Abstract: This experiment was conducted to study the mechanism of Escherichia coli (E. coli) infection on mitochondrial damage of mammary epithelial cells and mouse mammary gland. In this study, bovine mammary epithelial cells and mouse mammary gland were used as the research object. Bovine mammary epithelial cells and mouse mammary gland were randomly divided into three groups, including Control group, E.coli group and lipopolysaccharide (LPS) group, respectively. No treatment was conduceted in the Control group, boving mammary epithelial cells and mouse mammary gland tissue in the E.coli group were infected with E.coli (MOI=5) for 6 h or 10⁶ CFU E. coli for 24 h, respectively. In the LPS group they were treated with 1 μg·mL^-1 lipopolysaccharide (LPS) for 6 h or 20 mg·kg^-1 body weight LPS for 24 h, respectively. with three replicates in each group, 6 mice in each group. The results showed that: 1) Cytokeratin 18 in bovine mammary epithelial cells was dyed green and evenly distributed in the cytoplasm, and the nucleus was dyed blue by DAPI. 2) The mitochondria in untreated-bovine mammary epithelial cells were intact and the cell connections were tight. In E.coli group, the space between bovine mammary epithelial cells was increased, and mitochondria were swollen with disappeared mitochondrial cristae and blurred partial mitochondria cristae. 3) E. coli infection or LPS treatment caused the appearance of neutrophils in the mammary glands of mice. E.coli infection or LPS significantly decreased (P<0.05 or P<0.01) mitochondrial energy metabolism (decreased D(520 nm) absorption value, ATP concentration, and the activities of mitochondrial electron transport chain complex Ⅰ, Ⅱ, Ⅲ, Ⅳ and Ⅴ), mitochondrial fusion and division (reduced Drp1, Fis1, Mfn1, Mfn2, and OPA1 mRNA expressions) and mitochondrial biogenesis (inhibited PGC-1α, NRF1, TFAM, and D-Loop gene expression). These results indicated that E.coli infection caused energy metabolism disorder of mitochondria, inhibited mitochondrial division and fusion, and reduced mitochondrial biogenesis mainly through LPS in bovine mammary epithelial cells and mouse mammary gland, thus causing mitochondrial damage and eventually leading to mastitis. Therefore, it can be inferred that E.coli infection could cause bovine mastitis through inducing mitochondrial damage, which play an important role in mammary gland inflammation.

Key words: bovine mastitis, Escherichia coli, mitochondrion, bovine mammary epithelial cell, mouse mammary gland