Abstract: Background: Bovine clinical mastitis continues to pose a significant challenge in dairy farms, particularly due to the rise of antibiotic-resistant bacteria. This study aimed to investigate the prevalence and antimicrobial resistance profiles of key bacterial pathogens isolated from mastitic milk in Sanandaj, Iran. Methods: A total of 250 milk samples were collected from clinically affected quarters of lactating cows in industrial dairy farms located around Sanandaj during 2024. Bacterial isolates were identified to the genus and species level. Antimicrobial susceptibility testing was carried out using the disc-diffusion method following CLSI guidelines, focusing on antibiotics commonly used in veterinary practice. Results: Among the 250 milk samples, Staphylococcus aureus was the most frequently detected pathogen, present in 110 samples (44%), followed by coagulase-negative staphylococci (CNS) in 55 samples (22%) and Escherichia coli in 35 samples (14%). Other bacteria, including Streptococcus agalactiae, Klebsiella pneumoniae, and Enterococcus faecalis, were identified in 20 samples (8%). Mixed bacterial growth was observed in 30 samples (12%), while 50 samples (20%) showed no growth after repeated culture. Antimicrobial testing revealed high resistance rates to penicillin G (78%), ampicillin (72%), and tetracycline (61%). Moderate resistance was noted for erythromycin (47%) and chloramphenicol (38%), whereas gentamicin (24%) and trimethoprim-sulfamethoxazole (28%) exhibited lower resistance. Overall, the results indicate a widespread presence of multidrug-resistant bacteria among mastitis cases in this region. Conclusion: Staphylococcus aureus emerged as the predominant mastitis pathogen in Sanandaj, showing marked resistance to penicillin and several other antibiotics. These findings underscore the importance of judicious antibiotic use and the implementation of alternative control measures in dairy farms.
