Characterization of Staphylococcus aureus CC1 and CC1660 of Human and Equine Origin

<b>Background/Objectives</b>: <i>Staphylococcus aureus</i> isolates from humans and horses of the equine-associated clonal complexes (CCs) CC1 and CC1660 were comparatively investigated for their genomic relationships. <b>Methods</b>: A total of 91 <i>S. aureus</i> isolates (64 human, 27 equine) were subjected to whole-genome sequencing (WGS), sequence analysis, and antimicrobial susceptibility testing. <b>Results</b>: WGS confirmed 75 CC1 and 16 CC1660 isolates, comprising nine sequence types (STs) in CC1 and four STs in CC1660. Ten <i>spa</i> types were present in CC1 and five in CC1660. In the <i>arcC</i> gene of three CC1 isolates, a 285 bp deletion was detected, and a nucleotide deletion causing a premature stop codon was found in one CC1660 isolate. Core genome (cg) MLST revealed a minimum difference of 1398/1492 alleles between the two CCs. All CC1 isolates harbored <i>agr</i> group III and capsule type 8 alleles, whereas all CC1660 isolates had <i>agr</i> group II and capsule type 5 alleles. Antimicrobial susceptibility testing revealed 18 phenotypic and 19 genotypic resistance patterns. All isolates were susceptible to vancomycin, linezolid and quinupristin-dalfopristin. Several virulence genes were detected in different combinations. The equine leukocidin genes <i>lukP/lukQ</i> were found in 22 isolates from horses and 38 isolates from humans, of which 35 had confirmed contact with horses. No Panton-Valentine leukocidin genes were found. Three human CC1660 isolates carried the toxic shock syndrome toxin-1 gene <i>tst-1</i>. <b>Conclusions</b>: The analysis of the 91 isolates might suggest intra- and interspecies transmission among and between humans and horses, which should be monitored in the future.