Sulfide indole motility (SIM) medium is a semisolid agar used to determine hydrogen sulfide (H₂S) production, indole formation, and motility.  SIM medium is used to differentiate members of the family Enterobacteriaceae.  Haziness that spreads from the stab line indicates a positive test for motility.  Tubes must be compared to an uninoculated tube to discriminate between faint haziness and motility.  A red color development after addition of Kovács reagent indicates indole production.  A black precipitate indicates H₂S production.  Test tubes:  (A) uninoculated tube, (B) contains the nonmotile and indole-negative bacterium Klebsiella pneumoniae, (C) contains the motile and indole-positive bacterium Escherichia coli, and (D) contains the motile, indole-negative, and H₂S-producing bacterium Proteus mirabilis.  (Renee Wilkins, University of Mississippi Medical Center, Jackson, MS)