Inoculum effects on Etests and agar dilution minimum inhibitory concentrations. Piperacillin and...

Inoculum effects notoriously occur with beta-lactamase-positive staphylococci in dilution susceptibility tests with substrate beta-lactams. Etests, like dilution experiments, demonstrated such effects with piperacillin and, to a lesser extent, with its tazobactam combinations. When Etests were used with the recommended inoculum of approximately 10(6) colony-forming units (CFU/plate, the minimum inhibitory concentrations (MICs) of piperacillin-tazobactam found for beta-lactamase-positive Staphylococcus aureus isolates resembled those found by agar dilution for inocula of 10(4) CFU/spot but were two- to eightfold below those found by agar dilution for inocula of 10(6) CFU/spot; MICs of piperacillin itself by Etest were about fourfold below even those found by agar dilution with inocula of 10(4) CFU/spot. Inoculum effects for beta-lactamase-negative S. aureus were minimal by both methods.