Staphylococcus aureus is not only a frequent nasal coloniser of all mammals and birds, but also is a common cause of a wide range of infections in both hospitals and the community. In humans, S. aureus is associated with a wide spectrum of diseases from skin infections to life-threatening endocarditis. Infections caused by methicillin-resistant S. aureus (MRSA) are of particular concern due to limited treatment opportunities. Since the early 2000s, a particular MRSA clonal complex (CC398) was

more »

... dely disseminated as a coloniser and pathogen in economically important live-stock and companion animals worldwide. Since then, CC398 is of increasing concern to pose a risk to public health. In this study, we investigated the population structure of CC398 through mutation discovery at 97 genetic housekeeping loci, which are distributed along the S. aureus chromosome within 195 CC398 isolates, collected from 11 different host species and various countries. Furthermore, we investigated the phenotypic characters of a certain sub-clone within CC398 by studying its ability to adhere to fibronectin and to invade neutrophils from various host species. To gain a better insight into the genetic determinants of this CC398 sub-clone, we have applied whole genome sequencing approach on eight representative CC398 isolates. This study provided a novel insight into the phylogeny of CC398 concerning the spread of a specific MRSA-CC398 sub-clone within equine settings, which causes infections in horses and nasal colonisation of humans that are in close contact with these horses. Of note, it remained extremely rare among S. aureus isolates from hu-man infections. Furthermore, this MRSA-CC398 sub-clone can initiate colonisation in both human and horse efficiently; however, it was not generally protected from the host immune system response. Lastly, the comparative genomic analysis of CC398 revealed a novel pathogenicity island and two prophages that were harboured by certain CC398 isolates.