Clonal distribution of livestock-associated methicillin-resistant Staphylococcus aureus in pig farms of Korea: Emergence of a novel variant staphylococcal chromosome cassette mec.

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) is a considerable concern in pig farming due to its antimicrobial resistance and zoonotic potential. However, data on the genetic characteristics of LA-MRSA in Korean pig farms remain limited. This study investigated the prevalence, antimicrobial resistance profiles, and genetic diversity of LA-MRSA in Korean pig farms using whole-genome sequencing (WGS). Overall, 1515 samples were collected from 74 pig farms nationwide, yielding 96 MRSA isolates from 17 farms. All isolates exhibited multidrug resistance, with CC398 (ST398, spa type t571, SCCmec type V) as the dominant lineage. Notably, 35 isolates carried an SCCmec type III + V combination, which has not been reported in Korea. WGS analysis of 28 representative isolates identified six clonal clusters and seven singletons, with potential inter-farm transmission observed in four clusters spanning different provinces. Complete genome analysis revealed a novel SCCmec variant, found in a distinct cluster of six isolates from two separate farms purchasing pigs from the same breeding company. These findings highlight the ongoing clonal expansion of CC398, emergence of novel SCCmec elements, and potential role of breeding networks in LA-MRSA dissemination. Enhanced surveillance, biosecurity measures, and antimicrobial stewardship are necessary to mitigate the spread of LA-MRSA within pig farms and its potential impact on public health.