Misclassification of susceptible strains of Staphylococcus aureus as methicillin-resistant S-aureus by a rapid automated susceptibility testing system

Eight Staphylococcus aureus strains initially identified by Vitek GPS-BS or GPS-SA cards as resistant to oxacillin, but susceptible to most non-beta-l actam antibiotics, were found on further testing to be susceptible to oxaci llin and ceftizoxime by disk diffusion tests. For all these strains, the MI Cs of oxacillin were less than or equal to 0.5 mu g/ml by agar dilution tes ts, and the strains,were oxacillin susceptible when tested by the BBL Cryst al MRSA ID and a Vitek machine with GPS-101 cards. None grew on oxacillin-s alt agar screening plates. None were positive for mecA gene sequences by PC R When S. aureus strains tested by Vitek GPS-SA or GPS-BS cards appear resi stant to only penicillin and oxacillin, a confirmatory test such as the oxa cillin-salt agar screening method should be performed.