Ultrastructural study of the behavior of macrophages toward parasitic mycobacteria

Mycobacterium lepraemurium and M. microti (causal agent of vole tuberculosis) were isolated from tissues of experimentally infected mice and used to infect normal mouse peritoneal macrophage cultures. The cellular response to these bacteria up to 4 days after infection was studied quantitatively by electron microscopy. Prelabeling with ferritin was used to facilitate observation of fusion between secondary lysosomes in the cells and phagosomes containing the bacteria. All bacteria were intraphagosomal, and a high proportion of them was morphologically "intact." Nearly all phagosomes containing morphologically damaged (presumed nonviable) bacteria also contained ferritin, having fused with secondary lysosomes. Fusion of lysosomes had also occurred with most phagosomes containing intact M. lepraemurium but was infrequent with phagosomes containing intact M. microti. This tendency of multiplying mycobacteria of the tubercle type to avoid contact with lysosomal contents has already been reported for M. tuberculosis strain H37Rv. The different intracellular circumstances of the parasites may reflect different means of intracellular survival.