Antibiotic susceptibility of the subspecies of Bacteroides fragilis.

Strains (115) of Bacteroides fragilis were identified at the subspecific level and were tested for susceptibility to ampicillin, cephalothin, chloramphenicol, clindamycin, penicillin, and tetracycline using an agar dilution technique. We tested the following strains: B. fragilis subsp. distasonis, 12; B. fragilis subsp. fragilis, 39; B. fragilis subsp. ovatus, 10; B. fragilis subsp. thetaiotaomicron, 32; B. fragilis subsp. vulgatus, 10; and B. fragilis subsp. "other," 12. There were no marked differences in susceptibility between the subspecies. One strain of B. fragilis subsp. thetaiotaomicron had a minimal inhibitory concentration of 12.5 mug/ml for clindamycin, but all other strains were susceptible. All the strains were susceptible to chloramphenicol. Susceptibility to tetracycline was variable. Only a few strains were susceptible to the penicillins and cephalothin.