Phylogenetic analysis of Staphylococcus aureus CC398 reveals a sub-lineage epidemiologically associated with infections in horses

Abstract

In the early 2000s, a particular MRSA clonal complex (CC398) was found mainly in pigs and pig farmers in Europe. Since then, CC398 has been detected among a wide variety of animal species worldwide. We investigated the population structure of CC398 through mutation discovery at 97 genetic housekeeping loci, which are distributed along the CC398 chromosome within 195 CC398 isolates, collected from various countries and host species, including humans. Most of the isolates in this collection were received from collaborating microbiologists, who had preserved them over years. We discovered 96 bi-allelic polymorphisms, and phylogenetic analyses revealed that an epidemic sub-clone within CC398 (dubbed 'clade (C)') has spread within and between equine hospitals, where it causes nosocomial infections in horses and colonises the personnel. While clade (C) was strongly associated with S. aureus from horses in veterinary-care settings (p = 2 × 10(-7)), it remained extremely rare among S. aureus isolates from human infections.

Figure 1. Minimum spanning tree (MST) represent the clustering of 195 CC398 isolates based on genome-wide SNPs; The ancestral node was determined by comparing concatenated sequences from the investigated loci of all investigated CC398 isolates with the concatenated sequences of N315 as an out-group.

A) host origin of the 195 CC398 isolates, B) plotting of the geographical distribution on the MST, C) shows the different spa types, D) the SCCmec types and the susceptibility of the 195 CC398 isolates and E) the isolation date.