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


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:

Streptococcus pyogenes is negative for motility and sulfur reduction.
Serratia marcescens is positive for motility and negative for sulfur reduction.
Salmonella typhimurium is positive for motility and for sulfur reduction.
The tube on the far right has not been inoculated.

With the Kovac's reagent added:

Streptococcus pyogenes is negative for indole production. 
Escherichia coli is positive for indole production.
Salmonella typhimurium is negative for indole production. 
Proteus vulgaris is positive for indole production.