Staphylococcus aureus is recognized as the primary pathogen responsible for contagious mastitis in dairy cattle, whereas non-aureus staphylococci and mammaliicocci (NASM) represent the bacterial group most frequently recovered from milk samples. Apart from their role in animal disease, NASM are known to possess antimicrobial resistance (AMR) determinants that can potentially be exchanged bidirectionally with S. aureus. The occurrence of methicillin-resistant (MR) staphylococci (MRS) poses notable One Health implications. This investigation determined the prevalence and molecular features of MRS in bulk tank milk (BTM) collected from 88 dairy farms situated in the livestock-intensive province of Lodi, Lombardy (northern Italy). S. aureus was detected in 32.95% of BTM samples, with the Ribosomal Spacer PCR (RS-PCR) genotype B found to be the dominant strain, appearing in 37.93% of S. aureus-positive holdings. All isolates harbored ica genes (icaA, icaB, icaC, icaD), suggesting their ability to produce biofilm. MRS were present in 56.81% of farms. MALDI-TOF MS identification revealed S. epidermidis (MRSE, 35.59%) as the most abundant MR species, followed by S. aureus (MRSA, 18.64%), M. sciuri (15.25%), S. saprophyticus (11.86%), S. borealis (6.78%), S. haemolyticus (5.08%), M. fleurettii (3.39%), S. cohnii, and S. pettenkoferi (1.70% each). The majority of MR isolates carried the mecA gene, while mecC was not detected. The staphylococcal cassette chromosome mec (SCCmec) was mainly type V in MRSA (45.45%) and type IV in MRSE (61.90%). Considering their One Health significance, continuous AMR surveillance among all staphylococcal and mammaliicoccal isolates from milk is vital to elucidate prevalence, characteristics, and the dissemination patterns of methicillin-resistance genes in dairy herds.
