A Study of the Neutral Red Reaction for Determining the Virulence of Mycobacteria

Abstract

A study of the Dubos neutral red cytochemical reaction as an indicator of the virulence of strains of mycobacteria isolated from clinical material has yielded information which suggests that the method measures the virulence of a strain as satisfactorily as the conventional inoculation of guinea pigs. 195 acid-fast organisms were examined, 180 of them isolated from patients; 168 of the 180 were culturally consistent with Mvcobacterium tuberculosis and 12 were acid-fast chromogens. 13 of the remaining 15 acid-fast organisms were lyophilized stock mycobacteria, comprised of 4 human, 3 avian, 2 bovine strains, and 4 non-pathogens. The 4 non-pathogens included M. laticola, M. stercoria, M. leprac and M. phlei. The human virulent strain, H37Rv, and its avirulent variant, H37Ra, completed the group studied. 163 of the 168 acid-fast strains culturally consistent with M. tuberculosis were neutral red positive and produced characteristic disease in the guinea pig. Only 2 of the 165 neutral red positive acid-fast strains isolated from clinical material failed to infect the guinea pig in repeated tests and these 2 strains, when inoculated into chickens, gave results which indicated that they were avian variants. Three of the 168 isolates were negative by both the neutral red and animal virulence tests. The 12 chromogenic strains were negative by both tests. Thus, for the application of the 2 methods in the determination of virulence, 166 of 168 acid-fast strains culturally identified as M. tuberculosis gave identical results. All the virulent stock strains, whether human, avian, or bovine, were positive by the cytochemical test. One human, 1 bovine, and 2 avian strains failed to produce characteristic disease in the guinea pig although the human and bovine strains were virulent for this animal when first isolated. Guinea pigs, of course, are not usually susceptible to avian strains. None of the known non-pathogens gave a neutral red reaction nor induced detectable disease in the exptl. animal.