Narachasima L.Phokhaphan P.Phornsiricharoenphant W.Ruangchai W.Palittapongarnpim P.Kondo S.Mahidol University2026-06-062026-06-062026-05-01Journal of the Medical Association of Thailand Vol.109 No.5 (2026) , 449-45701252208https://repository.li.mahidol.ac.th/handle/123456789/117107Background: Methicillin-resistant Staphylococcus aureus (MRSA) remains a major cause of healthcare-associated infections with substantial morbidity and mortality. Optimizing treatment and infection-control strategies is crucial to minimizing its spread in healthcare settings. Objective: To characterize MRSA isolates from two tertiary hospitals in Thailand, including Thammasat University Hospital (TU) and Songklanagarind Hospital (PSU) at the molecular level, and to identify clinical and microbiological factors associated with treatment failure. Materials and Methods: Whole genome sequencing (WGS) was performed using the Illumina MiSeq. SCCmecFinder, PubMLST, and spaTyper were used to determine SCCmec, MLST, and spa types, respectively. Phylogenetic trees were constructed with RaxML (54 samples). Vancomycin MICs were determined by the broth microdilution (BMD) method, and susceptibility data for other antibiotics (by disk diffusion method) were obtained from hospital medical records. Clinical and microbiological data were analyzed using STATA, version 17. The isolates and medical records were provided from both hospitals with official permission. Results: The predominant genotypes were SCCmec type IV, ST239, and t037, with CC8 being the most common clonal complex in both hospitals. Most isolates were vancomycin-susceptible S. aureus (VSSA). However, a few heteroresistant vancomycin-intermediate S. aureus (hVISA) strains were detected. Isolates were highly susceptible to fusidic acid and linezolid but resistant to β-lactams, macrolides, and clindamycin. Pneumonia and acute kidney injury were independently associated with treatment failure. Conclusion: The findings revealed multiple genetic lineages and identified clinical and microbiological characteristics of MRSA-infected patients associated with treatment outcomes. These results highlight the need for prompt and appropriate therapy, continuous susceptibility surveillance, and rigorous infection-control measures to prevent the spread of MRSA. In addition, the phylogenetic tree provided valuable insights into the evolutionary relationships among the isolates, facilitating the identification of clonal lineages and transmission patterns, which are essential for infection control and epidemiological surveillance.MedicineArticleSCOPUS10.35755/jmedassocthai.2026.5.039072-s2.0-10504037575024081981