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[edit] Triple Sugar Iron (TSI) test

The Triple Sugar Iron test is a microbiological test roughly named for its ability to test microorganism's ability to ferment sugars and utilize iron to produce hydrogen sulfide. Is is often used in the selective identification of enteric bacteria.

The TSI slant is a test tube which contains agar, a pH-sensitive dye, 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).

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 to a yellow color. Position of the color change distinguishes acid production from glucose from acid production from lactose or sucrose. Many bacteria that can ferment sugars in the anaerobic butt of the tube are enterebacteria.

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. This is the Sulfide.

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.