In vitro activity of dicloxacillin against methicillin-susceptible and methicillin-resistant Staphylococcus-aureus

A total of 54 strains of Staphylococcus aureus, 37 methicillin-susceptible (MSSA) and 17 methicillin-resistant (MRSA), were investigated for their susceptibility to dicloxacillin as compared to methicillin and oxacillin by agar plate dilution at two different temperatures of incubation (30 degrees C and 37 degrees C). Against MSSA strains we found a slight but significant increase (0-2 dilution steps) in minimal inhibitor concentrations (MICs) for all three antibiotics with decrease in incubation temperature. Against MRSA strains methicillin and oxacillin showed a 3-6 fold increase in median MIC with decrease incubation temperature. For dicloxacillin, in contrast, there were no significant differences in median MICs (i.e. 0.4 mg/l) against MSSA strains at 30 degrees C or MRSA strains at either incubation temperature. Population-analysis of the MRSA strains revealed, however, that a highly dicloxacillin-resistant subpopulation appeared with a frequency of 10(-6) to 10(-7). Such heterogenous resistance of MRSA strains to dicloxacillin probably prohibits the use of dicloxacillin against serious infections caused by these pathogens.