Survey of methicillin-resistant clinical strains of coagulase-negative staphylococci for mecA gene distribution

A total number of 125 methicillin-resistant (MIC, greater than or equal to 16) coagulase-negative Staphylococcus strains isolated in Japan were surveyed for the distribution of the mecA gene, the structural gene for penicillin-binding protein 2', which is the causative genetic element for the intrinsic resistance of methicillin-resistant Staphylococcus aureus. Screening with colony hybridization by using a cloned mecA gene probe revealed that 121 strains (96.8%) belonging to the nine coagulase-negative Staphylococcus species (S. epidermidis, S. haemolyticus, S. saprophyticus, S. sciuri, S. simulans, S. hominis, S. capitis, S. warneri, and S. caprae) carried mecA in their genome, indicating wide distribution of the gene among coagulase-negative Staphylococcus species. Most (93.4%) of the mecA-carrying strains were producers of penicillinase. Four strains, including two S. haemolyticus and two S. saprophyticus strains, did not carry mecA in spite of their resistance to methicillin. One of them was of low-level resistance (MIC, 16), but three of them had moderate- to high-level resistance to methicillin (MIC, 64). Analysis of gel electrophoretic banding patterns of penicillin-binding proteins of these strains showed absence of penicillin-binding protein 2' but some alterations in signal intensities of the other penicillin-binding proteins. The result indicated that about 3% of methicillin-resistant coagulase-negative staphylococci in these hospitals had a resistance mechanism different from that associated with the production of penicillin-binding protein 2', as has been reported in the case of a borderline methicillin-resistant strain of S. aureus.