Objective: To examine in vitro resistance to azithromycin and moxifloxacin in bacterial conjunctivitis isolates. Methods: MIC90s (Minimum Inhibitory Concentration) and resistance rates to azithromycin and moxifloxacin were determined based upon microtiter broth dilution and/or antimicrobial gradient test strips in a multicenter phase III study and confirmed externally. Results: The most common isolates collected from bacterial conjunctivitis patients in the phase III study were Haemophilus influenzae (40.6%), followed by Staphylococcus epidermidis (19.3 %), Propionibacterium acnes (17.3%), Streptococcus pneumoniae (16.8%), and Staphylococcus aureus (0.06%). MIC 90s for all of these organisms were well below established resistance breakpoints for moxifloxacin, indicating no bacterial resistance. On the other hand, the MIC90 for H. influenzae was 3-fold higher than the resistance breakpoint for azithromycin, ≥ 128-fold higher for S. epidermidis, 16-fold higher for S. pneumoniae and ≥ 128-fold higher for S. aureus, indicating moderate to very high bacterial resistance to azithromycin. Conclusions: Resistance to azithromycin is more common than resistance to moxifloxacin in clinical isolates causing bacterial conjunctivitis.
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