Further Investigation on the Usefulness of the Direct Qualitative Micro-Niacin Test for Distinguishing Human Tubercle Bacilli from Other Mycobacteria

The study described here was undertaken to compare further three recently reported micro-niacin tests using both freshly isolated and stored cultures of various types of mycobacteria. When performed on the fresh cultures, it was observed that all of the human-type tubercle bacilli yielded slightly to intensely positive tests, whereas, all of the other strains, with two exceptions, gave either negative or doubtful results. As the period of incubation increased, there was a corresponding increase in both the number and degree of positive results. The bovine-type tubercle bacilli exhibited slightly positive results with each test after the 10, 14, and 21 day periods, respectively. When used on cultures of human-type tubercle bacilli from seven to ten days old, the sensitivity of the tests was observed to be as follows: (1) the benzidlne-cyanogen bromide and the o-tolidine-cyanogen bromide tests were slightly better than the aniline-cyanogen bromide method with four of the strains tested; (2) the aniline-cyanogen bromide and the o-tolidine-cyanogen bromide methods were slightly better than the benzidine-cyanogen bromide technic with six of the strains tested; and (3) the aniline-cyanogen bromide and the benzidine-cyanogen bromide procedures were slightly better than the o-tolidine-cyanogen bromide test with 47 of the strains tested. When used on the cultures of human-type tubercle bacilli from 14 to 21 days old, however, the sensitivity of all three niacin tests was virtually the same. When performed on the stored cultures, it also was observed that all of the human type tubercle bacilli exhibited slightly to intensely positive results irrespective of the period of storage or the dryness of the cultures. The drug resistant organisms yielded similar results to those of the drug susceptible strains. With two exceptions, all of the other mycobacteria exhibited either negative or doubtful tests. In the bovine group of tubercle bacilli, all of the cultures yielded slightly positive results at each test period. In the Battey group of organisms, 7 out of 100 cultures exhibited slightly positive results within the 50 to 156 week test periods. The sensitivity of each niacin test under these conditions was virtually the same. The tests are simple, efficient, safe, and economical. Reproducible and consistent results can be obtained on cultures as old as three years. In most instances, the results exhibited by human tubercle bacilli are sufficiently different in character from the results produced by other mycobacteria to enable a clear-cut differentiation of these organisms. For greater effectiveness, it is recommended that the niacin test be used in conjunction with other differential procedures.