Microscopic counts carried out on Mycobacterium leprae and M. tuberculosis suspensions. A comparison of three...

Standard smears of heat-killed Mycobacterium leprae and M. tuberculosis H37Rv were counted microscopically following staining by the Ziehl-Neelsen, auramine, and silver-methenamine methods. The numbers of stained bacillary bodies were consistently higher in the silver-methenamine stained smears compared to the Ziehl-Neelsen and auramine stained smears. The auramine smears were examined under ultraviolet illumination and permitted the enumeration of the brightly fluorescent bacilli against a black background. The auramine counts were not as high as those obtained using the silver-methenamine stained preparations but were consistently higher than those obtained with the Ziehl-Neelsen preparations. Both the auramine and silver-methenamine stained preparations clearly outlined the cell walls of the bacilli in the smears and this greatly facilitated the counting process, especially if the cell suspension was badly clumped.