Molecular characterization of methicillin-susceptible/resistant Staphylococcus aureus (MSSA/MRSA) from bloodstream infections in northern Japan : the dominance of CC1-MRSA-IV, the emergence of human-associated ST398 and livestock-associated CC20 and CC97 MSSA

Objectives: Staphylococcus aureus is a major cause of bloodstream infections. The recent epidemiological features and antimicrobial resistance trend were analyzed for methicillin-resistant and susceptible S. aureus (MRSA/MSSA) isolates from blood samples in northern Japan.

Methods: S. aureus isolates from blood culture were screened by MALDI-TOF and genotyped by the schemes of MLST, coa, agr, spa, and SCCmec types. Antimicrobial resistance genes and virulence factors were detected by multiplex/uniplex PCR. Antimicrobial susceptibility was examined using a broth microdilution test.

Results: A total of 301 isolates (163 MRSA and 138 MSSA) were isolated from bloodstream infections in 2023 (From Apr. to Dec.). The MRSA isolates were classified into three groups, i.e., CC1-SCCmec-IV (CC1-IV) (52%), CC5-II (36%), and CC8-IV (12%). The prevalence of CC1 was significantly higher than those in our previous studies (2017-2021). Four CC8-IVa isolates with PVL genes on ΦSa2usa were considered to be the USA300 clone (ST8/spa-t008/coa IIIa/agr I) or its variants that were genotyped as those closely related to ST8/t008 or lacking ACME. In contrast, MSSA was genetically highly divergent and classified into 22 STs, with CC1 (ST1, ST188) being the most common (25%). It was notable that 29 MSSA isolates (21%) were classified into livestock-associated (LA) genotypes, ST20, ST97, and CC398 (ST398, ST291). Genetic characterization of the CC398 isolates suggested that these belong to human-adapted MSSA clones.

Conclusions: The present study revealed the increasing trend of CC1 MRSA surpassing CC5, and the emergence of MSSA representing human-adapted CC398, and LA types ST97 and ST20 from bloodstream infections in Japan.