[Staphylococcus aureus: new detection of intrinsic resistance using the diffusion method]

Because of an heterogeneously-expressed resistance among methicillin-resistant Staphylococcus aureus strains the conditions for antibiogram determination had rapidly to be modified so as to improve their detection. The newly recommended conditions (incubation at +30 degrees C or on hypersalted agar medium) remain widely used at the moment, although they appear to be more and more often badly adapted, particularly because of the recently-observed renewed outbreak of wild strains with a weak in vitro phenotypic expression. It is the reason why we searched for a new and more reliable phenotypic method although still accessible for any laboratory. Sixty-five strains of Staphylococcus aureus entered the study. The absence or presence of mecA gene was previously investigated by gene amplification. These strains were of various origins and had often caused difficulties for the detection of intrinsic resistance to methicillin on the antibiogram. Our results confirm the failures of the classical methods (false negative results at +30 degrees C, false negative or positive results on hypersalted agar medium incubated at +37 degrees C). They also allow to propose a new method which relies on the determination of the susceptibility to cefoxitin using the usual conditions for antibiogram determination. In our series of strains, this new method proved to widely improve both the sensitivity and the susceptibility for the detection of methicillin-resistance by diffusion on the antibiogram.