Discrepant Tests for Hydrogen Sulfide.

Kligler and Triple Sugar Iron (TSI) media have been considered to be interchangeable for the purpose of detecting hydrogen sulfide production in the family Enterobacteriaceae. Contrary evidence is presented in this report. From various sources, 11 cultures were isolated which yielded a positive hydrogen sulfide reaction on Kligler medium and a negative reaction on TSI. Biochemically, these cultures appear to belong to the Citrobacter group. In Kligler medium, these cultures react like the Salmonella and Arizona groups. On TSI medium, the reactions resemble the coliform group. Therefore, the classification of these cultures depends on which of the two media is employed. The chief difference between the media is the presence of sucrose in TSI medium. Presumably, other differences in the formulas are not significant. All 11 cultures ferment sucrose, but not lactose, promptly. Therefore, the utilization of sucrose appears to be responsible for the discrepancy. The acid products of sucrose fermentation may suppress the enzyme mechanism which forms hydrogen sulfide. To test this, lead acetate strips were placed in the tops of TSI cultures. Blackening of the strips indicated that hydrogen sulfide gas was being produced, although no blackening developed in the butts. Therefore, it appears that the iron sulfide indicator in the medium has been masked. Since many strains of Proteus vulgaris ferment sucrose and produce hydrogen sulfide, it seemed desirable to determine whether these cultures would yield the same results as the 11 Citrobacter cultures. The two media were inoculated with 18 strains of P. vulgaris. Blackening occurred in both media, although it was visibly more intense in Kligler medium. It was observed that, when these cultures were incubated several days longer, the iron sulfide disappeared gradually until all was gone, from TSI medium but not from Kligler medium. When the Proteus and Citrobacter cultures were inoculated into sucrose Purple Broth Base, acid production was evident in all of the cultures in a few hours. Therefore, the rate of utilization of sucrose did not account for the negative hydrogen sulfide tests.