Final answer:
The indicator for the reaction where L-cystine reduces oxygen to water could be a redox-sensitive dye such as N,N-diethyl-p-phenylenediamine (DPD), which changes color upon oxidation or reduction.
Step-by-step explanation:
The student is asking about an indicator for the reaction in which L-cystine in the media reduces oxygen to water. In microbiology, this type of reaction is often monitored using various indicators that change color in response to chemical changes, such as the redox state of the media. One common approach to detect chemical reactions involving oxygen is the use of redox indicators.
A potential indicator for the reduction of oxygen to water by L-cystine might be something similar to the reagent used in the oxidase test, which relies on the color change of a substance when oxidized. In the given reaction, an indicator like N,N-diethyl-p-phenylenediamine (DPD), which turns red upon oxidation, can be utilized to visually demonstrate the reaction. This substance is used for free chlorine detection, where it is oxidized by chlorine to a red color and then back-titrated to its colorless form. However, since L-cystine is acting as a reducing agent, the indicator would lose its color upon reduction by L-cystine.