SIM Medium is a semisolid medium used for the determination of indole formation, sulfide production, and motility of the Enterobacteriaceae, especially Salmonella and Shigella. Ferrous Ammonium Citrate and Sodium Thiosulfate are used to detect hydrogen sulfide production by Salmonella. H2S gas reacts with Ferrous Ammonium Citrate to produce ferrous sulfide, a black precipitate. Casein Peptone contains tryptophan, which is converted to indole by Shigella. Indole is detected after incubation by the addition of Kovac’s reagent, which reacts with indole to produce a red color. The medium is semi-solid due to the low concentration of agar, and motility is easily seen by growth radiating from original stab as opposed to non-motile organisms, which grow along the inoculation line.