Genotypic and Phenotypic Characterization of MRSA Colonization and Persistence in Residential Aged Care Facilities in Adelaide, Australia.

BACKGROUND

Residential aged care facilities (RACFs) are potential reservoirs for methicillin-resistant Staphylococcus aureus (MRSA), with residents at increased risk due to compromised immunity, comorbidities, frequent hospital visits, and close living quarters.

OBJECTIVES

This study is aimed at assessing MRSA prevalence in RACFs and analyzing antimicrobial resistance mechanisms both phenotypically and genotypically.

METHODS

A total of 106 participants were recruited: 84 from three RACFs and 22 from retirement villages (as control samples). MRSA was isolated from nasal, wound, and urine samples using selective media. Identification was confirmed via MALDI-TOF MS. Antimicrobial susceptibility testing and MRSA screening were conducted using ISO broth microdilution and EUCAST disc diffusion. Whole-genome sequencing and bioinformatics were used to explore resistance mechanisms and clonal transmission.

RESULTS

Nasal carriage of MRSA was 8.2% (n = 7/85) among RACF residents; none were found in retirement village participants. All isolates formed strong biofilms, and 66.6% (10/15) showed multidrug resistance. Genotypic analysis revealed ST22 as the predominant clone. Genotypic resistance largely matched phenotypic profiles, except for unexplained tetracycline resistance. No isolates carried Panton-Valentine leukocidin (PVL), consistent with previously reported data on MRSA in long-term care facilities. SNP-based phylogenetic analysis indicated persistent colonization and circulation of multidrug-resistant clones within the facilities. Nasal carriage of MRSA was associated with UTIs and/or wound infections in some cases, whereas a UTI caused by the same clone in a resident within the same facility without nasal carriage indicates possible intrafacility clonal dissemination.

CONCLUSION

This study highlights persistent colonization with hospital-associated MRSA and potential clonal transmission among residents of aged care facilities despite several courses of antibiotics. The predominance of the epidemic ST22 clone and high rates of multidrug resistance underscore the urgent need for targeted infection control, routine genomic surveillance, and antimicrobial stewardship in long-term care settings to prevent MRSA dissemination.