ELECTRON-MICROSCOPIC STUDY OF EXPERIMENTAL INFECTIONS WITH ACID-FAST BACILLI

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Abstract
52 mice were infected intravenously with Mycobacterium tuberculosis (H37Rv) and another 52 were infected with a nonchromogenic unclassified Mycobacterium isolated from a patient with pulmonary disease. Two mice in each group were killed at various intervals beginning three hours after infection and extending to 150 days for the H37Rv and to 365 days for the nonchromogen. The H37Rv infection progressed more rapidly, but the histopathologic pattern was the same in both, consisting of collections of histiocytes and various degrees of necrosis. Electron microscopy revealed myco-bacteria in various forms in tissue cells. The earliest to be recognized were bacilli in a clear zone surrounded by a well-defined membrane. Spherules with homogeneous or granular content were seen in bacilli. These eventually became free within the cytoplasm of the host cell. Spherules were embedded in what appeared to be lysosomes. Rarely, bacillary forms were seen in lysosomes. In pulmonary lesions there were numerous large, lamellated, osmiophilic bodies, some of which were the size of nuclei of the host cell. These were not seen by light microscopy in sections stained with hematoxylin and eosin or with acid-fast stains. These, and additional studies by electron microscopy, disclosed no qualitative differences in vivo among M. tuberculosis (H37Rv), two strains of nonchromogenic mycobacteria, and a strain of photochromogenic Mycobacterium. However, spherules were more prominent in nonchromogens.