Activity of combinations of antimicrobial agents against Bacteroides fragilis

Thirty-two clinical isolates of Bacteroides fragilis were tested against nine pairs of antimicrobial agents by means of an agar dilution technique. A synergistic effect was observed with the combination of clindamycin and metronidazole. End points were achieved with 17 strains. Of these strains, 13 (76%) were inhibited by the combination of clindamycin and metronidazole; each drug was present at a concentration of less than or equal to 25% of its minimal inhibitory concentration when tested alone. This combination also showed synergistic bactericidal activity against three of six strains examined by a tube dilution technique. No antagonism was noted with any strain. The other eight combinations tested failed to show a consistent synergistic effect, although no antagonism was observed. These in vitro data indicate that antagonism is not likely to be encountered when combination therapy is used for B. fragilis infections. For selected B. fragilis infections, the combination of clindamycin and metronidazole may be useful.