Improved ability to utilize lactose and grow in milk as a potential explanation for emergence of the novel bovine Staphylococcus aureus ST5477

Staphyloccous aureus belonging to sequence type 5477 have recently been identified as a predominant clone causing bovine mastitis in Rwanda and Tanzania. We compared nine S. aureus ST5477 to 17 isolates belonging to other sequence types by their biochemical profile and ability to acidify milk and grow in minimum media containing lactose. We found that ST5477 isolates all were positive in ONPG (o-nitrophenyl-β-D-galactopyranoside) test and negative for mannitol fermentation potentially challenging the correct identification of this sequence type as S. aureus. In addition, ST5477 isolates were all much faster in acidifying milk and grew faster in minimal media with lactose compared to other strains suggesting an increased lactose utilization and thereby adaptation to the bovine udder environment as a possible reason for the recent successful emergence. Comparison of the lac gene region of the genome of a recently sequenced ST5477 and that of S. aureus reference genome showed that both strains contained the known lacABCD genes involved in the lactose degradation, but that ST5477 had a 12 amino-acid deletion and two amino-acid differences in the lac gene transcription regulator, suggesting that increased transcription might play a role. In conclusion, these preliminary data suggests that improved lactose utilization and the ability to grow faster in milk may have been a key feature for the recent success of ST5477 as a bovine adapted clone.