New insights in mastitis pathogens through comprehensive "-omic" approaches: the example of Staphylococcus aureus

S. aureus is one of the main pathogens involved in ruminant mastitis worldwide. The severity of staphylococcal infection is highly variable, ranging from subclinical to gangrenous mastitis. This talk will present an in-depth characterization of S. aureus mastitis isolates to identify bacterial factors putatively involved in severity of mastitis infection and to reveal staphylococcal proteins that are produced during the infection of the mammary gland. We employed genomic, transcriptomic and proteomic approaches to comprehensively compare two clonally related S. aureus strains that were responsible for severe (strain O11) and milder (strain O46) mastitis in ewes, respectively. Variation in the content of mobile genetic elements, iron acquisition and metabolism, transcriptional regulation and exoprotein production was observed. In particular, O11 produced relatively high levels of exoproteins, including toxins and proteases known to be important in virulence. A characteristic we observed in other S. aureus strains isolated from clinical mastitis cases. In parallel, experimental mastitis were induced on primiparous ewes using O11 and O46. Strains induced mastitis with drastically distinct clinical symptoms when tested in ewe and mice experimental infections. Notably, they reproduced mild (O46) or severe (O11) mastitis in ewes. Ewe sera were sampled and used to identify staphylococcal immunoreactive proteins commonly or differentially produced during infections of variable severity and to define core and accessory seroproteomes. Such SERological Proteome Analysis (SERPA) allowed the identification of 89 immunoreactive proteins, of which only 52 (58.4%) had previously been identified as immunogenic proteins in other staphylococcal infections. Our data are consistent with a dose-dependant role of some staphylococcal factors in the hypervirulence of strains isolated from severe mastitis. Mobile genetic elements, transcriptional regulators, exoproteins and iron acquisition pathways constitute good targets for further research to define the underlying mechanisms of mastitis severity. These findings open avenues in S. aureus mastitis studies as some of these proteins, expressed in vivo, are likely to account in part for the success of S. aureus as a pathogen of the ruminant mammary gland.