SIM Medium

SIM medium is a combination differential medium that tests three different parameters, which are represented by the three letters in the name:

• Sulfur Reduction

• Indole Production

• Motility

The sulfur reduction test is useful in differentiating enteric organisms.  The indole test is a component of the IMViC series of tests, which is used for differentiating the Enterobacteriaceae.  The motility test is useful for testing a wide variety of organisms.  As a whole, the SIM test is primarily useful for differentiating Salmonella and Shigella.

SIM medium contains nutrients, iron, and sodium thiosulfate. One of the nutrients is peptone, which contains amino acids, including tryptophan.

If an organism can reduce sulfur to hydrogen sulfide, the hydrogen sulfide will combine with the iron to form ferric sulfide, which is a black precipitate. If there is any blackening of the medium, it indicates the reduction of sulfur and is a positive result.

The sulfur and motility test results should be determined before you perform the indole test.

Some bacteria possess the ability to produce the enzyme tryptophanase, which hydrolyzes tryptophan.  The end products of this hydrolyzation are indole, pyruvic acid, and ammonia, by way of deamination. The Kovac’s reagent that you add to the SIM medium to test for indole contains hydrochloric acid, p-dimethylaminobenzaldehyde (DMABA), and n-amyl alcohol. DMABA reacts with indole to produce a red quinoidal compound.  If the reagent turns red, the indole test is positive.

Before the Kovac's reagent has been added:

The stab line in the SIM medium on the left is sharp; the Klebsiella pneumoniae is non-motile. The SIM medium in the middle is cloudy throughout; the Escherichia coli is motile. The SIM medium on the right is cloudy and black; the Salmonella typhimurium is motile and H2S positive.

The Kovac’s reagent in the tube on the left stayed yellow; the Klebsiella pneumoniae is indole negative. The Kovac’s reagent in the tube on the right turned pink; the Escherichia coli is indole positive.