Comparative genomics of bovine mastitis-origin Staphylococcus aureus strains classified into prevalent human genotypes.

Humans may serve as a reservoir host of Staphylococcus aureus, resulting in transmission to animals. Previously, we used RNA polymerase beta subunit gene (rpoB)-based genotyping and classified S. aureus strains into rpoB sequence types (RSTs). According to our previous work, the predominant genotypes of S. aureus in humans and cows differ in Korea, but some predominant genotypes (RST4-1 and RST2-1) in humans have been isolated from bovine mastitis. Therefore, it needs to be determined whether some strains of the predominant human genotypes have adapted to or caused occasional infections in cows. We determined the whole genome sequences of 2 bovine mastitis-origin strains, PMB179 (RST4-1) and PMB196 (RST2-1), and performed comparative genomics with the corresponding RST4-1 and RST2-1 S. aureus strains in the NCBI database. We identified 257 and 180 pseudogenes among 131 RST4-1 and 54 RST2-1 strains, respectively, for the comparison of pseudogene profiles. RST4-1 strains shared more common pseudogenes than RST2-1 strains, and some epidemiologically related strains shared common pseudogenes. However, most of the pseudogenes were strain-specific, and diverse pseudogene profiles were apparent in both the RST4-1 and RST2-1 strains. Furthermore, analysis of the mobile genetic elements, virulence genes, and antibiotic resistance genes revealed no molecular markers to differentiate PMB179 and PMB196 from human strains. Interestingly, the collective comparison of RST4-1 or RST2-1 strains revealed cumulative acquisition steps of genomic islands and antibiotic resistance genes. In conclusion, our data support PMB179 and PMB196 causing occasional infections that result in bovine mastitis.