A total of 120 raw meat product samples (pork, n = 90; beef, n = 30) were randomly collected from 30 retail grocery stores of seven supermarket chains in Baton Rouge, LA. Sampling visits were made weekly for 6 weeks (February to March 2008). On each sampling day, five stores were chosen based on geographical proximity to each other. Three prepackaged pork chops and one beef steak in the refrigerated fresh meat section were collected from each store. After the exterior surface of the package was cleaned with paper towels moistened with 70% alcohol, the meats were mixed with equal volumes of buffered peptone water (BD Diagnostic Systems, Sparks, MD) and aseptically massaged for 5 min. A 50-ml aliquot of the rinse was enriched in an equal volume of double-strength enrichment broth (Trypticase soy broth supplemented with 10% NaCl and 1% sodium pyruvate). After 24 h of incubation at 35°C, the enrichment broth was streaked in duplicate on Baird-Parker (BP) medium and spread-plated on BP with cefoxitin (4 μg/ml [Sigma, St. Louis, MO]). Following 48 h of incubation, three to six presumptive S. aureus and MRSA colonies per meat sample (black colonies surrounded by 2- to 5-mm clear zones) were transferred to Trypticase soy agar plates followed by confirmation by a tube coagulase test (Remel, Lenexa, KS).
Confirmation of S. aureus
and MRSA was conducted using a multiplex PCR for the species-specific 442-bp fragment with unknown coding potential (13
) and mecA
encoding a penicillin-binding protein with a low affinity for β-lactams (3
). MRSA isolates were then characterized by antimicrobial susceptibility testing using broth microdilution in accordance with Clinical and Laboratory Standards Institute guidelines (1
), PFGE with SmaI digestion (14
), detection of the PVL gene by PCR (12
), and characterization of the SCCmec
). Multilocus sequence typing (MLST) (5
) and single-locus DNA sequencing of the repeat region of the Staphylococcus
protein A gene (spa
) were performed for three representative MRSA isolates from each MRSA-positive store.