Comparison of azithromycin and moxifloxacin against bacterial isolates causing conjunctivitis

Headnote

Key words: Azithromycin - Conjunctivitis - Fluoroquinolone - Macrolide - Moxifloxacin - Resistance

ABSTRACT

Objective: To examine in vitro resistance to azithromycin and moxifloxacin in bacterial conjunctivitis isolates.

Methods: MIC^sub 90^s (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^sub 90^s for all of these organisms were well below established resistance breakpoints for moxifloxacin, indicating no bacterial resistance. On the other hand, the MIC^sub 90^ 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.

Introduction

Bacterial conjunctivitis is a common childhood illness. A recent study demonstrated the need to exclude children with infectious conjunctivitis from school until it has resolved1. This requires the selection of the most effective topical antibiotic, so that the child may return to school as quickly as possible. The recent FDA approval of an ophthalmic topical formulation of azithromycin (AzaSite*)2 requires that the antibiotic choices be re-evaluated.

The most common pathogens reported in previous bacterial conjunctivitis literature are H. influenzae, S. pneumoniae, and to a lesser extent, S. aureus3-5. The introduction of the H. influenzae type b (HiB) vaccine in 1985 has not affected the total number of cases of conjunctivitis caused by non-typeable H. influenzae, but has likely decreased the prevalence of serotype b conjunctivitis6, as it has decreased systemic infection7 and preseptal and orbital cellulitis caused by this subtype8. On the other hand, the introduction of the heptavalent pneumococcal vaccine (Prevnart) has decreased the frequency of conjunctivitis due to S. pneumonia9. Recent epidemics of conjunctivitis due to non-typeable strains of S. pneumoniae demonstrate that this organism remains an important cause of epidemic conjunctivitis10-13.

Non-typeable H. influenzae and S. pneumoniae are among the...