Effects of purification and fluorescent staining on viability of Mycobacterium leprae

Over the years, researchers have carried out experiments with Mycobacterium leprae obtained from either human multibacillary lesions, or infected armadillo tissues, or infected footpad tissues of conventional mice as well as athymic nu/nu mice. In general, these sources of leprosy bacilli are satisfactory for most biochemical and mouse footpad studies, but less than satisfactory for studies in cell biology and immunology where contaminating host tissues pose a serious problem. We examined the utility of a procedure for eliminating mouse footpad tissue from M. leprae suspension using sodium hydroxide solution and its subsequent effect on the viability of the organism by determining the rate of palmitic acid oxidation, bacterial membrane integrity, and growth in the mouse footpad. We found that treating M. leprae suspension, obtained from infected nu/nu mouse footpad, with 0.1N NaOH for 3 min was sufficient to remove the majority of mouse tissue without adversely affecting the viability of the organism. This is a simple and rapid method to get suspensions of nu/nu footpad-derived viable M. leprae essentially free of host tissues, which can be a research reagent for studying the host-pathogen relationship in leprosy. We also report here a method for labeling M. leprae with the fluorescent dye PKH26, without compromising on the viability of the organism. This method may be useful in intracellular trafficking studies of M. leprae or in other cell biology studies that require tracking of the bacteria using fluorescent tag. We observed the staining to be stable in vitro over considerable lengths of time and did not affect the viability of the bacteria.