TSI slant

The Triple Sugar Iron (TSI) test is a microbiological test roughly named for its ability to test microorganism's ability to ferment sugars and to produce hydrogen sulfide. It is often used in the selective identification of enteric bacteria including Salmonella and Shigella.

The TSI slant is a test tube that contains agar, a pH-sensitive dye (phenol red), 1% lactose, 1% sucrose, 0.1% glucose, as well as sodium thiosulfate and ferrous sulfate or ferrous ammonium sulfate.

All of these ingredients are mixed together and allowed to solidify in the test tube at a slanted angle. The slanted shape of this medium provides an array of surfaces that are either exposed to oxygen-containing air in varying degrees (an aerobic environment) or not exposed to air (an anaerobic environment).

TSI agar medium was developed based on Kligler's iron agar, which had been used for the determination of lactose fermentative bacteria, by addition of sucrose to be able to detect sucrose fermentative bacteria also.

Bacteria that ferment any of the three sugars in the medium will produce byproducts. These byproducts are usually acids, which will change the color of the red pH-sensitive dye (phenol red) to a yellow color. Position of the color change distinguishes the acid production associated with glucose fermentation from the acidic byproducts of lactose or sucrose fermentation. Many bacteria that can ferment sugars in the anaerobic butt of the tube are enterobacteria.

Some bacteria utilize thiosulfate anion as a terminal electron acceptor, reducing it to sulfide. If this occurs, the newly formed hydrogen sulfide (H
2S) reacts with ferrous sulfate in the medium to form ferrous sulfide, which is visible as a black precipitate. Examples of sulfide-producing bacteria include Salmonella, Proteus, Citrobacter and Edwardsiella species. The blackening of the medium is almost always observed in the butt (bottom) of the medium.

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