Variability in susceptibilities of Haemophilus influenzae to clarithromycin and azithromycin due to medium pH

The National Committee for Clinical Laboratory Standards (NCCLS) methods for susceptibility testing of Haemophilus influenzae in Haemophilus test medium allow a pH range of 7.2 to 7.4. However, it is known that bacteria may appear to be less susceptible to macrolides at lower pHs. Forty-four strains of H. influenzae were tested for their susceptibilities to clarithromycin and azithromycin by the disk diffusion and broth microdilution methods. The isolates appeared to be less susceptible at pH 7.2 than at pH 7.4 by both methods. Clarithromycin was less active at pH 7.2 against 43% of the isolates by the disk diffusion method and against 52% of the isolates by the broth microdilution method. Similarly, azithromycin was less active at pH 7.2 against 41 and 45% of the isolates by the disk diffusion and broth microdilution methods, respectively. Forty-two isolates were classified as clarithromycin susceptible and all isolates were classified as azithromycin susceptible by the disk diffusion method, regardless of the medium pH. However, only 21 isolates were clarithromycin susceptible at pH 7.2 and 34 isolates were susceptible at pH 7.4 by the broth microdilution method, even though quality control results indicated valid testing at both pHs. This study indicated that the results of tests of the susceptibility of H. influenzae with clarithromycin and azithromycin are highly dependent on the pH of the medium. Test results and their interpretations varied even when the medium pH was within the NCCLS-approved range and, coupled with the current NCCLS breakpoint of 8 microg/ml in the case of clarithromycin, may explain some of the observed discordances between the disk diffusion and broth microdilution methods.