Electron microscopic observations of the interaction of casein micelles and milk fat globules with bovine polymorphonuclear leucocytes during the phagocytosis of staphylococci in milk

Abstract

Polymorphonuclear leucocytes (PMN) isolated from bovine blood or milk were examined in the electron microscope before and after incubation with staphylococci in either milk or a synthetic salts medium supplemented with serum. Milk PMN on isolation contained prominent milk fat globules and a few small vacuoles containing granules resembling casein micelles. Neither of these were seen in blood PMN. Milk PMN contained fewer lysosomes than blood PMN, and it is suggested that phagocytosis of fat globules and casein micelles in vivo, with consequent degranulation, accounts for their previously observed deficient phagocytosis of staphylococci. On incubation in milk in vitro with staphylococci, both blood PMN and milk PMN ingested apparently large quantities of granules resembling casein micelles, forming phagocytic vacuoles separate from those containing staphylococci. The ingested staphylococci appeared largely intact, despite the loss of lysosomes from the PMN cytoplasm. In contrast, the staphylococci ingested by blood PMN in synthetic medium appeared to be dead after 2 h. These observations support previous findings that casein inhibits the bactericidal activities of PMN, and suggest possible mechanisms for this inhibition.