Divergent Immune Responses to Minor Bovine Mastitis-Causing Pathogens

Simple Summary Bovine mastitis can be caused by both bacterial and non-infectious agents. Well-established mastitis pathogens include Staphylococcus aureus, Streptococcus agalactiae, and Streptococcus uberis. Traditionally, non-aureus staphylococci and mammaliicocci (NASM), including Staphylococcus chromogenes, and lactic acid bacteria (LAB) were not considered major contributors to bovine mastitis. However, their role in subclinical mastitis is increasingly recognized. This study investigated the involvement of NASM and LAB strains (Weissella paramesenteroides) through the lens of bovine neutrophil responses. This study suggests that S. chromogenes and Weissella exhibit divergent immunomodulatory effects on bovine neutrophils via coordinated functional and gene expression alterations. These findings may necessitate a re-evaluation of their respective roles in bovine mastitis etiology.Abstract Traditionally, non-aureus staphylococci and mammaliicocci (NASM) were not considered significant players in bovine mastitis. This study investigated the involvement of NASM (Staphylococcus hominis and Staphylococcus chromogenes) and lactic acid bacteria (LAB) strains (Weissella paramesenteroides) through bovine neutrophil responses. Bovine neutrophils displayed minimal apoptosis upon NASM and LAB challenge. Neutrophils expressed high TLR2 after challenge, but TLR6 expression varied and remained low in NASM pathogen recognition. Bovine neutrophils effectively engulfed and killed LAB, but their activity was significantly impaired against NASM. This was evident in S. chromogenes, where reduced TLR6 recognition and a weakened phagocytic response likely contributed to a lower bactericidal effect. Regardless of the bacteria encountered, intracellular ROS production remained high. S. chromogenes-challenged neutrophils displayed upregulation in genes for pathogen recognition (TLRs), ROS production, and both pro- and anti-apoptotic pathways. This response mirrored that of Weissella. except for CASP9 and BCL2, suggesting these bacteria have divergent roles in triggering cell death. Our findings suggest that S. chromogenes manipulates bovine neutrophil defenses through coordinated changes in functional responses and gene expression, while LAB strains have a weaker influence on apoptosis.