USA300 (sequence type 8) has become a major clone of methicillin-resistant Staphylococcus aureus in northern Taiwan

Background: Most methicillin-resistant Staphylococcus aureus (MRSA) isolates in Taiwan are resistant to clindamycin. MRSA USA300 (sequence type 8), usually susceptible to clindamycin and prevalent in North America, was rarely identified in Taiwan until October 2015. The increasing susceptibility of clinical MRSA isolates to clindamycin prompted the authors to investigate the changing molecular epidemiology of MRSA. Methods: Reviewing the antibiograms of clinical MRSA isolates in the study hospital from 2008 to 2018 showed a significant increase in clindamycin susceptibility (CLI-S) from 4.9% (293/5979) in 2008 to 33.2% (1058/3184) in 2018 (P_trend<0.001). MRSA isolates identified from bloodstream and paediatric samples, preserved systematically in 2008, 2013, 2017 and 2018, were selected for characterization. Results: Molecular analysis revealed that of the 496 CLI-S MRSA isolates, 0%, 6.5%, 44.4% and 48.8% were USA300 [characterized as SCCmec type IV/ Panton–Valentine leukocidin (PVL)-gene-positive/arginine catabolic metabolic element (ACME)-positive] in 2008, 2013, 2017 and 2018, respectively, in paediatric isolates (P_trend<0.001); and 0%, 7.1%, 47.1% and 50.6% were USA300 (characterized as SCCmec type IV/PVL-gene-positive/ACME-positive) in 2008, 2013, 2017 and 2018, respectively, in bloodstream isolates (P_trend<0.001). Whole-genome sequencing revealed that 80% of 74 domestic USA300 isolates were clustered into three clades, and all but one isolate belonged to a global clone that diverged in 1994, which further diverged in Taiwan between 2003 and 2006. Conclusions: CLI-S of MRSA increased significantly from 2008 to 2018 in northern Taiwan, mainly due to the increase in USA300. Most USA300 strains circulating in Taiwan have been imported on multiple occasions.