In a TSI slant, Triple Sugar Iron test, bacteria that ferment any of the three sugars in the medium will produce byproducts and these byproducts are usually acids.
The Triple Sugar Iron (TSI) test is a microbiological test roughly named for its ability to test a microorganism's ability to ferment sugars and to produce hydrogen sulfide. 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. 
