In vitro activity of oral beta-lactams against 40 beta-lactamase producing strains of Branhamella (Moraxella) catarrhalis correlated to the inoculum size

Inhibitory activity of amoxicillin alone and in combination with 2 mg/l or 4 mg/l clavulanate, ampicillin alone and in combination with 4 mg/l or 8 mg/l sulbactam, cefaclor, cefuroxime-axetil, and cefixime were determined against 40 beta-lactamase-producing strains of Branhamella (Moraxella) catarrhalis. MIC were assessed, using an agar dilution method, with bacterial inoculum size ranging from 10(8) to 10(10) CFU per mi (10(5), 10(6), and 10(7) CFU/spot). Aminopenicillins and cefaclor expressed very strong inoculum effects (2 mu g/ml less than or equal to MIC 90 % less than or equal to 8 mu g/ml at 10(5) CFU/spot i.e. MIC 90 % > 64 mu g/ml at 10(7) CFU/spot). Inoculum effect is at its highest with cefuroxime-axetil and decreases progressively to its lowest with cefixime and aminopenicillins in combination with beta-lactamase inhibitors. The most active compound appeared to be amoxicillin in association with clavulanate (2 or 4 mg/l), with no significant difference between MIC 50 % and MIG: 90 % values, as opposed to ampicillin in combination with sulbactam and other beta-lactams tested.