Intracellular Localization of Mycoplasma bovis in the Bronchiolar Epithelium of Experimentally Infected Calves

Lung tissues from calves infected experimentally with Mycoplasma bovis were examined by immunohistochemistry and electron microscopy. All inoculated calves had dark red areas of consolidation affecting both left and right lungs, which were characterized microscopically by subacute purulent bronchiolitis with hyperplasia of the surrounding lymphoid tissue. Immunohistochemically, M. bovis antigen was detected on the surface and inside the cytoplasm of bronchiolar epithelial cells in the pneumonic foci. The antigen was also found in the cytoplasm of phagocytes at the margin of bronchiolar exudates. Electron microscopically, numerous organisms were demonstrated in the immunohistochemically-positive sites. These findings suggest that M. bovis organisms adhere to the bronchiolar epithelium and at least some of them invade the epithelium.