Prevalence of Variant GTRIStaphylococcus aureus Isolated from Dairy Cow Milk Samples in the Alpine Grazing System of the Aosta Valley and Its Association with AMR and Virulence Profiles

<b>Background/Objectives</b>: In the Aosta Valley, the alpine grazing system integrates livestock production and land management. Valdostana breeding has adapted to this mountainous region, but the spread of <i>Staphylococcus aureus</i> within pastures may impact animal health. The aim of this study was to provide an overview of <i>S. aureus</i> genotypes associated with antimicrobial resistance (AMR) and virulence profiles in four dairy herds in the Aosta Valley from July 2022 to August 2023. <b>Methods</b>: A total of 468 composite milk samples were collected at three timepoints: T1 (pasture-livestock system), T2 (farm-livestock system), and T3 (pasture-livestock system). <i>S. aureus</i> isolates were characterized by antimicrobial susceptibility testing, ribosomal spacer (RS)-PCR, multilocus sequence typing (MLST), PCR analysis for 28 virulence genes and 6 AMR genes, and <i>adlb</i>-targeted real-time PCR. <b>Results</b>: RS-PCR analysis of 82 <i>S. aureus</i> strains revealed 12 genotypes (GT) in eight clusters (CL). The most prevalent variant was GTR^I (61%), followed by GTB (15%). Resistance to penicillin was high (69%), with CLR strains showing 88% resistance, and 51% resistance to amoxicillin plus clavulanate. All strains were susceptible to cephalosporins and oxacillin. Macrolide resistance was low (4%), and multi-drug resistance was 6%. AMR gene presence corresponded with susceptibility, with <i>blaZ</i> detected in 94% of CLR strains. CLR strains also possessed genes for biofilm formation and virulence factors. <b>Conclusions</b>: This study highlights the presence of AMR and virulence factors in <i>S. aureus</i> strains from alpine grazing systems, underscoring the need for ongoing monitoring to mitigate risks to animal health.