Electron microscopic observations on the interaction of Listeria monocytogenes and peritoneal macrophages of normal mice.

Acid phosphatase histochemistry was performed to study ultrastructurally the intracellular survival and digestion of Listeria monocytogenes in normal mouse peritoneal macrophages. Acid phosphatase activity proved to be a useful marker for digestive processes of the macrophages, as this activity could be seen in the cytoplasm of killed listeria before disintegration, but not in viable listeria. Almost all killed listeria and the majority of the viable listeria were found in phagosomes of the loosely fitting type. The fusion of lysosomes and phagosomes containing killed bacteria was completed very quickly; the fusion involving phagosomes containing viable listeria was delayed. This delay was abolished by opsonization of bacteria. Disintegration of killed listeria in macrophages could be seen. No such effects were observed when viable listeria were used. Increased numbers of listeria within the phagosomes upon longer incubation suggest multiplication within macrophages.