SIM Media

SIM media is a 3-in-1 test. It includes Sulfur, Indole, and Motility testing. It is one of the few media that requires an inoculating needle instead of a loop. The needle is inserted at the top middle of the media with organism, the needle is moved to the bottom of the tube, then out the top in as straight a line as possible. This semi-solid media is created with casein and animal tissue for protein, an iron compound, and sodium thiosulfate.

Indole is a molecule created as a byproduct to tryptophan digestion. Kovac's reagent (p-Dimethylaminobenzaldehyde + HCl +Amyl Alcohol) is added to the top of the media. The combination of all of these reagents causes the production of rosindole dye and a red color in the reagent fluid. This red color confirms Indole production.

Thiosulfate is reduced to hydrogen sulfide gas by certain bacteria. This gas is colorless but will react with some iron compounds to form a jet black coloration in the media.

Motility can be seen in this media as it is semi-solid, and has reduced effect from random motion and diffusion. A motile organism will cause a cloudy appearance to the media while a non motile organism will be clear everywhere other than the stab line. A positive sulfur reduction test can mask the results of the motility test. This is often not a problem, as sulfur reduction is often better for identifying organisms than motility is.