Mastitis-Associated Bacterial Isolates and Antimicrobial Resistance in a Single Dairy Herd in the Akmola Region, Kazakhstan

Background/Objectives: Bovine mastitis is a common infection in cattle causing significant economic losses globally in the dairy industry. While bacterial profiles and antimicrobial resistance (AMR) patterns are generally consistent worldwide, comprehensive epidemiological and AMR data from Kazakhstan are lacking. This study characterized mastitis-associated bacterial isolates and their AMR profiles from a single commercial Simmental dairy herd in the Akmola Region, Kazakhstan. Milk samples from 41 cows with clinical mastitis and documented treatment histories were collected. Methods: Bacterial identification was performed using MALDI-TOF MS, and antimicrobial susceptibility was tested phenotypically following EUCAST criteria. Whole genome sequencing (WGS) on a subset of isolates analyzed resistance and plasmid content using AMRFinderPlus v3.11.14 and Abricate v1.4.0, with the ResFinder and PlasmidFinder databases, respectively. Results: All samples yielded bacterial growth, resulting in 58 isolates across 11 species and five genera, from which Staphylococci predominated (50.0%), followed by Streptococci (28.0%), Escherichia coli (19%), one Aerococcus (1.72%) and one Corynebacterium (1.72%) strain. Phenotypic resistance to at least one antimicrobial class was found in 65.5% of tested isolates. Tetracycline resistance was the most frequent (71.7% of tested isolates), with multidrug resistance detected in 6.9%. WGS of a randomly selected subset of 33 isolates revealed that 27 (82%) carried at least one acquired AMR gene, predominantly tetracycline resistance genes (26/33), followed by lincosamide/macrolide (12 and 11 isolates) and aminoglycoside resistance genes (9 isolates). Plasmid replicon screening identified plasmid replicons in 21 of the 33 sequenced isolates, mainly of the repUS43 and repUS76 staphylococcal replicon families, suggesting a potential role of plasmids in AMR gene dissemination within the herd. Conclusions: This study offers a comprehensive phenotypic and genomic characterization of mastitis-associated pathogens and their AMR determinants within this single herd, highlighting the co-circulation of tetracycline, lincosamide/macrolide, and aminoglycoside resistance genes. As this study was limited to a single dairy herd, these findings should be regarded as descriptive, farm-level observations rather than being representative of the Akmola Region or Kazakhstan more broadly. These findings support farm-level mastitis surveillance that combines species identification, phenotypic AMR testing, and targeted molecular screening.