It is well known that bacteria producing β-lactamases in general show marked inoculum effect in susceptibility testing. The aim of this study is to determine the effect of inoculum size on the susceptibility of β-lactamase positive strains of M. catarrhalis to oral β-lactam and non β-lactam antibiotics. MICs of antibiotics were determined by a twofold microdilution technique with two different inoculum sizes were tested: 5x 105 CFU/ml -standard inoculum and 5 x107 CFU/ml -high inoculum. The highest increase (4-fold) was observed with penicillins alone (amoxycillin and ampicillin) or combined with inhibitor and ceftibuten, followed by older cephalosporins, erithromycine and chloramphenicol ( 2-fold). Tetracycline did not show a significant increase in MIC when a higher inoculum size was applied. In spite of the increase in MIC with high inoculum all strains were still susceptible to amoxycillin combined with clavulanate. MICs of cephalosporins were also below the resistance breakpoint for most of the strains at the higher inoculum. Based on that, we can conclude that therapeutic implications of the inoculum effect were not significant.These data suggest that high inocula should be used to determine MICs of ampicillin and amoxycillin for M. catarrhalis but that this precaution is unnecessary with the cephalosporins tested or with amoxycillin/clavulanat
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