Mycobacterium smegmatis bacteria. Coloured scanning electron micrograph (SEM) of Mycobacterium smegmatis bacteria. These bacteria feed on dead or decaying material. They are found in soil or in smegma (secretions from genital glands of animals). Most Mycobacteria are harmless, but a few are pathogenic, such as Mycobacterium tuberculosis, the cause of the lung disease tuberculosis. M. smegmatis is used as a model for studies of tuberculosis. Magnification: x4000 when printed 10cm wide.