Polymorphonuclear neutrophils and Escherichia coli proteases involved in proteolysis of casein during experimental E. coli mastitis

An Escherichia coli mastitis model was used to characterize enzymes involved in bovine mammary tissue damage and proteolysis in milk. One-quarter each of four cows were inoculated with a suspension (104 cfu mL−1) of E. coli P4:O32. Blood and milk were collected before inoculation and for 216 h afterwards. Intracellular elastase, collagenase and cathepsin activities were measured by flow cytometry of peripheral blood leukocytes and milk polymorphonuclear neutrophils (PMNs). Leukopenia occurred in peripheral blood 9 h after infection, concomitant with an increase in somatic cell count in milk. Milk PMNs had lower activity of cathepsins and collagenase than peripheral blood PMNs. In parallel, milk samples were studied by zymography, and several proteases were detected in mastitic milk. These activities increased after infection, to reach a peak in 6 h. However, total protease profiles and plasmin activities differed. It was concluded that proteases released by PMNs and E. coli contribute to proteolysis of casein during mastitis, as well as plasmin.