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

Methods: The S. aureus isolates from blood culture were screened by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) and genotyped by the schemes of multilocus sequence typing (MLST), coa, agr, spa, and SCCmec types. Antimicrobial resistance genes and virulence factors were detected by multiplex/uniplex polymerase chain reaction (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 April to December). The MRSA isolates were classified into three groups, that is, clonal complexes (CC)1-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 (sequence type [ST]8/spa-t008/coa IIIa/agr I) or its variants that were genotyped as those closely related to ST8/t008 or lacking arginine catabolic mobile element (ACME). In contrast, MSSA was genetically highly divergent and classified into 22 STs, with CC1 (ST1 and 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 and 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 people in Japan. © 2024 The Author(s). Published by Elsevier Ltd on behalf of International Society for Antimicrobial Chemotherapy.