In vivo killing and degradation of Mycobacterium aurum within mouse peritoneal macrophages

We studied the in vivo killing and degradation of Mycobacterium aurum, a nonpathogenic, acid-fast bacillus, within macrophages after inoculation into the peritoneal cavity of CD-1 mice. The degradative process could be divided in five successive steps that were characterized on ultrastructural and cytochemical grounds and the relative contributions of which were determined by quantitative electron microscopy of samples taken at different times. The main ultrastructural alterations observed during the degradative process were ribosome disaggregation, coagulation of the cytoplasmic matrix, and change in the membrane profile from asymmetric to symmetric, with loss of the polysaccharide components from the outer layer, followed by membrane solubilization and intracellular clearing, followed by digestion of the innermost (peptidoglycan) layer of the cell wall, and at the end of the process, disorganization and collapse of the remaining layers of the cell wall. The correlation between viability and morphology indicated that the first ultrastructural signs of viability loss are cytoplasmic coagulation, change in the membrane geometry, and disappearance of ribosomes. The labeling of lysosomes of peritoneal macrophages with ferritin or by the cytochemical demonstration of inorganic trimetaphosphatase showed that fusion of lysosomes with phagosomes containing mycobacteria occurs in the phagocytes in the mouse peritoneal cavity and is already extensive as soon as 1 h after the inoculation of the bacilli.