Changes in epidemic methicillin-resistant Staphylococcus aureus strains in a Japanese hospital in Tokyo, Japan

In recent years, methicillin-resistant Staphylococcus aureus (MRSA) isolates from Japanese hospitals have predominantly comprised community-acquired MRSA (CA-MRSA) carrying staphylococcal cassette chromosome mec (SCCmec) type IV in most regions. Certain CA-MRSA strains produce toxins, notably, Panton-Valentine leukocidin (PVL), a leukocyte-lytic toxin, and may cause infections in otherwise healthy individuals. This study aimed to clarify the current epidemiologic status of MRSA in a Japanese hospital.

In total, 1479 MRSA isolates collected from a Japanese university hospital between 2016 and 2023 were analyzed. SCCmec types and various genes were detected using polymerase chain reaction. Molecular epidemiologic analyses were performed using multilocus sequence typing and pulsed-field gel electrophoresis. Antimicrobial susceptibility was tested using the agar dilution method.

The proportion of SCCmec type IV MRSA cases increased from 45.9% to 86.4% over 8 years. The PVL-positive MRSA isolation rate ranged from 6.4% to 15.9%. PVL-negative strains predominated throughout the study period. Among the PVL-negative SCCmec type IV strains, the USA400/J clone belonging to sequence type 1 accounted for 40.6% of the isolates, whereas the sequence type 8 CA-MRSA/J clone carrying SCCmec type IVl accounted for 15.1%. The USA400/J clone exhibited high resistance to levofloxacin but remained susceptible to clindamycin and minocycline, displaying a characteristic antimicrobial susceptibility pattern.

The USA400/J clone of CA-MRSA was predominant in the studied hospital. Strains with characteristics consistent with the USA400/J clone have also been reported in other regions of Japan, suggesting that this clone may be widely disseminated in Japan.