Antimicrobial susceptibility of Moraxella catarrhalis isolated from children in Kalgoorlie-Boulder, Western Australia.

Aim: To investigate antimicrobial susceptibility of Moraxella catarrhalis isolated from a cohort of children being followed in a study of the natural history of otitis media in a semi-arid region of Western Australia.

Methods: In the Kalgoorlie Otitis Media Research Project nasopharyngeal aspirates were collected from children up to seven times between the age of 1 week and 2 years. A total of 261 M. catarrhalis strains from 50 Aboriginal and 50 non-Aboriginal children were tested against 14 antibiotics using the Clinical and Laboratory Standards Institute (CLSI) agar dilution method.

Results: All strains were susceptible to amoxicillin/clavulanate, cefuroxime, azithromycin, ciprofloxacin, moxifloxacin, levofloxacin, erythromycin and minocycline. While no criteria exist for assessment of susceptibility to roxithromycin, minimum inhibitory concentrations (MICs) were low. Approximately 46% of strains from Aboriginal children and 27% from non-Aboriginal children appeared susceptible to ampicillin. A small number of strains was intermediately resistant to cefaclor (9/261, 3.4%), while the bulk of strains was intermediately resistant to co-trimoxazole. A low prevalence of tetracycline resistance (3/261, 1.1%) was noted. beta-lactamase production was observed in 97.7% of strains.

Conclusions: While M. catarrhalis strains from children of the Kalgoorlie region were susceptible to many of the antibiotics used to treat respiratory tract infections, a large proportion of strains were resistant to ampicillin and/or co-trimoxazole. Current therapeutic guidelines, which recommend amoxicillin for treatment of otitis media, may need to be revised.