Use of pyridine for differentiating Mycobacterium leprae from other mycobacteria in direct microscopy

The loss of acid-fastness by M. leprae after two-hour pyridine extractions, reportedly a specific test for differentiating M. leprae from all other mycobacteria, was verified on different materials obtained from leprosy patients, histologic sections from a fatal post-BCG vaccination case and smears prepared from pure cultures of 32 strains of 18 different mycobacterial species. Under the conditions used, pyridine extraction led to complete loss of acid-fastness in M. leprae only in histologic sections of biopsy specimens from leprosy patients, whereas in direct smears from skin lesions containing M. leprae the number of acid-fast rods after pyridine extraction was either equal to or only slightly smaller than in control preparations. Moreover, since smears from pure cultures of M. avium, M. diernhoferi, M. fortuitum, M. scrofulaceum, M. vaccae and especially M. phlei displayed a smaller or greater number of nonacid-fast cells as well (in some instances only 10% to 20% of cells were found stained whereas control slides contained 90% to 100% acid-fast rods), loss of acid-fastness after two-hour pyridine extraction cannot be considered a property typical of M. leprae only.