Relationship Between the Staining Quality of Mycobacterium leprae and Infectivity for Mice

The proposal has been made that only solidly staining forms of Mycobacterium leprae are viable. On the basis of a previous study, solidly staining bacilli were defined as those that stained completely and darkly throughout their length. A study was carried out to correlate the proportion of solidly staining bacilli in inocula with the infectivity and rate of appearance of bacillary growth in inoculated mice. The inocula originated in skin biopsy specimens of patients and in mouse passage material; there were 347 inocula suitable for study. The rate of appearance of bacillary growth in inoculated mice confirmed the hypothesis that nonsolid bacilli are inert and nonviable and that all growth originates from the solidly staining organisms. The minimal infectious dose was determined in four suspensions and was found to range from 40 to 3 solidly staining bacilli.