Host-pathogen interactions during early stages of bovine mastitis: divergent macrophage responses to distinct bovine-associated Staphylococci species and strains.

Abstract

BACKGROUND: Bovine mastitis is the most economically significantdisease in dairy farming, with Staphylococcus aureus and Staphylococcus chromogenes asprominent agents, the former posing a major threat. As principal immune sentinels in themammary gland, macrophages orchestrate early pathogen recognition and immune activation,critically influencing the trajectory and outcome of infection. Thus, this study aimed tocharacterize the early macrophage responses to distinct bovine-associated S. aureus and S.chromogenes strains. METHODS: Here, RAW 264.7 cells were challenged with four differentstrains: S. aureus [isolated from nose (SN), and intramammary infection (IMI)] and S.chromogenes [IMI, and teat apex (TA)] were evaluated after 90- and 180-min. Nitric oxide (NO)production was analyzed in the supernatants, and mRNA levels of IL-1β, IL-18, NLRP3, NOS2,Arg1, Bax, and Bcl2 were assessed in the cells. RESULTS: Macrophages challenged with S. aureusIMI strains showed elevated Nos2 expression but negligible NO production, indicating a potentialimmune evasion mechanism. The commensal S. aureus SN strain uniquely maintained arginaseexpression, suggesting M2-like polarization that may promote immune tolerance and bacterialcolonization. Both S. aureus strains significantly upregulated the anti-apoptotic Bcl2 gene, atranscriptional response that may be associated with host cell survival, which may facilitatebacterial intracellular persistence. In contrast, S. chromogenes strains induced strong NOS2expression, robust NLRP3 inflammasome activation, and increased IL-1β production, indicatingM1 polarization and a pro-inflammatory response. The pro-apoptotic Bax gene showed an earlydecrease followed by a later increase exclusively in S. aureus-infected macrophages, indicating atime-dependent transcriptional modulation of apoptosis-related genes. CONCLUSIONS: Thesegenotype-dependent macrophage responses reveal complex immune modulation shaping mastitis pathogenesis. However, our findings are based solely on transcriptional data on the murine cells and require further validation.