[Methods for the detection of resistance to oxacillin in strains of Staphylococcus aureus]

Methods suitable for detection of resistance of staphylococci to oxacillin were tested in a group of 77 strains of Staphylococcus aureus (39 strains sensitive and 38 strains resistant to oxacillin). The influence of the composition of the medium, the growth phase of the inoculum and time of incubation on detection of resistant strains was investigated. By none of the methods resistance to oxacillin was proved in the control group of sensitive strains. For evidence of oxacillin resistance after 24 hours' incubation the standard micro-method is suitable and the diffuse method with 1 microgram disc of oxacillin which both detected 100% of resistant strains, and the screening method with a yield of 97% strains. The growth phase of the inoculum and the incubation period do not influence the results of these methods. The micromethod in MH broth, the diffuse method with a 10 micrograms oxacillin disc and the dilution plate method were influenced to a considerable extent by the composition of the medium, the growth phase of the inoculum, the incubation period and they revealed a small number of resistant strains.