Mycobacterium is a genus of Actinobacteria, given its own family, the Mycobacteriaceae. Over 190 species are recognized in this genus. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae) in humans. The Greek prefix myco- means "fungus," alluding to the way mycobacteria have been observed to grow in a mold-like fashion on the surface of cultures. It is acid fast and cannot be stained by the Gram stain procedure.