Rifampicin-resistant strains of Mycobacterium leprae may have reduced virulence

A strain of Mycobacterium leprae resistant to rifampicin (RMP) failed to infect normal mice when injected into the foot pads (FP) at a dose of 10 or 100 bacilli/FP, although it could be maintained by serial passage in mice by the use of inocula of 10(4) bacilli/FP; normal mice can be infected by RMP-sensitive M. leprae at a dose of 10 bacilli/FP. By contrast, nude (athymic) mice could be infected with an inoculum of 10 bacilli/FP of the RMP-resistant strain. It is suggested that the strain concerned possessed reduced virulence for normal mice, and the implications of this for the probability of occurrence of human disease caused by RMP-resistant strains of M. leprae are discussed.