Cinétiques des variations de la composition protéique du lait au cours d'une mammite expérimentale à Escherichia coli

Abstract

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 soluble proteins were significantly higher after 14 h and a max- imum was reached at 48 h after infection (+44 %, + 104 % and +574 % respectively). Casein content independently from time was reduced of 20 %. Proportions of 13- and œ-caseins were significantly lower at 24 h (-49 %) and between 48 and 72 h (--D2 %) respectively. Proportion of pH 4.6 insoluble pep- tides including y-caseins reached a maximum between 24 and 72 h (+580 %). These different kinet- ics can be explained by sequential origin ofproteolysis; plasmin activity could be maximal before max- imal activity of somatie cell proteases. Protein composition in milk from control quarters was also greatly modified. Regardless of time, soluble protein content increased (+68 %) and casein content decreased (-21 %). Significative proteolysis of l3-casein occurred at 72 h (-25 %) accompanied by increase in relative proportion of pH 4.6 insoluble peptides (+50 %); this was probably due to higher milk plasmin activity levels. Therefore, interdependance was notieed between infected and unin- fected quarters, probably through systemic response to E. coli infection. © InralElsevier, Paris.