Bovine mastitis, an inflammation of the udder, results in reduced milk production and poor milk quality. Mastitis is usually, but not always, a response to pathogen infection. High milk yield can produce oxidative stress in the mammary tissue. High milk yield is also known to be associated with bovine mastitis. Thus, in the current study, we hypothesised that oxidative stress increases inflammatory responses in bovine mammary cells. To examine the hypothesis, we produced cellular oxidative stress and investigated resulting inflammatory responses in bovine mammary alveolar cells (MAC-T). To produce oxidative stress, cells were treated with the reactive oxygen species (ROS; e.g., superoxide anion)-producing agent, menadione (MD; 0–10 µm; 6 h). To ensure the ROS-induced responses, cells were pretreated with an antioxidant NAC (0–10 mm; 1 h). Results showed that MD elevated intracellular ROS levels and protein expression of cyclooxygenase-2 (COX-2), a biomarker of inflammation. Pretreatment of cells with NAC attenuated MD-induced COX-2 expression by scavenging intracellular ROS and enhancing intracellular glutathione levels. MD-induced COX-2 expression was mediated by activation of extracellular signal receptor-activated kinase 1/2 (ERK1/2), Akt, and nuclear factor-kappa B (NF-κB). NAC attenuated activation of these intracellular signalling molecules. Treatment of cells with pharmacological inhibitors for ERK1/2, Akt, and NF-κB confirmed the association of these signalling pathways in MD-induced COX-2 expression. These results support our hypothesis that oxidative stress, which is found in high-yielding dairy cows, can produce cellular inflammation in bovine mammary alveolar cells and prevention of oxidative stress can attenuate such pathological responses. This may be relevant for cases of clinical mastitis for which no pathogen can be isolated.

Persistent or difficult-to-treat Staphylococcus aureus infections in animals and humans may be related to small colony variants (SCVs) that can hide inside host cells and modulate host defenses. S. aureus SCVs have gained much attention in human medicine but have been underestimated and overlooked in veterinary medicine. Recently, an SCV isolated from a dairy cow with a history of chronic mastitis was shown to possess similar phenotypic and transcriptomic properties to those of human SCVs. SCVs form small, colorless, non-hemolytic colonies after 48 h, are only slowly coagulase positive, fail to ferment mannitol, and can revert to the parental phenotype. The phenotype of SCVs is mostly related to alterations in hemin and/or menadione biosynthesis or to thymidine deficiency. Transcriptomic analysis of SCVs shows up-regulation of genes involved in glycolytic and arginine–deiminase pathways, capsular biosynthesis; increased sigma B activity; and down-regulation of genes for α-hemolysin, coagulase and effector molecule RNA III of the global virulence regulator Agr. Similar results are reported at the protein level. SCVs are less virulent but successful persisters in infection models. SCVs persist longer and at higher numbers within non-phagocytes than do their parents. SCVs survive within spacious vacuoles up to 24 h within cultured bovine mammary epithelial cells, likely due to up-regulation of protective mechanisms that counteract the lethal acidic environment of the phagolysosome. Persistence of SCVs within host cells may explain failures in antimicrobial therapy and vaccinations.