Question

2 Explain why the TSI medium contains a lower concentration of glucose than of lactose and sucrose. 3. Explain the purpose of the phenol red in the medium. 4. Explain the purpose of thiosulfate in the medium.

Answer 1

2. The medium contains 10 times as much lactose and sucrose as glucose. Bacteria that ferment glucose produce a wide variety of acids, turning the color of the medium from red to yellow. When any sugar is metabolized under anaerobic conditions (fermentation), there is increased acid production, thus the butt of the tube remains acid for a longer period of time. When glucose only is fermented, acid is produced throughout the medium but due to the relatively low concentration of glucose in the medium and the aerobic metabolism that is present on the slant, nitrogenous metabolic end products soon change the reaction of the slant to alkaline. These alkaline products neutralize the small amounts of acids present in the slant but are unable to neutralize the large amounts acid present in the butt. Thus, the appearance of an alkaline (red) slant and an acid (yellow) butt after 24 hours incubation indicates that the organism is a glucose fermenter but is unable to ferment lactose and sucrose. Bacteria that ferment lactose or sucrose (or both), in addition to glucose, produce such large amounts of acid that the oxidative deamination of protein that may occur in the slant does not yield enough alkaline products to cause a reversion of pH in that region. Thus, these bacteria produce an acid slant and acid butt .
3.To detect the fermentation of carbohydrates by change of the medium from an orange-red to a yellow color.
-phenol red pH 7,
changes to yellow at slightly acidic 6.8
4. A substrate for hydrogen sulfide production. Terminal electron acceptor, reducing bacteria to sulfide and H2S .