Rapid detection, differentiation and typing of methicillin-resistant Staphylococcus aureus harbouring either mecA or the new mecA homologue mecALGA251

Abstract

The recent finding of a new mecA homologue, mecA(LGA251) , with only 70% nucleotide homology to the conventional mecA gene has brought the routine testing for mecA as a confirmatory test for methicillin-resistant Staphylococcus aureus (MRSA) into question. A multiplex PCR was designed to differentiate mecA(LGA251) from the known mecA together with detection of lukF-PV and the spa gene fragments, enabling direct spa typing by sequencing of the PCR amplicons. The PCR analysis and subsequent spa typing were validated on a large collection (n=185) of contemporary MRSA and methicillin-sensitive S. aureus isolates, including 127 isolates carrying mecA(LGA251) . The mecA(LGA251) gene was situated in staphylococcal cassette chromosome mec type XI elements, and sequence variation within a 631-bp fragment of mecA(LGA251) in 79 isolates indicated a very conserved gene sequence. Following a successful validation, the multiplex PCR strategy was implemented in the routine testing of MRSA for national surveillance. Over a 2-month period, among 203 samples tested, 12 new MRSA cases caused by isolates carrying mecA(LGA251) were identified, emphasizing the clinical importance of testing for these new MRSA isolates.