Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has emerged as a major public health concern worldwide (8). In the United States, among 8,987 cases of invasive MRSA reported in nine communities in 2005, 1,234 (13.7%) were due to CA-MRSA, and the remainder involved health care-associated (HA [85%]) and unclassified (1.3%) infections (10). Among the eight pulsed-field gel electrophoresis (PFGE) types (USA100 to USA800) originally identified in the United States, USA300 and USA400 have been associated with CA-MRSA infections and the remaining types have been associated with HA-MRSA (14). In particular, USA300 is the most common CA-MRSA clone in the United States (8, 15), and common HA-MRSA clones include USA100, USA800, and USA700 (16). The widely recognized association of CA-MRSA with type IV staphylococcal cassette chromosome mec (SCCmec) and the genes encoding Panton-Valentine leucocidin (PVL) has greatly facilitated strain differentiation between CA-MRSA and HA-MRSA (4).

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).

Table ​Table11 lists the prevalence of S. aureus and MRSA strains in the 120 meat samples examined. We recovered 121 S. aureus isolates from 47 meat samples. A total of 99 isolates (derived from 43 samples) were methicillin-susceptible S. aureus, and 22 isolates (from 6 samples) were MRSA. The overall prevalences of S. aureus were 45.6% in pork and 20% in beef, whereas MRSA was found in five (5.6%) pork samples and one (3.3%) beef sample. Two pork samples containing MRSA also yielded methicillin-susceptible S. aureus strains that lacked mecA. The use of BP medium supplemented with cefoxitin facilitated MRSA isolation; however, this medium missed the recovery of MRSA from pork in three instances (Table ​(Table1).1). This demonstrates the importance of including an antibiotic-free medium even when screening for MRSA only.

Major genotypic and phenotypic characteristics of representative MRSA isolates are summarized in Table ​Table2.2. Among 22 confirmed MRSA isolates, 3 isolates recovered from two pork samples in stores 4 and 5 were positive for PVL, a two-component staphylococcal membrane toxin that targets leukocytes (Table ​(Table2).2). Primarily associated with CA-MRSA but not HA-MRSA, PVL is considered the principal virulence factor responsible for the spread of CA-MRSA in skin and soft-tissue infections (4). SCCmec typing identified two structure types among the 22 MRSA isolates, type IVa (a type IV subtype) for isolates from stores 4 and 5, and type II for all 19 isolates from store 11 (Table ​(Table2).2). SCCmec type IV is predominately associated with CA-MRSA. It is characterized by its smaller size (20 to 24 kb) and carriage of a limited number of antimicrobial resistance genes (22). Type II is larger (52 kb) and contains additional resistance genes (22). Susceptibility testing revealed that isolates belonging to the two SCCmec types differed in the numbers of antimicrobials to which they were resistant (Table ​(Table22).

To our knowledge, this is the first survey in the United States examining the prevalence and characteristics of MRSA in retail meats. Six samples (5%) contained MRSA strains, which were determined to be of two unique human epidemic clones, USA100 and USA300. Additionally, nearly 40% of the meats examined contained S. aureus, which was comparable to the prevalence rate recently reported in The Netherlands (19).