Lack of intracellular degradation of Mycobacterium paratuberculosis by bovine macrophages infected in vitro and in vivo: light microscopic and electron microscopic observations.

Blood-derived macrophages from clinically healthy cows and cows infected with Mycobacterium paratuberculosis were cultured in vitro and inoculated with heat-killed M paratuberculosis. Five weeks after infection, acid-fast bacteria continued to be demonstrated in the macrophages. Macrophage cultures also were infected with living M paratuberculosis and were examined by electron microscopy. Four weeks after infection, intact bacteria were found in all cultures. Electron microscopic examinations of intestinal macrophages from infected cows also demonstrated intracellular intact bacteria.