Straph. aureus is a major pathogen in dairy cattle worldwide. Our study confirms that the Staph. aureus spread within a farm and the consequent impact on the prevalence of intramammary infections (IMI) are mainly linked to the genetic characteristics of the strain itself. In particular, we demostrated the pivotal role of adlb gene in the contagiousness of Staph. aureus and its usefulness as a genetic marker in diagnostic testing for the control of Staph. aureus IMI. Interestingly, when adlb-positive strains circulated, even environmental and milking management factors lost importance, highlighting the importance of a prompt identification of these strains.