In vitro activity of roxithromycin against 16 species of atypical mycobacteria and effect of pH on its radiometric MICs.

The antimycobacterial spectrum of roxithromycin, a semisynthetic 14-membered ring macrolide, was determined against 28 strains belonging to 16 species of atypical mycobacteria by measuring radiometric MICs by BACTEC methodology at two different pH values, i.e., 6.8 and 7.4. The MICs obtained at pH 7.4 were 1 to 2 dilutions lower or more than those obtained at pH 6.8 for some of the species. Roxithromycin possessed promising MICs against such potential pathogens as the Mycobacterium avium complex, M. scrofulaceum, M. szulgai, M. malmoense, M. xenopi, M. marinum, M. kansasii, and rare pathogens like M. chelonei subsp. chelonei and M. chelonei subsp. abscessus but not against M. simiae. Roxithromycin showed lower MICs against M. fortuitum var. peregrinum than M. fortuitum var. fortuitum.