Effect of pH on the in vitro potency of clarithromycin against Mycobacterium avium complex.

Employing 7H11 agar medium at pH 6.6, the MICs of clarithromycin for 50% (MIC50) and 90% (MIC90) of 19 strains of Mycobacterium avium complex were 8 and 16 micrograms/ml, respectively. However, the MICs were 2 to 3 log2 dilutions lower in the 7H11 medium adjusted to pH 7.4, and the MICs on 10% OADC (oleic acid-albumin-dextrose-catalase)-enriched Mueller-Hinton agar at pH 7.3 were also 2 log2 dilutions lower than those measured on 7H11 agar at pH 6.6. Therefore, clarithromycin is more active at a physiologic than at an acidic pH.