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-lactam antibiotics, were found on further testing to be susceptible to oxacillin and ceftizoxime by disk diffusion tests. For all these strains, the MICs of oxacillin were </=0.5 microg/ml by agar dilution tests, and the strains were oxacillin susceptible when tested by the BBL Crystal MRSA ID and a Vitek machine with GPS-101 cards. None grew on oxacillin-salt agar screening plates. None were positive for mecA gene sequences by PCR. When S. aureus strains tested by Vitek GPS-SA or GPS-BS cards appear resistant to only penicillin and oxacillin, a confirmatory test such as the oxacillin-salt agar screening method should be performed.
Ribeiro
J
J
Hospital de Base, Brasilia, Brazil.
Vieira
F D
FD
King
T
T
D'Arezzo
J B
JB
Boyce
J M
JM
D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods*; D010068:Oxacillin / Q000494:pharmacology; D013211:Staphylococcus aureus / Q000187:drug effects*
D010068:Oxacillin