Introduction: Methicillin-resistant Staphylococcus aureus (MRSA) is an important clinical problem. In 2005, a livestock-associated MRSA clone was described, named CC398, being mostly associated with pigs, and causing colonization and infection in pigs and in related humans. The prevalence of these strains in food-producing pigs raised concerns about the possibility of MRSA-CC398 being a foodborne pathogen. The objective of this study was to investigate the presence of S. aureus and MRSA in 141 carcasses of pigs at three slaughterhouses of Portugal, discarded from the food chain by signs of infection, and to characterize the recovered isolates. Methods: S. aureus isolates were identified by matrix-assisted laser desorption/ionization time-of-flight and they were typed (spa, CC398-clone, and SCCmec). The study of antibiotic resistance and virulence genes, and the detection of immune evasion cluster genes and prophages were performed by PCR and sequencing. Results: Twenty-eight S. aureus were obtained from 141 samples (one/sample, 19.9%), being 22 MRSA and 6 methicillin-susceptible S. aureus (MSSA). All MRSA strains were typed as CC398 and were ascribed to three spa types (t011, t108, and t1451). The SCCmec detected differed according to the spa types of MRSA isolates (SCCmecV: t011 and t108; SCCmecIVa: t1451). The MSSA strains were classified as spa-t1491-ST1-CC1. All the strains contained a wide range of antimicrobial resistance genes, the resistance to tetracycline being the prevalent one. In contrast, the strains contained only a few virulence genes. Among the 6 integrases of phages tested, three were detected: SΦ1, SΦ2, and SΦ7, with variations between MRSA and MSSA strains. Conclusions: MRSA-CC398 is not only a habitual pig colonizer but also an opportunistic pathogen in these animals, and must be controlled at the level of producers and slaughterhouses because of its impact on public health.

Vânia Santos

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