Anti-anaerobic activity of erythromycin, azithromycin and clarithromycin: effect of pH adjustment of media to compensate for pH shift caused by incubation in CO2

The activity of erythromycin, azithromycin and clarithromycin against 112 anaerobes was tested by the Oxyrase agar dilution MIC method at pH 7.2 without supplemental CO2 and by an agar dilution MIC method in CO2 with media adjusted to pH 8.0. MICs (mg/L) of erythromycin, azithromycin and clarithomycin against Bacteroides fragilis ATCC 25285 and Bacteroides thetaiotaomicron ATCC 29741 were similar by the two methods. MICs for 94 clinical isolates tested by the two methods were within two dilutions of each other. Eighteen additional isolates required CO2, and did not grow in Oxyrase. With the exception of fusobacteria, with which azithromycin yielded the lowest MICs, clarithromycin had the lowest MICs with both methods. These results show that the pH effect of incubation in CO2 can be avoided by using the Oxyrase method, or by incubating in CO2 with pH adjusted to 8.0. The latter method has the advantage of allowing testing of strains requiring CO2.