Blood and Milk Bacterial Community Profiles Differ According to Seasonal Thermal Conditions in Clinically Healthy Holstein Cows from Northern Mexico

Bovine-associated microbial communities play important roles in animal physiology, immune regulation, and mammary health. However, knowledge regarding bacterial populations associated with blood and milk remains limited in healthy dairy cattle exposed to different environmental conditions. In this study, we characterized and compared blood- and milk-associated bacterial communities from clinically healthy Holstein cows during summer and winter in an intensive commercial dairy production system located in northern Mexico using 16S rRNA metabarcoding. Significant differences in alpha and beta diversity metrics were observed according to sample type and season. Milk samples exhibited higher microbial richness and diversity than blood samples, and comparative analyses revealed significant differences in bacterial community composition between sample types and seasonal sampling periods. Bacterial communities were mainly dominated by Proteobacteria, Firmicutes, Actinobacteriota, and Bacteroidota. Differential abundance analyses identified genera associated with environmental exposure or opportunistic infections, including Staphylococcus, Streptococcus, Corynebacterium, Pseudomonas, and Acinetobacter. Overall, the observed bacterial community patterns varied according to sample type and seasonal environmental conditions, highlighting the complexity of host-associated microbial ecosystems in dairy cattle.