Study on mec gene in methicillin-resistant staphylococci

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA), methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus epidermidis and methicillin-resistant Staphylococcus haemolyticus strains which were isolated in 1992 were surveyed for distribution of mecA, mecR1 (putative mecA gene inducer gene) and mecI gene (putative mecA gene repressor gene) by the method of polymerase chain reaction (PCR). All the 30 strains (100%) of MRSA carried mecA gene and mecR1 gene-1 (32-357 bp, closer to mecA gene). In contrast, mecI gene (putative mecA gene repressor gene) and mecR1 gene-2 (987-1221 bp, closer to mecI gene) were detected in only 23 (73%) of the MRSA strains. By RT-PCR, the mecI gene existing in these highly resistant MRSA strains was demonstrated to remain inactivated. In 28 strains of MSSA and two strains of methicillin with MIC of 6.25 micrograms/ml, all of the mec genes (mecA, mecR1 and mecI) were not detected. In contrast, in the other three strains, which were sensitive to methicillin in spite of the presence of mecA gene and defined as MSSA, the mec genes other than mecA were detected except for one strain. Methicillin-resistant (with MIC of 12.5 micrograms/ml or more) coagulase-negative staphylococci (15 S. epidermidis and 9 S. haemolyticus) strains were surveyed as well. As results, all the mec genes mentioned above were detected in all the strains except for one of S. epidermidis, but only mecA gene in the strains of S. haemolyticus. Thus, it was found that the composition of the mec genes of methicillin-resistant S. haemolyticus were more variant from that of methicillin-resistant strains of S. aureus and S. epidermidis.