Effect of inoculum size on the in-vitro susceptibility to -lactam antibiotics of Moraxella catarrhalis isolates of different -lactamase types

The effect of inoculum size on the results of agar dilution MIC tests was assessed for 20 Moraxella catarrhalis isolates with BRO-1 enzyme, 20 with BRO-2 enzyme and 15 isolates that did not produce beta-lactamase. The compounds tested were ampicillin, coamoxiclav, cefaclor, cefixime and cefetamet, and the inocula were 10(4), 10(5), 10(6) and 10(7) cfu/spot. The MICs of ampicillin for BRO-1 and BRO-2 producers were consistently higher than those for non-producers at inocula of 10(7) cfu/spot but overlapped with those for non-producers at lower inocula. A small beta-lactamase-related inoculum effect was seen with coamoxiclav; small inoculum effects also occurred with cefaclor and cefixime but were not related to enzyme presence or type. MICs of cefetamet were the least affected by the inoculum size. For all the compounds, the degree of correlation between MICs and the inhibition zones observed in disk diffusion tests was independent of the inoculum used in the MIC tests. These data suggest that high inocula should be used to determine MICs of ampicillin for M. catarrhalis but that this precaution is unnecessary with the cephalosporins tested or with coamoxiclav.