The morphologic appearance of Mycobacterium leprae in acid-fast stained sections of skin biopsy specimens from patients with lepromatous leprosy has been found to correlate well with the infectivity of the specimen for the mouse. Viable M leprae were demonstrated in 15 of the 16 patients with previously untreated lepromatous leprosy. Ten of 38 specimens obtained early in the course of dapsone therapy of previously untreated patients were found to contain viable M leprae; and the viability of the organisms was found to be much reduced in five of these ten specimens. By contrast, of 15 specimens obtained during dapsone therapy from five patients prove to harbor dapsone-resistant M leprae; 14 were demonstrated unequivocally to contain viable organisms.
Levy L, Fasal P, Murray LP. Morphology of Mycobacterium leprae in Tissue SectionsCorrelation With Results of Mouse Foot Pad Inoculation. Arch Dermatol. 1969;100(5):618-620. doi:10.1001/archderm.1969.01610290102023