|Description:||This image depicts two test tubes. The tube at left contained triple sugar iron (TSI) growth medium, and the tube at right contained O/F glucose (dextrose) medium. Based on this test result, each culture had been inoculated with an oxidative bacterial organism. The TSI slant tube revealed no color change post-inoculation, while the O/F glucose culture medium showed a color change from a dark greenish-brown, still visible towards the butt of the tube, to a bright yellow in the upper half of the medium.|
With TSI growth medium, the lack of any fermentative metabolism leaves the color of the medium a bright red color, for with no metabolic reactivity, there is no accompanying change in pH, and no change in the color of the pH sensitive dye indicator contained in the medium. In the case of O/F glucose medium, the glucose component was metabolized through an oxidative process. Glucose is a component of both media, however, there is 10x the concentration of glucose in the O/F glucose medium than within the TSI medium. Therefore, the acidity created by the oxidative carbohydrate metabolism in the O/F glucose medium was enough to change the innate bromthymol blue indicator yellow, while any change in the pH experienced in the TSI culture was insufficient to change the innate phenol red indictor to yellow. The required presence of oxygen in the oxidative process caused the agar in the upper half of the O/F glucose tube to turn yellow, while the bottom half remained unchanged.