SIM MEDIUM

SIM Medium is used to differentiate enteric bacilli particularly Salmonella and Shigella on the basis of sulphide production, indole formation and motility. Jordan and Victorson reported that Salmonella Paratyphi A and Paratyphi B can be distinguished on the basis of H2S production using lead acetate. Sulkin and Willett used Triple Sugar Iron Agar with 1% agar for motility along with H2S production and carbohydrate fermentation. Sosa described a peptone medium with low agar for motility and indole determination.

SIM Medium enables determination of three characteristics by which enteric bacteria can be differentiated. Motile organisms intensify the H2S reaction. Motile organisms grow away from the line of inoculation showing diffused growth while non-motile organisms grow along the stab line. Motility detection is possible due to the semisolid nature of the medium. Growth radiating out from the central stab line indicates that the test organism is motile. Inoculate fresh culture with a single stab using a straight needle through the center of the medium. Allow to incubate and look for motility (diffuse growth away from the stab line or turbidity of the medium) and H2S production (blackening of the medium). Indole production is detected by adding 3-4 drops of Kovacs reagent and looking for the appearance of red color (positive reaction). Determine motility and H2S production prior to determination of indole production. Peptone and beef extract provides nitrogenous and carbonaceous compounds, long chain amino acids, vitamins and other essential nutrients. Tryptophan from peptone is degraded by specific bacteria to produce indole. The indole is detected by the addition of chemical reagents following the incubation period. Peptonized iron and sodium thiosulphate are the indicators of H2S production. This H2S reacts with peptonized iron to form black precipitate of ferrous sulphide.

for determination of hydrogen sulphide production, indole formation and motility of enteric bacilli