The most important findings are: (1) some genes were always absent, or evenly distributed in all strains considered; (2) genes belonging to MSCRAMMs (fib, fnbA, fnbB, sdrD and sasG) and the serine proteases had an increased risk of being detected in more contagious strains; (3) a heterogeneous group of genes interacting with the host immune response, including sed, ser, sej, sak and scn was correlated with the herds characterized by the highest prevalence of S. aureus mastitis. This evidence concerns the gene SRI and mastitis.