In-vitro comparison of roxithromycin and erythromycin against 900 anaerobic bacterial strains

The in-vitro antibacterial activity of roxithromycin was assessed by an agar dilution method against 900 recent clinical anaerobic isolates by five laboratories in England, France, Germany and Japan. Roxithromycin had similar activity to erythromycin against most anaerobic bacteria, the latter being slightly more active against Gram-negative bacilli. Roxithromycin inhibited 53% of the Bacteroides fragilis group strains; the noticeable exception was Bact. thetaiotaomicron, only 17% of strains of which were inhibited by 4 mg/l roxithromycin. In contrast, all isolates of Bacteroides, other than the Bact. fragilis group, and all Mobiluncus isolates were inhibited by 2 mg/l of roxithromycin or less. This compound was inactive against half the Fusobacterium strains. Roxithromycin and erythromycin inhibited 65 and 72%, respectively, of 436 strains of Gram-negative anaerobes (MIC less than or equal to 4 mg/l). Roxithromycin suppressed 97% of non-sporulating Gram-positive to bacilli, 86% of Peptococcoaceae and 95% of clostridia. Two-thirds of Clostridium difficile strains were susceptible to roxithromycin (MIC less than or equal to 1 mg/l). The activity of roxithromycin against Gram-positive bacilli was identical to that of erythromycin. Overall a concentration of 4 mg/l roxithromycin and erythromycin inhibited 79 and 83%, respectively, of the strains investigated.