Proteus vulgaris, a motile organism, in a sulfide indole motility (SIM) deep. The black precipitate indicates hydrogen sulfide production by the organism. SIM medium is used to test for hydrogen sulfide production, indole production, and motility. An uninoculated SIM deep is a transparent yellow semisolid (3 g/liter of agar) medium. Inoculate the SIM deep with an inoculating needle, making the stab perpendicular to the agar surface. If the organism can produce hydrogen sulfide from sodium thiosulfate, then ferrous ammonium sulfate, an iron indicator, can combine with the hydrogen sulfide to produce a black precipitate in the agar. If the organism is nonmotile and produces hydrogen sulfide, the black precipitate is observed only along the stab line. Motile organisms that produce hydrogen sulfide will produce a black precipitate throughout the tube, wherever the organism grows. SIM medium contains tryptophan. If the organism can produce tryptophanase, it can degrade tryptophan to indole, pyruvate, and ammonia. After inoculation and incubation, Kovács reagent is added to the test tube. If indole is present, a dark pink-red liquid layer will be evident on top of the agar. In the absence of indole the reagent layer remains yellow. If the organism is nonmotile, the agar remains clear and the stab line is evident. Motile organisms grow outward from the stab line and produce turbidity in the medium that obscures the stab line. It is important that the motility and hydrogen sulfide reactions be recorded prior to adding Kovács reagent. Kovács reagent contains an aldehyde that "bleaches out" the black color and makes the medium cloudy. (Diane Hartman, Baylor University, Waco, TX)