Susceptibility and tolerance of beta-lactamase-producing, methicillin-sensitive strains of Staphylococcus aureus towards seven broad-spectrum penicillins.

The activity of penicillin G, ampicillin, carbenicillin, ticarcillin, azlocillin, mezlocillin and piperacillin against 102 beta-lactamase-producing, methicillin-sensitive strains of Staphylococcus aureus was determined by agar dilution (method A) and broth microdilution (method B) techniques. By NCCLS breakpoint criteria, 4% of the strains were "sensitive" to penicillin and ampicillin, and almost 100% were "sensitive" to the other drugs when method A was used. Results with method B were only significantly lower as far as the cumulative percentage of strains "sensitive" to azlocillin, mezlocillin and piperacillin was concerned (63-71%). Bactericidal effects at "sensitive" levels were observed in 0-2% (penicillin, ampicillin), 31-35% (carbenicillin, ticarcillin) and 10-14% (azlocillin, mezlocillin, piperacillin). While differences in MIC and MBC levels ranged from 0 to 8 dilution steps, tolerance (a greater than 32-fold difference) was seen in at least 9-22% of all strains (depending on the drug tested); experimental limitations, however, excluded a determination of tolerance in all our strains. In a semi-quantitative nitrocefin assay, "strong" beta-lactamase production was correlated to high MIC and/or MBC levels.