Indole is a by-product of the metabolic breakdown of the amino acid tryptophan used by some microorganisms. The presence of indole in a culture grown in a medium containing tryptophan can be readily demonstrated by adding Kovac's reagent to the culture. If indole is present, it combines with the reagent to produce a brilliant red color. If it is not present, there will be no color except that of the reagent itself. This test is of great value in the battery used to identify enteric bacteria, as you will see in Exercises 24 and 25.
Hydrogen sulfide is produced when amino acids containing sulfur are metabolized by microorganisms. If the medium contains metallic ions, such as lead, bismuth, or iron (in addition to an appropriate amino acid), the hydrogen sulfide formed during growth combines with the metallic ions to form a metal sulfide that blackens the medium (see colorplates 17 and 19).
The most convenient medium for testing for indole and/or hydrogen sulfide production is SIM medium (SIM is an acronym for sulfide, indole, and motility). This is a tubed semisolid agar that can also be used to demonstrate bacterial motility. It is inoculated by stabbing the wire loop (or preferably a straight wire inoculating needle) straight down the middle of the agar to about one-fourth the depth of the medium and withdrawing the wire along the same path.
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