SIM Medium is used to differentiate enteric bacilli on the basis of sulfide production, indole formation and motility.
SUMMARY AND EXPLANATION
Hydrogen sulfide production, indole formation and motility are distinguishing characteristics which aid in the identification of the Enterobacteriaceae, especially Salmonella and Shigella. SIM Medium, therefore, is useful in the process of identification of enteric pathogens.
The ingredients in SIM Medium enable the determination of three activities by which enteric bacteria can be differentiated. Sodium thiosulfate and ferrous ammonium sulfate are indicators of hydrogen sulfide production. The ferrous ammonium sulfate reacts with H2S gas to produce ferrous sulfide, a black precipitate. The casein peptone is rich in tryptophan, which is attacked by certain microorganisms resulting in the production of indole. The indole is detected by the addition of chemical reagents following the incubation period. 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.