Increased incidence of beta-lactamase-plasmid negative, high level methicillin-resistant Staphylococcus aureus (MRSA)

Recently isolated strains of methicillin-resistant Staphylococcus aureus (MRSA) have high levels of resistance to the agent and produce beta-lactamase less frequently than methicillin-susceptible strains (MSSA). This phenomenon has been observed in Japan. The majority of MSSA strains (26 of 30) isolated in 1992 produced beta-lactamase while only 18 of 31 and 25 of 67 MRSA strains, isolated in 1991 and in 1992 respectively, produced the enzyme. We analyzed the beta-lactamase gene (blaZ) in 30 strains each of MRSA and MSSA isolated from our hospital using the polymerase chain reaction. We found that 28 MSSA strains but only 14 MRSA strains possessed the gene. In addition, the present study indicates that many recently isolated MRSA strains, which lacked blaZ gene and were negative for beta-lactamase, consistently produced coagulase of type II (coagulase typing is a marker of epidemiology for Staphylococcus aureus). These beta-lactamase negative, type II coagulase producing and highly methicillin-resistant MRSA strains are considered to have an increased ability to successfully colonize individuals and an increased epidemic potential, probably because of the saving of the energy otherwise expended in beta-lactamase production and of the loss of beta-lactamase plasmid coding the mecA gene repressor, resulting in constitutive PBP2' production. These factors may contribute to the wide spread of these strains of MRSA in a nosocomial environment.