Susceptibility testing of 7 antibiotics against anaerobic bacteria: comparison of 2 different media and carbon dioxide concentrations.

Anaerobic agar (AA) and Danish Blood agar (DBA) were evaluated by a standardized agar diffusion and agar dilution test in 5% CO2. The activity of seven antibiotics (tetracycline, clindamycin, metronidazole, rifamycin, chloramphenicol, penicillin, erythromycin) was tested against 40 anaerobic bacteria, including 3 control strains (Cl.perfringens ATCC 13124, B.fragilis ATCC 25285, B.thetaiotaomicron ATCC 29741). 70% of the strains were resistant to erythromycin in 10% CO2, only 30% in 5% CO2. No evident CO2-effect could be seen with the other antibiotics. Mean MIC for tetracycline was twice as great on AA than DBA. In spite of that, tablet sensitivity testing with tetracycline on AA proved to be more accurate and completely separated the resistant and susceptible strains. For penicillin, the mean MIC was one dilution step higher on AA. No major differences could be seen with the other antibiotics. AA was superior to DBA in providing growth of anaerobes. Measurement on AA was easier, and it was more precise. Except for tetracycline. MIC on control strains fell well within range set by The National Committee for Clinical Laboratory Standards (NCCLS) on AA. Acceptable correlation coefficients were recorded between agar diffusion and agar dilution. Prediction of susceptibility based on zone diameter measurements was very good on AA. Only one discrepancy that could cause change of susceptibility status occurred on AA, while there were 12 on DBA. On DBA, there was poor correlation between MIC, compared with earlier results on the same agar. The 50% inhibitory concentration (IC50) was also measured, but offered no advantage over MIC.