Microbiologic activity of the newer macrolide antibiotics.

In vitro susceptibility testing has demonstrated good activity of the azalide azithromycin and the macrolide clarithromycin against Gram-positive and -negative pathogens as well as atypical organisms involved in the etiology of upper and lower respiratory tract infections. One difference between these drugs in terms of their antimicrobial spectrum is the activity of azithromycin against Haemophilus influenzae. This organism is 2 to 8 times more susceptible in vitro to azithromycin than to clarithromycin or to erythromycin, the prototypical macrolide antibiotic. A principal concern in the management of respiratory tract infections today is the emergence of penicillin-resistant strains of Streptococcus pneumoniae. Both azithromycin and clarithromycin are active against penicillin-susceptible S. pneumoniae, although the activity of azithromycin is somewhat less than that of erythromycin and clarithromycin. Results of susceptibility testing of resistant organisms have varied among centers; in some areas all of the intermediately and some of the highly penicillin-resistant S. pneumoniae isolates are susceptible to the newer macrolides, whereas in other areas they are not. High tissue antibiotic concentrations achieved with these drugs may contribute to their effectiveness against some of the resistant S. pneumoniae isolates.