title: In vitro activity of dicloxacillin against methicillin-susceptible and methicillin-resistant Staphylococcus-aureus.
abstract: 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.
authors.affiliation: Department of Antibiotics, Statens Seruminstitut, Copenhagen, Denmark.
authors.forename: N
authors.initials: N
authors.lastname: Frimodt-Møller
authors.forename: V T
authors.initials: VT
authors.lastname: Rosdahl
authors.forename: B
authors.initials: B
authors.lastname: Gahrn-Hansen
mesh_terms: D004009:Dicloxacillin / Q000494:pharmacology*; D008712:Methicillin / Q000494:pharmacology*; D008826:Microbial Sensitivity Tests; D010403:Penicillin Resistance; D013211:Staphylococcus aureus / Q000187:drug effects*
chemical_list: D004009:Dicloxacillin; D008712:Methicillin