[Detection of heterogeneous Staphylococci by the Autobac system using a nafcillin disk]

Detection of Methicillin Resistant Staphylococcus aureus (MRSA) is still a problem for automated system users. Forty-nine strains were studied--two kinds of tests were performed: the first by using a 1.0 microgram nafcillin disk with the Autobac system, the second by testing an oxacillin disk by diffusion on Mueller-Hinton agar base complemented with natrium chloride incubated at 37 degrees C. Between the two tests, we observed a concordance for 87.8% of the strains. Six strains showed a discrepancy with the reference method. But for five of them, the Light Scattering Index (LSI) showed a decreasing sensibility near for the cut off. Whereas, only one strain is considered as susceptible. The 1.0 micrograms nafcillin disk allow better results than the 5.0 micrograms oxacillin disk in the automated system, Autobac.