Triple Sugar Iron agar slants

These Triple Sugar Iron (TSI) agar slants contain sucrose, lactose, glucose and iron (ferrous sulfate). Lactose and sucrose are both present in TSI in very large amounts (1%). If either one is fermented large amounts of acid are produced and the whole tube turns from red to yellow. Some species produce gas which breaks up the agar, producing bubbles. Glucose is present in TSI in a very small amount (0.1%) and if it is the only sugar fermented only a very small amount of acid can be produced from it. This small amount of acid is oxidized at the surface in the presence of oxygen to CO2 and H2O. Therefore, the surface of the agar slant returns to a neutral pH (red) while the oxygen-free butt remains acid (yellow). Some species may also produce gas from glucose. Organisms that produce H2S react with the ferric sulfate to form black ferrous sulfide.