Molecular characteristics and pathogenicity-associated phenotypes of methicillin-resistant Staphylococcus aureus sequence type 398 clinical isolates from a women and children's hospital in Southwest China.

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) sequence type 398 (ST398), initially recognized as a livestock-associated lineage, has been increasingly reported in human infections. However, the molecular characteristics, antimicrobial resistance profiles, phylogenetic context, and pathogenicity-associated phenotypes of clinical MRSA ST398 isolates in China remain insufficiently defined. Here, we characterized such isolates from Southwest China using genomic, antimicrobial susceptibility, and phenotypic analyses.

METHODS

From January 2020 to June 2023, 23 nonduplicate MRSA ST398 isolates were identified among 305 nonduplicate clinical isolates from West China Second University Hospital. Whole-genome sequencing was performed for molecular typing, antimicrobial resistance determinant analysis, phylogenetic analysis, and virulence-associated gene screening. Growth, biofilm formation, cell adhesion, hemolysis, mouse skin abscess, and Galleria mellonella survival assays of selected clinical isolates within each lineage were performed to evaluate pathogenicity-associated phenotypes.

RESULTS

ST398-V-t011 was the dominant lineage in 15/23 of the isolates (65.2%). SCCmec type V and spa type t011 were detected in 22/23 (95.7%) and 16/23 (69.6%) of the isolates, respectively. All isolates were mecA-positive and were phenotypically resistant to penicillin G and oxacillin, but remained susceptible to vancomycin and linezolid. Erythromycin resistance and clindamycin resistance, mainly associated with ermC, were each detected in 15/23 of the isolates (65.2%). Phylogenetic analysis showed that 22/23 local isolates clustered within a China-associated clade, and LSD2-based dating estimated that the internal node of this clade was approximately 2008, providing temporal context for the observed clustering pattern. Virulence-associated gene screening revealed the conservation of many genes across isolates, whereas several accessory genes varied. Lineage-based phenotypic assays using selected isolates revealed heterogeneous pathogenicity-associated readouts: ST398-V-t2383 and ST398-V-t1580 displayed relatively higher biofilm formation and/or cell adhesion capacities, whereas ST398-V-t011 showed higher hemolytic activity and higher pathogenicity-associated readouts in mouse skin abscess and G. mellonella models.

CONCLUSION

The obtained clinical MRSA ST398 isolates predominantly belonged to the ST398-V-t011 lineage and had a mecA-positive beta-lactam-resistant phenotype with variable non-beta-lactam resistance. Although a China-associated genomic cluster was identified, this finding should not be interpreted as direct evidence of the transmission direction or a definite transmission chain. The observed phenotypic heterogeneity provides exploratory pathogenicity-associated data and supports continued genomic surveillance and multicenter validation of clinical MRSA ST398 in China.