Bacteria-dependent modulation of immune responses in the bovine udder.

Subclinical mastitis remains difficult to characterize due to the complex interplay between pathogens, local microbiota, and host immune responses. The aim of this study was to investigate how different bacterial species detected in the hindmilk influenced bovine udder immune response and its regulation. Quarter-level hind milk samples were collected from 24 healthy Norwegian Red cows before drying off and through a next full lactation period. Somatic cell populations, cytokines and host-proteome were assessed and combined with the presence of different pathogens. Quarters harboring Staphylococcus aureus and Streptococcus spp. showed significantly elevated somatic cell counts and inflammation associated patterns of somatic cell populations, while those with minor pathogens exhibited lower counts, particularly fewer granulocytes. Cytokines profiling revealed suppressed levels of IFN-γ, IL-10 and TNF-α in the presence of Staphylococcus chromogenes in contrast to the intensive inflammatory activity associated with S. aureus. Weighted gene co-expression network analysis of the host proteome identified two modules of proteins that were negatively correlated with increasing somatic cells count and several modules with strong positive correlation with the SCC. At protein level, we detected 67 proteins differentially expressed based on the pathogen present in the hindmilk. Of these, 19 were linked to immune system functions such as TLR2 and lactoferrin. Our study provides insights into host response in bovine mastitis and reveals a notable lack of interaction between S. chromogenes and the host body compared to an intense immune response in the presence of other pathogens such as S. aureus.