Bactericidal Activity of Five Antimicrobial Agents against Bacteroides fragilis

Abstract

Rifampin, 7-chlorolincomycin, vancomycin, and metronidazole (Flagyl® were evaluated for bactericidal activity against Bacteroides fragilis. Tetracycline, a bacteriostatic agent, was included for comparative purposes. Standard tube-dilution determinations of MICs were done with brucella broth (Pfizer) plus 5% laked blood, followed by subculture onto blood-agar plates for determination of minimal bactericidal concentration (MBC). Metronidazole and 7-chlorolincomycin inhibited all 19 strains tested at concentrations readily achieved in serum; however, only metronidazole was consistently bactericidal (MBC, 0.78–6.25 µg/ml). All strains were resistant to vancomycin and eight were resistant to tetracycline. Rifampin, although generally very active, demonstrated the “skip-tube phenomenon,” suggesting that resistance to this agent may be a problem with anaerobes as with other organisms. In conclusion, metronidazole and 7-hlorolincomycin were effective in vitro against 19 strains of B. fragilis. Only metronidazole was consistently bactericidal. This agent, therefore, might be of benefit in the therapy of infections due to B. fragilis, particularly endocarditis.