Occurrence and Antimicrobial Susceptibility of Major Bacterial Pathogens Associated with Subclinical Mastitis in Dairy Cows in Western Romania

Subclinical mastitis is a major but often overlooked constraint to dairy productivity, causing economic losses through reduced milk yield and quality. In Romania, comprehensive data on the bacterial etiology and antimicrobial resistance (AMR) of subclinical mastitis are limited. This study aimed to characterize the etiological agents and antimicrobial susceptibility profiles of major bacterial pathogens isolated from subclinical mastitis cases in dairy cows from Western Romania. Between 2021 and 2022, milk samples were collected from 117 lactating cows diagnosed with subclinical mastitis on three dairy farms. Bacterial isolation and differentiation were performed on ChromID^® CPS^® Elite Agar, and isolates were confirmed by standard biochemical tests. Antimicrobial susceptibility testing of <i>Staphylococcus aureus</i> and <i>Escherichia coli</i> isolates was conducted using the automated Vitek^®2 system, interpreted according to CLSI veterinary standards. Multidrug resistance (MDR) was defined as resistance to at least one agent in three or more antimicrobial classes. Bacterial growth occurred in 51 of 117 samples (43.6%). <i>S. aureus</i> subsp. <i>aureus</i> predominated (28.2%), followed by <i>E. coli</i> (4.3%), <i>Klebsiella pneumoniae</i> subsp. <i>pneumoniae</i> (2.3%), and <i>Streptococcus uberis</i> (2.3%). Mixed infections occurred in 6.0% of positive samples. Among <i>S. aureus</i>, the highest resistance rates were to fosfomycin (58.3%), penicillin (44.4%), clindamycin (44.4%), and tetracycline (41.7%), with 47.2% MDR isolates. <i>E. coli</i> showed resistance to amoxicillin/clavulanic acid (88.9%), ampicillin (55.6%), and cefotaxime (55.6%), with 66.6% MDR. <i>S. aureus</i> remains the leading cause of subclinical mastitis in Western Romania. The high MDR rates highlight the need for targeted antimicrobial stewardship, culture-based therapy, and continuous AMR monitoring under the "One Health" framework.