Epidemiology of Methicillin Resistant Staphylococcus aureus in a Japanese NICU

BACKGROUND: There have been few reports on the current population genetic structure of methicillin-resistant Staphylococcus aureus (MRSA) from neonatal intensive care units (NICUs) in Japan. In the present study, we conducted a molecular epidemiological analysis based on WGS against MRSA strains in a Japanese NICU. METHODS: Against fifty-seven MRSA strains from fecal or nasal specimens from NICU patients in Juntendo University Shizuoka Hospital in the period of 2013-2014, we performed genotypings by whole genome sequencing (WGS), PCR-based typing of Staphylococcal Cassette Chromosome mec (SCCmec) and PCR-based open reading frame typing (POT). RESULTS: Forty-nine MRSA strains (86.0%) exhibited a clonal complex (CC) 1, and were divided into three sequence types (STs); ST2725 (n=25), ST2764 (n=21), and ST1 (n=3). All CC1 MRSA strains had SCCmec IVa, and were resistant to new quinolones, which are limited in pediatric use, suggesting that these strains were derived from adult MRSA clones. SNP differences of both 100 nucleotides were observed by pairwise SNP analysis among ST2725 and ST2764 MRSA strains, respectively. Seven strains ST8 MRSA strains (12.2%) were isolated, and no strain exhibiting the Japanese hospital associated-MRSA genotype (ST5/SCCmec II) was isolated in this study. CONCLUSIONS: Our molecular epidemiological analysis suggested that ST2725 and ST2764 MRSA strains had genetic diversity that couldn't be explained by only a recent transmission event in the NICU. These MRSA clones might be disseminated in other Japanese hospital facilities as a new endemic clone. Our results are expected to contribute to the improvement of infection control measures of MRSA in NICUs.