Rapid detection and identification of methicillin-resistant Staphylococcus aureus directly from positive blood cultures exhibiting gram-positive cocci in clusters

Abstract

The rapid detection and identification of nosocomial or community-associated methicillin-resistant Staphylococcus aureus (MRSA) infection, especially bacteremia, is crucial to patient management and institution of infection control measures. In this study, we performed direct tests for coagulase and PBP2a from centrifuged aliquots of blood from BACTEC blood culture bottles showing gram-positive cocci in clusters, morphologically resembling staphylococci. Concomitantly, direct susceptibility tests were performed for cefoxitin and clindamycin resistance (D test). All test results were compared to those obtained after 24-h growth of the Staphylococcus species on agar media. Of 80 positive blood cultures, 32 were MRSA (coagulase positive, PBP2a positive, cefoxitin resistant), 17 MSSA (coagulase positive, PBP2a negative, cefoxitin susceptible), and 31 coagulase-negative staphylococci (CNS). Of the last, 12 were positive for PBP2a. Direct identification of MRSA was achieved within 4 h as contrasted to the 24 h required by the standard laboratory protocol. Inclusion of coagulase testing along with PBP2a allowed differentiation of CNS from MRSA.