Methicillin-Resistant Staphylococcus aureus Strains in Swiss Pigs and Their Relation to Isolates from Farmers and Veterinarians.

MRSA have emerged over the last decades as a One Health problem with an increasing prevalence in various animal species. The most notable are pigs, as asymptomatic carriers, and horses, where there is often an association with infections. The current study looked at the course of the MRSA prevalence in Swiss livestock since 2009 with a special focus on pigs followed by screening of veterinarians and farmers. Livestock isolates were obtained from the Swiss monitoring program and then characterized by spa typing. Concentrating on the year 2017, we analyzed the prevalence of MRSA in Swiss veterinarians and farmers followed by whole genome sequencing of selected human and animal strains. The phylogeny was assessed by applying core genome MLST and SNP analyses, followed by screening for resistance genes and virulence factors. The prevalence of MRSA in Swiss pigs showed a dramatic increase from 2% in 2009 to 44% in 2017. Isolates almost consistently belonged to CC398, split between spa type t011 and t034. The higher prevalence was mainly due to an increase in t011. Spa type t034 strains from farmers were found to be closely associated with porcine t034 strains. The same could be shown for t011 strains from horse and veterinarians. Spa type t034 strains had a high number of additional resistance genes and two strains had acquired the immune evasion cluster. However, all but one of the pig t011 strains clustered in a separate group. Thus, the increase in pig t011 strains does not directly translate to humans