Free Access
Reprod. Nutr. Dev.
Volume 45, Number 4, July-August 2005
Page(s) 485 - 496
Reprod. Nutr. Dev. 45 (2005) 485-496
DOI: 10.1051/rnd:2005033

E. coli proteolytic activity in milk and casein breakdown

Kahina Haddadi, Fatima Moussaoui, Imen Hebia, François Laurent and Yves Le Roux

Laboratoire de Sciences Animales, UC INRA 12 340, École Nationale Supérieure d'Agronomie et des Industries Alimentaires (ENSAIA), 2 avenue de la Forêt de Haye, BP 172, 54505 Vandoeuvre-Lès-Nancy, France

Abstract - Previous studies have focused on both LPS and E. coli experimental mastitis and underlined the respective roles of endogenous proteolysis (including plasmin from the blood stream and other proteases from milk leukocytes), as well as the presence of E. coli in a more intricate system. The aim of this study was to assess the role of E. coli in milk proteolysis and especially that of its proteases in casein breakdown. The first part consisted in the incubation of 104 cfu·mL-1 of the E. coli strain in raw milk at 37 °C for 24 h; the same milk was also incubated with 0.04% sodium azide. Several parameters were evaluated: CFU, plasmin activity, gelatinase activity and pH 4.6 insoluble peptides, including the proportion of $\gamma$-CN. The profile of gelatinase activity was determined by zymography and identified by immunoblotting. In the second part of the study, we examined the profile of CN ($\alpha$s-, $\beta$- and $\kappa$-CN) breakdown by E. coli lysate. The results suggest that E. coli proteases have a direct effect on CN, and the increase of $\gamma$-CN in inoculated milk may be generated by both plasmin and the gelatinase. Moreover, the gelatinase activity in the inoculated milk was higher after 24 h of incubation.

Key words: caseins / $\gamma$-CN / E. coli / gelatinase activity / milk / plasmin

Corresponding author: Kahina Haddadi

© INRA, EDP Sciences 2005