The in-vitro activity of roxithromycin, a new macrolide antibiotic, in comparison with that of erythromycin

A study was made of the in-vitro activity of roxithromycin in comparison with that of erythromycin on selected recent clinical isolates of a wide range of organisms. Minimum inhibitory concentrations (MICs) were determined by an agar dilution method with an inoculum of 10(4) cfu. Minimum bactericidal concentrations (MBCs) and the effect of pH were determined by a broth dilution method on selected strains. In general the in-vitro activity of roxithromycin mirrored that of erythromycin, but it was slightly less active. Most strains of streptococci, with the exception of enterococci, were highly sensitive to roxithromycin, as were isolates of Gardnerella vaginalis. Branhamella catarrhalis and Neisseria gonorrhoeae. Staphylococci and enterococci were only moderately sensitive to both agents as was Haemophilus influenzae. Most anaerobic bacteria were sensitive although Bacteroides fragilis, fusobacteria and some strains of some clostridial species were moderately resistant. The MBCs were usually at least 16-fold higher than MICs for most staphylococci, enterococci, alpha-haemolytic streptococci and B. fragilis; but for beta-haemolytic streptococci, pneumococci, H. influenzae and B. catarrhalis the MBCs were usually only 2- to 4-fold higher than MICs. Roxithromycin was most active at pH 8 and generally for each unit fall in pH there was a 4-fold diminution of activity. However, the effect was less marked for pneumococci, beta-haemolytic streptococci and H. influenzae.