TEM of negatively stained Mycobacterium intracellulare showing lipid-rich outer layer, the rope-like structure of mycolic acids and small dense granules. Mycobacteria are a genus of acid-fast organisms belonging to the Mycobacteriaceae family that includes the causative organisms of tuberculosis and leprosy. They are slender, nonmotile, gram-positive rods and do not produce spores or capsules. Mycobacterium Avium Intracellular (MAI) infection involves most commonly in the lymph nodes, followed by spleen, liver and lung. Infection is characterized by proliferation of histocytes with foamy or cavuolar cytoplasm containing acid-fast bacilli and formation of granulomatous nodules. Magnification = x41,000 at 70mm.