Which reagent is used to detect the presence of indole?

The presence of indole is detected using p-Dimethylaminobenzaldehyde, commonly known as Kovac's reagent. When this reagent is added to a culture medium that contains tryptophan, it reacts with indole, which is produced when tryptophan is broken down by certain bacteria. The reaction typically results in the formation of a red or pink ring at the top of the medium, indicating a positive test for the presence of indole.

The other reagents listed do not specifically react with indole. Bromcresol purple is used as a pH indicator and is not involved in indole detection. Methyl red serves as an indicator for mixed acid fermentation but does not interact with indole. Cytochrome oxidase is a reagent used to test for the presence of cytochrome c oxidase in certain bacteria, unrelated to indole detection. Thus, p-Dimethylaminobenzaldehyde is the appropriate choice for identifying indole production in microbial culture.