SIM Medium Butt

Principle And Interpretation

SIM Medium is used to differentiate enteric bacilli particularly Salmonella and Shigella on the basis of sulphide production, indole formation and motility (1,5). Jordan and Victorson (3) reported that Salmonella Paratyphi A and Paratyphi B can be distinguished on the basis of H2S production using lead acetate. Sulkin and Willett (7) used Triple Sugar Iron Agar with 1% agar for motility along with H2S production and carbohydrate fermentation. Sosa (6) 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. Peptonized iron and sodium thiosulphate are the indicators of H2S production. This H2S reacts with peptonized iron to form black precipitate of ferrous sulphide (6,7). Motile organisms intensify the H2S reaction. Motile organisms grow away from 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. Peptone and HM peptone B 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 (1). The indole is detected by the addition of chemical reagents following the incubation period. Inoculate fresh culture with a single stab using straight needle through the center of the medium. Following incubation, observe for motility (diffuse growth outward from the stab line or turbidity throughout the medium) and for H2S production (blackening of the medium). To detect indole production, add three or four drops of Kovacs reagent (1) and observe for development of red color (positive reaction). Determine motility and H2S production prior to determination of indole production.