What color indicates a positive oxidase test?

A positive oxidase test is indicated by the development of a dark purple color. This test is used to determine the presence of cytochrome c oxidase in bacteria. When the substrate, typically a reagent like tetramethyl-p-phenylenediamine, is added to the bacterial culture, if cytochrome c oxidase is present, it catalyzes a reaction that leads to the formation of a dark purple product. This color change signifies that the organism can perform aerobic respiration, often associated with the presence of certain bacteria that are oxidase-positive, such as Neisseria and Pseudomonas species.

The other color options do not indicate a positive reaction in the oxidase test. Blue and green are usually associated with various other processes or test reactions, while yellow does not correlate with oxidase activity. Thus, the dark purple color is both a clear and distinct indicator of a positive result in the oxidase test, confirming the presence of the enzyme and the organism's ability to utilize oxygen for respiration.