Antibiotic susceptibility of anaerobic bacteria isolated in Johannesburg.

In vitro susceptibilities of 198 anaerobic bacteria to seven antibiotics were evaluated by the agar dilution method. In addition to testing amoxycillin/clavulanic acid in a 2:1 ratio against the bacteria, the combination was also tested against 63 isolates using fixed concentrations of clavulanic acid and serial dilutions of amoxycillin. Penicillin and cefoxitin were not effective against beta-lactamase-producing Bacteroides isolates and only 50% of isolates were susceptible to the 2:1 amoxycillin/clavulanic acid combination. However, when varying concentrations of amoxycillin were used together with constant concentrations of clavulanic acid (4 micrograms/ml) only 9 of 55 amoxycillin-resistant Bacteroides were resistant to the combination. Two clostridia were found to produce beta-lactamases and as expected were resistant to penicillin. Of the non-beta-lactamase-producing clostridia 11% were resistant to penicillin and 5% resistant to cefoxitin. Imipenem was effective against the majority of anaerobes tested and only 5 Bacteroides isolates were resistant. All anaerobic strains were susceptible to chloramphenicol and only 6% of strains resistant to clindamycin. Eighty-five per cent and 51% of Bacteroides strains had minimum inhibitory concentrations within two dilutions of the breakpoints of chloramphenicol (16 micrograms/ml) and clindamycin (4 micrograms/ml) respectively. Three strains of Peptostreptococcus spp. were resistant to metronidazole.