SIM Medium is a semisolid medium used for the determination of indole formation, sulfide production and motility of the Enterobacteriaceae, especially Salmonella and Shigella. The SIM Medium is formulated to detect sulfide production, indole formation and production. Ferrous ammonium citrate and sodium thiosulfate are used to detect hydrogen sulfide production. H2S gas reacts with ferrous ammonium citrate to produce ferrous sulfide, a black precipitate. Casein peptone contains tryptophan, which is converted to indole. Indole is detected after incubation by the addition of Kovac’s reagent which reacts with indole to product a red color. The medium is semi-solid due to a low concentration of agar and motility is easily seen by growth a radiating from original stab.