Sequential changes in milk protein composition after experimental Escherichia coli mastitis

Milk protein composition was determined between 0 and 72 h after infection during experimentally induced E. coli mastitis of seven cows. In milk from infected quarters, contents of non protein nitrogen, total proteins and sol uble proteins were significantly higher after 14 h and a maximum was reache d at 48 h after infection (+44%, +104% and +574% respectively). Casein cont ent independently from time was reduced of 20%. Proportions of beta- and al pha-caseins were significantly lower at 24 h (-49%) and between 48 and 72 h (-62%) respectively. Proportion of pH 4.6 insoluble peptides including gam ma-caseins reached a maximum between 24 and 72 h (+580%). These different k inetics can be explained by sequential origin of proteolysis; plasmin activ ity could be maximal before maximal activity of somatic cell proteases. Pro tein composition in milk from control quarters was also greatly modified. R egardless of time, soluble protein content increased (+68%) and casein cont ent decreased (-21%). Significative proteolysis of beta-casein occurred at 72 h (-25%) accompanied by increase in relative proportion of pH 4.6 insolu ble peptides (+50%); this was probably due to higher milk plasmin activity levels. Therefore, interdependance was noticed between infected and uninfec ted quarters, probably through systemic response to E. coli infection. (C) Inra/Elsevier, Paris.