title: Inoculum effects on Etests and agar dilution minimum inhibitory concentrations. Piperacillin and piperacillin-tazobactam against Staphylococcus aureus.
abstract: 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.
authors: Department of Medical Microbiology, London Hospital Medical College, England.
authors: G
authors: G
authors: Bonfiglio
authors: D M
authors: DM
authors: Livermore
mesh_terms: D001431:Bacteriological Techniques; D015169:Colony Count, Microbial; D008826:Microbial Sensitivity Tests / Q000592:standards*; D010397:Penicillanic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D010878:Piperacillin / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000254:growth & development; D000078142:Tazobactam; D065093:beta-Lactamase Inhibitors; D001618:beta-Lactamases / Q000494:pharmacology*
chemical_list: D065093:beta-Lactamase Inhibitors; D010397:Penicillanic Acid; D001618:beta-Lactamases; D000078142:Tazobactam; D010878:Piperacillin