Susceptibility surveillance of US respiratory pathogen isolates to newer macrolide and azalide antibiotics

Azithromycin and clarithromycin are both highly active in vitro against Gram-positive respiratory pathogens, but azithromycin is substantially more potent against Haemophilus influenzae. We investigated the susceptibility of H. influenzae, Moraxella catarrhalis, Streptococcus pneumoniae and S. pyogenes to azithromycin and clarithromycin, and determined the prevalence of beta-lactamase production in H. influenzae and M. catarrhalis. Results from three geographic regions of the USA were compared. A significantly greater proportion of H. influenzae isolates were susceptible to azithromycin than to clarithromycin (95.7% vs 63.1%, P < 0.001). M. catarrhalis, S. pneumoniae and S. pyogenes were highly susceptible to both antibiotics with no significant differences in susceptibility. beta-Lactamase was produced by 94.8% of M. catarrhalis isolates and 35.6% of H. influenzae.