Purpose: To differentiate between bacteria based on three tests: sulfur reduction (cysteine desulfurase), indole production (tryptophanase), and motility.
Principle: If a bacterium can produce the enzyme tryptohanase then it can use the amino acid tryptophan as a carbon and energy source (as pyruvate). One of the biproducts of this conversion is indole, which is detected with Kovac's reagent. If the bacterium posseses the enzyme cysteine desulfurase, sulfur containing amino acids will be broken down into pyruvate, ammonia and hydrogen sulfide. Iron in the medium reacts with hydrogen sulfide producing the characteristic black percipitate. Motility is observed as growth away from the stab line, but cannot be detected if hydrogen sulfide is formed.
The figure to the left represents the following Sulfur-Indole-Motility Results:
A. Indole positive and hydrogen sulfide positive
B. Hydrogen sulfide positive
C. Indole positive and motility positive (note fuzzy growth away from stab line)
D. Negative Control