Genomic analysis of a penicillin-susceptible, methicillin-resistant Staphylococcus aureus (ST9332) harboring triple mecA substitutions

RESEARCH BACKGROUND: mecA-positive MRSA is generally assumed to be broadly non-susceptible to β-lactams. Reports of penicillin-susceptible MRSA (PS-MRSA) challenge this view. We describe CDSYY2023, a cefoxitin-resistant yet penicillin-susceptible MRSA carrying three mecA substitutions (S225R, K239E, E246G) and a novel sequence type, ST9332. METHODS: The strain was recovered from the sputum of a hospitalised older adult with dementia and identified as S. aureus by MALDI-TOF MS (Autobio MS1000). Susceptibility testing followed CLSI procedures on VITEK 2 with parallel Mueller-Hinton edge tests (cefoxitin, penicillin). Key readouts were verified by Etest and independently confirmed at a referral laboratory. Whole-genome sequencing (Illumina HiSeq 2500) was assembled de novo with SOAPdenovo2. Functional annotation used COG/GO/KEGG; resistance and virulence genes were queried against CARD and VFDB. MLST was assigned and registered via PUBMLST. RESULTS: The draft genome is ∼2 750 950 bp (GC 32.94%), with no plasmids, encoding 2521 CDSs, 55 tRNAs and 19 rRNAs. MLST designated ST9332. Phenotypically, VITEK 2 and MH edge tests agreed on cefoxitin resistance (17 mm), while penicillin remained susceptible (25 mm; Etest MIC 0.032 µg/mL), contrary to expectations for a mecA-positive background. Genomics revealed the triple mecA substitutions noted above and no definitive evidence of blaZ. The resistome/virulome included multiple efflux and regulatory elements. CONCLUSIONS: CDSYY2023 is a PS-MRSA with triple-substituted mecA on a novel MLST background (ST9332). Its penicillin-susceptible/cefoxitin-resistant profile likely reflects a specific mecA sequence/regulatory context, highlighting the limits of relying solely on cefoxitin-based screening to infer β-lactam behaviour.