Koch's postulates
Genetically, "M. leprae" and "M. lepromatosis" lack the genes that are necessary for independent growth. "M. leprae" and "M. lepromatosis" are obligate intracellular pathogens, and can not be grown (cultured) in the laboratory. The inability to culture "M. leprae" and "M. lepromatosis" has resulted in a difficulty definitively identifying the bacterial organism under a strict interpretation of Koch's postulates.