Which IMViC test requires the addition of reagents to detect specific metabolic activity?

The Voges-Proskauer test is designed to detect the ability of certain bacteria to produce acetoin, a metabolic product formed from glucose fermentation. After the culture has incubated, specific reagents—commonly alpha-naphthol and potassium hydroxide—are added to the culture. These reagents react with acetoin to produce a color change, which indicates a positive result. This color change provides a visual confirmation of metabolic activity, specifically the production of acetoin, allowing for the differentiation of bacterial species based on their metabolic pathways.

The other tests mentioned do not require the addition of reagents for this purpose. The citrate test involves a color change in the medium itself as a result of the growth of the organism on citrate as its sole carbon source. The indole test uses Kovac's reagent, but it is aimed at detecting indole production from tryptophan and not specifically measuring metabolic activity in the same manner as Voges-Proskauer. The methyl red test uses methyl red indicator to assess the pH of the medium after incubation, indicating mixed acid fermentation but doesn’t specifically involve the addition of reagents to indicate metabolic activity like acetoin production does.