Heat stress enhances Klebsiella pneumoniae -induced cellular injury and impairs milk fat synthesis in bovine mammary epithelial cells

Heat stress (HS) poses a significant threat to dairy production by reducing milk yield and elevating the incidence of mastitis. Concurrently, mastitis caused by Klebsiella pneumoniae (K. pneumoniae) occurs more frequently during the summer months, further exacerbating udder health challenges. Our study demonstrates that combined HS and K. pneumoniae exposure significantly exacerbates cellular damage of bovine mammary epithelial cells (BMECs). Mechanistically, HS potentiates K. pneumoniae-induced NF-κB pathway activation and reactive oxygen species (ROS) accumulation, leading to upregulated pro-inflammatory cytokines, chemokines, and antimicrobial peptides. Simultaneously, HS disrupts tight junction integrity, downregulating ZO-1, Occludin, and Claudins, and compromises the epithelial barrier. Critically, HS further suppresses lipid accumulation and the expression of milk fat synthesis-related genes and proteins in K. pneumoniae-challenged BMECs. This metabolic suppression is linked to the downregulation of key lipogenic regulators (SREBP1, PPAR-γ) and enzymes (FASN, ACC), alongside upregulated fatty acid oxidation (CPT1A). These findings elucidate how HS aggravates K. pneumoniae-induced mastitis by amplifying inflammation, disrupting barrier function, and inhibiting lactation, providing new insights into summer mastitis pathogenesis.