Here, the organism that produces enzyme thiosulfate reductase, which reduces the Sulphur into hydrogen sulfide gas.

This occurs when strain degrades the amino acid cysteine while the degradation process of protein or during the process of protein degradation or during the process of anaerobic respiration, which shuttles the electrons to Sulphur instead of oxygen.

In Sulphur indole motility tubes, the medium contains casein and the animal proteins as the source of amino acid, sodium thiosulphate, as a source of sulfur and the ferrous ammonium sulphate, which acts as the hydrogen sulphide indicator.

Cysteine is a Sulphur containing amino acids that is present in the SIM medium.

The enzyme cysteine desulfurizes and thiosulfate reductase catalysis the reactions of hydrolysis and it produces hydrogen sulphide.

Thus, the produced hydrogen sulphide gas, combines with the ferrous ammonium sulfates and forms an insoluble, black color ferrous sulfide precipice.

This black color acts as an indicator during the presence of a hydrogen sulphide.

In the laboratory, a fresh medium of the cultured organisms is inoculated in the center of the medium in a single stab with the help of a straight needle.

After the process of incubation, the tube is further observed for the production of hydrogen sulphide gas, which is observed by the blackening of the medium.

SIM medium is generally used for this Sulphur reagent test, it consists of nutrients, iron and sodium thiosulphate.

One of the vital nutrient present in the medium is peptone, which is made up of amino acids like those of tryptophan.

However, if the organism has the capability to reduce Sulphur into hydrogen sulfide, the reduced hydrogen sulphide will combine with the iron and results in the formation of the ferric sulphide, which is shown as a black precipitate.

This is the reason for blackening of the medium and it indicates the reduction of the Sulphur in the medium and gives a positive result.