Two-hospital study of Staphylococcus aureus susceptibility to penicillin and ampicillin by Autobac I

A total of 322 clinical isolates of Staphylococcus aureus were tested with ampicillin and penicillin G by Autobac I. Of these, 74 non-beta-lactamase-producing strains were isolated, of which only 32 were initially reported susceptible to both antibiotics by Autobac I. Thirty-two strains were ampicillin susceptible but penicillin resistant by Autobac I, and nine others were "resistant" to both antibiotics but had light-scattering index values that neared the "susceptible" breakpoints established by Pfizer Diagnostics for ampicillin and penicillin. Two strains were intermediate by disk diffusion test. A method was devised by which accurate results for non-beta-lactamase-producing S. aureus could be determined by Autobac I. Substitution of the 0.22-microgram ampicillin disk for the 0.2-U penicillin disk, coupled with extended incubation beyond the 3-h recommended interval, improved results. Alternative methods such as beta-lactamase enzyme assay or disk diffusion testing could be reserved for the occasional remaining cases of borderline light-scattering index-producing S. aureus (those nearing the susceptible break point, greater than or equal to 0.60). Substitution of the ampicillin disk coupled with extended incubation time did not result in reporting of false susceptibility for beta-lactamase-producing strains, a clinically hazardous situation.