Inhibition of the bactericidal activity of bovine polymorphonuclear leucocytes and related systems by casein.

Polymorphonuclear leucocytes (PMN) obtained from lactating cows' udders were deficient in their ability to kill Staphylococcus aureus compared with PMN isolated from blood. However, blood PMN suspended in separated milk or in the presence of casein were similarly impaired. Casein was found to inhibit in vitro the bactericidal activities of histone, the lactoperoxidase-H2O2-KI system and PMN lysates. Electron microscopy showed that casein was ingested by PMN with the formation of phagocytic vacuoles. These observations provide the basis of a hypothesis explaining the bactericidal deficiency of milk PMN and the consequent susceptibility of the udder to infection.