Identification of Methicillin Resistant Staphylococcus aureus (MRSA) and Methicillin Resistant Coagulase-Negative Staphylococcus (CoNS) in Clinical Settings

To evaluate different methods for the identification of methicillin-resistant Staphylococci and their reliability, 112 Staphylococcal isolates (32 Staphylococcus aureus isolates and 80 coagulase negative Staphylococci isolates "CoNS") were collected from 118 nasal swab cultures and were subjected to three methods to detect oxacillin susceptibility of the isolates. The three methods were oxacillin disk diffusion method; the Epsilometer- test (E-test) and polymerase chain reaction (PCR). For the S. aureus strains, the E-test and the PCR methods showed discrepant results in two isolates (6.25%); that showed susceptible patterns with the E-test, but were resistant by the disk diffusion method. Both isolates were negative for the presence of mecA gene. Seven (8.75%) out of the 80 CoNS isolates showed conflicting results where four isolates showed resistance with the disk diffusion and the E-test methods, and had negative mecA gene by PCR. Three of the 7 CoNS with the conflicting results showed a susceptible pattern to oxacillin by the E-test method, while the PCR method showed the presence of mecA gene. We concluded that combination of molecular and conventional methods should be used to assess methicillin resistance of Staphylococci in clinical practices.