THE ANTIMICROBIAL SUSCEPTIBILITY OF MORAXELLA-CATARRHALIS ISOLATED IN ENGLAND AND SCOTLAND IN 1991

Between 1 January and 31 March 1991, 20 laboratories in England and Scotland sent a total of 413 consecutive clinical isolates of Moraxella catarrhalis to The London Hospital Medical College (LHMC). After confirmation of identity, the susceptibility of all isolates to 11 antimicrobial agents was determined. Of the 375 (90·8%) isolates which were found at LHMC to be β-lactamase-positive, 174 produced zones of inhibition around 2 μg ampicillin disc which were ≥ 20 mm in diameter and 252 were inhibited by ≤ 0·5 mg/L of ampicillin. However, 71 of these 375 had been reported to be ampicillin-susceptible by peripheral centres. While β-lactamase had not been detected in 35 of these 71 isolates, the other 36 had been reported to be ampicillin-susceptible and β-lactamase-positive. All 38 β-lactamase negative isolates produced zones ≥ 30 mm in diameter and were inhibited by ≤ 0·06 mg/L of ampicillin. No M. catarrhalis isolate was found to be resistant to co-amoxiclav, tetracycline, chloramphenicol or cefaclor. Two strains showed intermediate susceptibility to erythromycin (MIC 1 mg/L) and 27 required ≥ 32 mg/L of sulphamethoxazole for inhibition. Resistance to trimethoprim was uniform (MICs 2-128 mg/L). Two isolates showed intermediate susceptibility to cefixime (MIC 2 mg/L) but none was resistant to the new oral cephalosporin cefetamet or to the oral carbacephem loracarbef. Aprt from β-lactamase production and inherent resistance to trimethoprim. the results indicate that there are few problems of acquired resistance to antimicrobials amongst M. catarrhalis isolates in England and Scotland. © 1992 The British Society for Antimicrobial Chemotherapy.