Zinc Resistance within Swine-Associated Methicillin-Resistant Staphylococcus aureus Isolates in the United States Is Associated with Multilocus Sequence Type Lineage.

Zinc resistance in livestock-associated methicillin-resistant (LA-MRSA) sequence type 398 (ST398) is primarily mediated by the gene colocated with the gene, encoding methicillin resistance, within the type V staphylococcal cassette chromosome (SCC) element. Because and are located within the same mobile genetic element, it has been suggested that the use of zinc in feed as an antidiarrheal agent has the potential to contribute to the emergence and spread of methicillin-resistant (MRSA) in swine, through increased selection pressure to maintain the SCC element in isolates obtained from pigs. In this study, we report the prevalence of the gene and phenotypic zinc resistance in U.S. swine-associated LA-MRSA ST5 isolates, MRSA ST5 isolates from humans with no swine contact, and U.S. swine-associated LA-MRSA ST398 isolates. We demonstrated that the prevalence of zinc resistance in U.S. swine-associated LA-MRSA ST5 isolates was significantly lower than the prevalence of zinc resistance in MRSA ST5 isolates from humans with no swine contact and swine-associated LA-MRSA ST398 isolates, as well as prevalences from previous reports describing zinc resistance in other LA-MRSA ST398 isolates. Collectively, our data suggest that selection pressure associated with zinc supplementation in feed is unlikely to have played a significant role in the emergence of LA-MRSA ST5 in the U.S. swine population. Additionally, our data indicate that zinc resistance is associated with the multilocus sequence type lineage, suggesting a potential link between the genetic lineage and the carriage of resistance determinants. Our data suggest that coselection thought to be associated with the use of zinc in feed as an antimicrobial agent is not playing a role in the emergence of livestock-associated methicillin-resistant (LA-MRSA) ST5 in the U.S. swine population. Additionally, our data indicate that zinc resistance is more associated with the multilocus sequence type lineage, suggesting a potential link between the genetic lineage and the carriage of resistance markers. This information is important for public health professionals, veterinarians, producers, and consumers.