Detection of methicillin-resistant Staphylococcus aureus using double duplex real-time PCR and dye Syto 9

A screening method for methicillin-resistant Staphylococcus aureus (MRSA) using real-time polymerase chain reaction (PCR) and dye Syto 9 was developed and evaluated. The assay was based on the two duplex reactions run simultaneously. The detection reaction amplified staphylococcal cassette chromosome mec (SCCmec) right extremity sequences and S. aureus-specific 442-bp DNA (Sa442). The control reaction amplified S. aureus-specific nuclease gene nuc and a marker of methicillin resistance, mecA. The method was evaluated by analyzing 214 clinical S. aureus isolates yielding 98.7 % sensitivity, 100 % specificity, 100 % positive predictive value and 96.6 % negative predictive value for detection of MRSA. The detection limit was determined to be 15-80 genome copies per real-time PCR. It was able to discriminate between MRSA, methicillin resistant coagulase negative staphylococci and methicillin susceptible S. aureus (MSSA) isolates containing only small fragments of the right extremity of the SCCmec (MSSA revertants).