Molecular typing of the methicillin resistance determinant (mec) of clinical strains of Staphylococcus based on mec hypervariable region length polymorphisms.

We used a method for molecular typing the methicillin resistance determinant (mec) based on the size of the mec-associated hypervariable region amplified by the polymerase chain reaction (PCR) to examine 61 methicillin-resistant Staphylococcus aureus (MRSA), 15 methicillin-resistant (Mcr) S. epidermidis, and 11 Mcr S. haemolyticus clinical isolates. In the 61 MRSA isolates, five sizes of PCR products were observed. The MRSA isolates were grouped into five hypervariable region (HVR) genotypes on the basis of the size of the PCR product. Three different sizes were detected among 15 isolates of Mcr S. epidermidis and two sizes among 11 isolates of Mcr S. haemolyticus. The PCR products amplified from 14 of 15 Mcr S. epidermidis isolates were the same as products amplified from MRSA isolates, which was confirmed by the PCR-SSCP (single-strand conformation polymorphism) method. In methicillin-susceptible isolates, the target gene was not amplified. This method is thought to be useful in epidemiologic investigations of nosocomial infections caused by MRSA. This is the first typing method capable of comparing the mec determinants of MRSA isolates and Mcr coagulase-negative staphylococcal isolates to establish the origin of the mec determinant.