Effect of temperature on the in vitro susceptibility of Staphylococcus aureus to penicillinase-resistant penicillins.

Heteroresistant (methicillin-resistant) and nonheteroresistant strains of Staphylococcus aureus were tested for their susceptibility to penicillinase-resistant penicillins at incubation temperatures of 37, 35, and 30 C. Susceptibilities were determined by agar dilution and by the standard Kirby-Bauer agar diffusion tests. Minimal inhibitory concentrations were higher at 35 and 30 C than at 37 C. Heteroresistance could be detected with the Kirby-Bauer test if the incubation temperature was 30 or 35 C instead of 37 C, when tests were performed against methicillin, oxacillin, or nafcillin, because the resistant organisms grew up to the disks even though the susceptible organisms were inhibited. At 37 C, the resistance was detectable with some strains but not with others. When cloxacillin disks were used, the temperature effect was not seen. The incubation temperature did not affect results with nonheteroresistant strains. Therefore, it is recommended that all Kirby-Bauer tests be incubated at a temperature of 35 C to insure detection of methicillin-resistant S. aureus strains. Detection of these strains is of increasing importance because the incidence of infections with these organisms is increasing, particularly in hospitalized patients.