Abstract
Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) of clonal complex (CC) 398 has become a rising issue for public health. While it is known that >80% of pig farmers are colonized with LA-MRSA, only a few studies have assessed the situation for humans with occasional livestock contact. Recently it was shown that over 75% of scientific fieldworkers visiting pigsties were temporarily carrying LA-MRSA. To find out whether they were transiently or permanently colonized, we used whole-genome sequencing (WGS) data to analyze the relatedness of isolates from these recurrently LA-MRSA-positive fieldworkers and from corresponding pigsties. Sequences were analyzed using in silico typing (spa and core genomic multilocus sequence typing [cgMLST]), and the BEAST software package was used to examine phylogeny. In total, 81 samples from three fieldworkers on eight different pigsties over a period of 2.5 years were sequenced. All isolates belonged to spa type t011, t034, or t2011, with different types found in the same fieldworker at different time points. Analysis of cgMLST revealed nine genotypic clusters, mostly correlating with the pigsty on which they were sampled. Fieldworker isolates clustered with the samples from farms that were visited on the same day. BEAST analysis corroborated the cgMLST-based clustering and suggests an origin of the lineage about 22 years ago. We conclude that nasal LA-MRSA colonization among humans with occasional livestock contact is common but most likely only temporary. Furthermore, we showed that the Western German LA-MRSA CC398 originated in the late 1990s and diversified into farm-specific genotypes, which stay relatively consistent over time.