Extreme Genetic Diversity of Methicillin-Resistant Staphylococcus epidermidis Strains Disseminated among Healthy Japanese Children

For the past few years, we have been observing the dissemination of methicillin-resistant staphylococci in the community. From 2001 to 2003, an evaluation of nasal samples from 1,285 children in five day-care centers and two kindergartens in three districts in Japan revealed that methicillin-resistant coagulase-negative staphylococci (MRC-NS) have been widely disseminated in the Japanese community. Their prevalence is much greater than community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). Forty-nine children (3.81%) were colonized with MRSA, whereas 390 children (30.35%) were colonized with MRC-NS. These MRC-NS strains predominantly harbored a pair of cassette chromosome recombinase types A2 and B2 (ccrAB2). Of these, 40.8% harbored type IVa staphylococcal cassette chromosome mec (SCCmec) elements, a distinct/characteristic type of SCCmec in pandemic clones of CA-MRSA. Interestingly, there was also a high frequency of nontypeable strains which possessed atypical structures compared to previous SCCmec types. Among the MRC-NS, the majority of strains (63.59%) were methicillin-resistant Staphylococcus epidermidis (MRSE). Their genotypes, as judged from pulsed-field gel electrophoresis (PFGE), were highly diverse. They were so diverse that there was no sign of an immediate transmission of any MRSE clone among children in the same institutions. In a previous report, we expounded that a few CA-MRSA clones with distinct SCCmec types were disseminated among children in the same institutions. Au contraire, with the case of CA-MRSE, there was no single genotype of CA-MRSE disseminated among children even in the same institution or class.