Susceptibility of Moraxella catarrhalis to 21 antimicrobial drugs: validity of current NCCLS criteria for the interpretation of agar disk diffusion antibiograms

Abstract

Ninety-four clinical isolates of Moraxella catarrhalis were examined for susceptibility to 21 antimicrobial drugs; 67 isolates (= 71.3%) produced beta-lactamase(s). In terms of antibiotic resistance, the number of isolates resistant to penicillin G, ampicillin, and cotrimoxazole were 56, 32, and 1, respectively. The number of isolates with intermediate susceptibility to penicillin G, ampicillin, ciprofloxacin, ofloxacin, cotrimoxazole, and fosfomycin were 11, 34, 1, 2, 2, and 47, respectively. All 94 isolates proved susceptible to ampicillin + 10 micrograms/ml of sulbactam, amoxicillin + 4 micrograms/ml of clavulanic acid, cefuroxime, cefotaxime, cefepime, cefepime, cefixime, imipenem, meropenem, chloramphenicol, doxycycline, tetracycline, fusidic acid, erythromycin, clarithromycin, and rifampin, as based on currently valid NCCLS criteria, where applicable. There were no very major or major discrepancies between agar dilution and agar disk diffusion test results. There were only a few minor discrepancies between test results, specifically: penicillin G (category IV = 4, category VI = 1); ampicillin (category IV = 4, category V = 1, category VI = 7), amoxicillin + clavulanic acid (category III = 11), cotrimoxazole (category IV = 1, category V = 1, category VI = 1), ciprofloxacin (category V = 1), and ofloxacin (category VI = 2). The sole exception was fosfomycin, with a total of 25 minor discrepancies encountered (category III = 14, category V = 9, category VI = 2). Wilkins-Chalgren agar compared favorably with Mueller-Hinton agar following examination with 11 selected antimicrobial drugs against 31 representative isolates of M. catarrhalis.