Susceptibility of Moraxella catarrhalis isolates to beta-lactam antibiotics in relation to beta-lactamase pattern

Moraxella catarrhalis isolates (n = 413) were collected from 20 clinical laboratories in England and Scotland in 1991 and were examined for beta-lactamase production by isoelectric focusing. beta-Lactamases were found in 375 isolates of which, 349 (93.1%) had BRO-1 enzyme and 26 (6.9%) had BRO-2. Minor variation in electrofocusing pattern occurred within both enzyme types. Ampicillin MICs for BRO-1 producers were 25-fold higher than for non-producers, but those for BRO-2 producers were raised only four-fold. MICs of cefaclor, cefixime, loracarbef, co-amoxiclav and cefetamet generally were two- to four-fold higher for BRO-1 producers than for BRO-2 producers and enzyme non-producers. Similarly, the inhibition zones of discs containing cefaclor, cefixime, loracarbef or co-amoxiclav were smaller for BRO-1 producers than for non-producers. Amongst the compounds tested, cefetamet seemed the least affected by beta-lactamase production in both MIC and disc tests. Overall, these results indicate that BRO-1 enzyme predominates amongst M. catarrhalis isolates from the UK, as in other countries, and suggest that BRO-1 production gives slight protection against many of the newer oral beta-lactams as well as causing ampicillin resistance.