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.

Leprosy in the nude mouse

The continued failure to grow the leprosy bacillus in vitro and the limited and localized nature of the infection resulting from the inoculation of Mycobacterium leprae into normal mice emphasise the need for an experimental animal which readily permits growth of the bacillus.

Mycobacterium leprae infection in nude mice: bacteriological and histological responses to primary infection and large inocula.

Previous studies have demonstrated that congenitally athymic, nude mice are highly susceptible to infection with Mycobacterium leprae. In this study, we showed that footpad inoculation of nude mice with different inoculum sizes of M. leprae resulted in exponential growth of bacilli until bacillary numbers reached approximately 10(10) bacilli per footpad. There was dissemination of the infection from approximately 10 months after inoculation. When nude mice were compared with thymectomized and irradiated mice and normal intact mice for the ability to detect growth from large inocula of low viability, nude mice were the most sensitive, permitting the detection of 10(2) viable M. leprae among 10(7) irradiation-killed organisms. There was widespread dissemination of the infection throughout the reticuloendothelial system and the tissues of the cooler body sites from approximately 10 months after inoculation. Histologically, the lesions resembled those seen in lepromatous leprosy, although the bacillary load appeared larger and was similar to that seen in heavily infected tissues of the nine-banded armadillo. An unusual feature was the presence of numerous foci of neutrophil polymorphs in the footpads and liver of infected nude mice.

The effect of body temperature and cell-mediated immunity on the growth of Mycobacterium marinum and Mycobacterium leprae in mice.

Evidence is presented that the high susceptibility of armadillos to infection with Mycobacterium leprae cannot be explained solely in terms of body temperature because mutant mice maintained with a body temperature similar to that of armadillos do not become heavily infected with M. leprae. The depression of cell-mediated immunity accompanying the low body temperature is not sufficient to produce an overwhelming infection. The results obtained with M. marinum suggest that whereas lack of cell-mediated immunity or a low body temperature result in a moderately enhanced infection in the mouse a combination of both of these factors is required to produce an overwhelming infection involving the internal organs.

Intermittent chemotherapy of experimental leprosy in mice.

In this study we assess the degree of prolonged bacteriostasis of Mycobacterium leprae after temporary exposure to ehtionamide or thiacetazone, and relate this to their efficacy when administered intermittently to mice with experimental leprosy infections. The results show that temporary exposure of M. leprae to either of these drugs results in a prolonged bacteriostatic effect, but that efficacy is rapidly lost as the interval between doses is increased. Using the mouse foot pad system, growth of M. leprae is not inhibited by thiacetazone when the frequency of administration is less than three times weekly. When ethionamide is administered once weekly, growth of M. leprae is inhibited but bactericidal activity is lost. When ethionamide is administered in combination with continuous dapsone therapy, either continuously or three times weekly, the bactericidal activity of the drug combination is greater than when either drug is administered alone. However, when ethionamide is administered once weekly in combination with continuous dapsone treatment, the bactericidal effect is identical to that when dapsone is given alone: that is, ethionamide makes no contribution to the combination.

The effect of freezing and storage in liquid nitrogen on the viability and growth of Mycobacterium leprae.

The effects of rapid and slow rates of freezing in liquid nitrogen, storage in liquid nitrogen for 12 months, and the rate of subsequent thawing on the viability and growth of M. leprae in the mouse footpad were studied. Some loss of viability of M. leprae was detected, and this was found to be associated with the freezing process, rather than with storage or thawing. Slow freezing was less deleterious than quick freezing, with a loss of viability of 90% compared with 98%. The growth pattern of M. leprae was unaffected except for a delay in the appearance of growth caused by the loss of viability, though there was some evidence of an increased lag phase of one strain, possibly due to the repair of sub-lethally damaged organisms.

Effect of rifampin, clofazimine, and B1912 on the viability of Mycobacterium leprae in established mouse footpad infection.

Continuous dietary administration of rifampin to mice with an established Mycobacterium leprae footpad infection reduced the bacillary solid ratio, with an estimated survival half-life of 5-6 days. In rifampin-treated immunosuppressed animals the survival half-life of solid bacilli, in the absence of host immunity, was 12-13 days. Clofazimine and B1912 produced a significant effect on solid ratio only after a lag period of apparently 100 days. The rate of action was considerably slower than that of rifampin. Intermittent (once monthly) administration of both drugs produced effects similar to those of continuous administration.

Effect of rifampin, clofazimine, and B1912 on the viability of Mycobacterium leprae in established mouse footpad infection

Continuous dietary administration of rifampin to mice with an established Mycobacterium leprae footpad infection reduced the bacillary solid ratio, with an estimated survival half-life of 5-6 days. In rifampin-treated immunosuppressed animals the survival half-life of solid bacilli, in the absence of host immunity, was 12-13 days. Clofazimine and B1912 produced a significant effect on solid ratio only after a lag period of apparently 100 days. The rate of action was considerably slower than that of rifampin. Intermittent (once monthly) administration of both drugs produced effects similar to those of continuous administration.