Hereditary Characteristic that Varies Among Isolates of Mycobacterium leprae

Isolates of Mycobacterium leprae in mouse foot pads were found to differ in two related properties, the average rate of growth between inoculation and harvest (G) and the number of bacilli in the harvest (H). For "fast" strains the median values for G were less than 25 days per generation, and the median values for H were above 10(6.1). For "slow" strains the median values for G were above 30, and the median values for H were below 10(5.6). The G and H values for the 59 isolates for which data were available formed a continuous spectrum between the two extremes; there was no correlation with dapsone resistance. The fastness characteristic was stable; it did not change on passage in mice and was in agreement when more than one isolate had been made from the same patient. No important differences were apparent according to geographic origin of the infection of the patient. Histological studies showed that fast strains grew to a higher level without inducing the infiltrate of lymphocytes and macrophages that appears at the end of the logarithmic phase of growth in mouse foot pads. Although fast strains often had higher ratios of solidly staining (and presumably viable) bacilli in the inoculum, the fast-slow difference was not accounted for by the solid ratio. Slow strains differed from fast by having longer times until harvest and by having fewer generations of growth, even when their frequently lower solid ratios were taken into account.