Evolutionary Dynamics of Pandemic Methicillin-Sensitive Staphylococcus aureus ST398 and Its International Spread via Routes of Human Migration

Methicillin-susceptible Staphylococcus aureus (MSSA) accounts for the majority of S. aureus infections globally, and yet surprisingly little is known about its clonal evolution. We applied comparative whole-genome sequencing (WGS) analyses to epidemiologically and geographically diverse ST398-MSSA, a pandemic lineage affecting both humans and livestock. Bayesian phylogenetic analysis predicted divergence of human-associated ST398-MSSA ~40 years ago. Isolates from Midwestern pigs and veterinarians differed substantially from those in New York City (NYC). Pig ST398 strains contained a large region of recombination representing imports from multiple sequence types (STs). Phylogeographic analyses supported the spread of ST398-MSSA along local cultural and migratory links between parts of the Caribbean, North America, and France, respectively. Applying pairwise single-nucleotide polymorphism (SNP) distances as a measure of genetic relatedness between isolates, we observed that ST398 not only clustered in households but also frequently extended across local social networks. Isolates collected from environmental surfaces reflected the full diversity of colonizing individuals, highlighting their potentially critical role as reservoirs for transmission and diversification. Strikingly, we observed high within-host SNP variability compared to our previous studies on the dominant methicillin-resistant Staphylococcus aureus (MRSA) clone USA300. Our data indicate that the dynamics of colonization, persistence, and transmission differ substantially between USA300-MRSA and ST398-MSSA. Taken together, our study reveals local and international routes of transmission for a major MSSA clone, indicating key impacts of recombination and mutation on genetic diversification and highlighting important ecological differences from epidemic USA300. Our study demonstrates extensive local and international routes of transmission for a major MSSA clone despite the lack of substantial antibiotic resistance. IMPORTANCE Unlike methicillin-resistant Staphylococcus aureus (MRSA), surprisingly little is known about the clonal evolution of methicillin-susceptible S. aureus (MSSA), although these strains account for the majority of S. aureus infections. To better understand how MSSA spreads and becomes established in communities, we applied comparative bacterial whole-genome sequencing to pandemic ST398-MSSA, a clone of clinical importance affecting humans and livestock in different geographic regions. Phylogeographic analyses identified that ST398-MSSA spread along local cultural and migratory links between parts of the Caribbean, North America, and France, respectively. We observed high within-host SNP variability compared to our previous studies on the dominant MRSA clone USA300. Our data indicate that the dynamics of colonization, persistence, and transmission differ substantially between USA300 MRSA and ST398 MSSA.