Electron microscopic study of milk sediments

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Abstract
Sediment of milk from ovine mammary glands infected experimentally with coagulase‐negative staphylococci was examined by transmission electron microscopy. The proportions of the various particles and cells present in milk and of macrophages and neutrophils involved in phagocytosis were determined 6 h, 18 h and 49 days after infection. All cell types together predominated over cytoplasmic bodies in approximately 54% of normal milk samples. The proportion of macrophages in normal milk was higher than that of neutrophils in approximately 69% of samples examined. After infection and in the early phase of inflammation, the proportion changed in favour of neutrophils. In both the early and late phases of infection, greater numbers of macrophages (3–27.2%) than neutrophils (0–2.7%) contained phagocytized cocci. Moreover, greater numbers of cocci were observed in macrophages than in neutrophils (mean values 8.1 vs 1.2), indicating the importance of the macrophage in the maintenance of ovine subclinical mastitis caused by coagulase‐negative staphylococci.