TSI slant
The Triple Sugar Iron or TSI test is a microbiological test roughly named for its ability to test microorganism's ability to ferment sugars and to utilize iron to produce hydrogen sulfide. It is often used in the selective identification of enteric bacteria including but not limited to Salmonella and Shigella.
Composition
The TSI slant is a test tube which contains agar, a pH-sensitive dye (phenol red), high concentrations of lactose and sucrose, and a low concentration of glucose as well as sodium thiosulfate and ferric citrate. 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 also sucrose fermentative bacteria.
Interpretation of results
Bacteria which 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 are among those that can reduce the thiosulfate anion to sulfide, using the thiosulfate as an electron acceptor to create an iron sulfide precipitate, which appears black. Salmonella is one sulfide-producing bacteria.
Under anaerobic conditions (as occur toward the bottom of the tube) some bacteria use H+ as an electron acceptor and reduce it to hydrogen gas. This is not very soluble and may accumulate as bubbles along the inoculation track, between the agar and the glass, or in the fluid which accumulates at the bottom of the slant. Hydrogen production may lift the agar from the butt of the tube or fracture the agar. Carbon dioxide, if produced, may not manifest as bubbles because it is far more soluble in the medium.