. fragilis was detected in only 45% of specimens. Asaccharolytic species were isolated in smaller numbers from 14 (70%) subjects, but only five strains of the B. melaninogenicus/oralis group were isolated and fusobacteria were not detected.