Antimicrobial treatment of mastitis has led to the selection of resistant staphylococci, among which the Methicillin Resistant S. aureus (MRSA) are the most frequently studied ones. Still, MR has also been described for non-S. aureus (NSA) species. Bovine veterinary MR(N)SA represent not only a problem in the treatment of mastitis, but also a potential hazard in public health via the inter-Staphylococcus transferability of the mobile genetic « Staphylococcal Cassette Chromosome » (SCC) that carry the mec gene(s) encoding MR and the zoonotic potential of some Staphylococcus species.

The overall aim of this proposal is to study the potential of collection and wild-type phages isolated from farm slurry and milk in different countries for the therapy or the prophylaxis of staphylococcal mastitis in cows, focusing more specifically on their potential activity against MR(N)SA. The potency of those phages as alternative to antimicrobial administration will be unravelled by: a) testing their lytic activity on bovine MR(N)SA isolated from cows with mastitis during in vitro growth (=lysotyping); (b) assessing their therapeutic or prophylactic activity compared to antimicrobials by conducting challenge experiments in mice.

The additional specific aims are to: (a) study the transferability of the SCCmec cassettes identified in the different MRS(A) ; (b) decipher the basis of the activity of the phages through comparison of their Whole Genome Sequencing (WGS); and (c) compare the (phylo)genetics of the MRS(A) on the basis of their clonal complexes, resistotypes, virulotypes and WGS.

This project is divided into 4 workpackages (WP):

i) WP1: identification to the species of MRS belonging to existing collections and newly isolated from milk and of their mec genes and SCCmec cassettes;

ii) WP2: lysotyping of MR(N)SA during in vitro growth and efficiency of phages to treat/prevent the infection compared to classical antimicrobial treatment during in vivo challenge experimental models in mice;

iii) WP3: typing of the MR(N)SA by their clonal complexes, antibiotic resistance profiles ("resistotypes") and combinations of virulence properties-encoding genes ("virulotypes");

iv) WP4: WGS of active phages and of MR(N)SA.

At the end of the project, the main achievements will be: (i) assessment of the potential of phages asalternative therapy or prophylaxis of MR(N)SA mammary gland infections and (ii) identification of the genetic criteria of identification of the most effective phages