Occurrence of Serratia marcescens in bovine intramammary infections: identification and antimicrobial susceptibility in a dairy herd

Abstract Introduction Mastitis is one of the most economically significant diseases in dairy cattle, and its causation by atypical environmental pathogens is gaining attention. The study investigated the involvement of Serratia marcescens in intramammary infections occurring in a dairy herd and evaluated the bacterium’s antimicrobial resistance profile. Material and Methods Between February 2024 and June 2025, 481 quarter-milk samples collected from cows on a dairy farm in eastern Poland were subjected to routine bacteriological examination. Isolates were identified by biochemical, proteomic and molecular methods and tested for antimicrobial susceptibility by automated biochemical analysis. Results Serratia marcescens was isolated from the milk samples of 10 cows during different sampling periods. All isolates were consistently identified as S. marcescens , confirming the reliability of the applied comprehensive approach. Molecular analyses demonstrated high sequence similarity with reference strains. Antimicrobial susceptibility testing revealed uniform resistance to amoxicillin/clavulanic acid, cefuroxime, colistin and tigecycline. Conclusion Serratia marcescens should be considered a relevant opportunistic pathogen in dairies, capable of causing sporadic or epidemic mastitis episodes. The findings are consistent with previous reports linking S. marcescens mastitis to environmental reservoirs and milking equipment contamination. Comprehensive diagnostic strategies, strict hygiene practices and targeted herd management measures are important to prevent and control mastitis caused by uncommon environmental pathogens.