You were distracted by another student in the classroom and forgot your oxidase test for 15 minutes. When you check on the result, you notice a dark blue ring but no added coloration to the colony smear. What can you conclude from this result?

The blue ring around was caused by the reaction of the oxidase reagent with the filter paper

Explanation:

Generally the student must have performed filter paper method of oxidative test. Based on standard, when the colony was applied to the filter paper, within 10 - 30s a deep-blue or purple coloration should indicate oxidation, and catalysis of reduction by oxidase enzyme, to indicate the bacteria that produce the cytochrome oxidase.

However, when the reaction was left for long minutes, the oxidative reagents reacts with the paper based medium of the reaction. These reagents are tetramethyl-p-phenylenediamine (which the filter paper was soaked in) and iodophenol. The oxidase enzyme catalyses tetramethyl-p-phenylenediamine to iodophenol; which gives the usual purple or blue coloration end products. However, when the reaction was left for long these reagents reacted with the filter paper to give dark blue ring around each of the microorganism.

The result is a Positive Oxidase test

Explanation:

The oxidase test is used to identify bacteria that produce cytochrome c oxidase, an enzyme of the bacterial electron transport chain. (note: All bacteria that are oxidase positive are aerobic, and can use oxygen as a terminal electron acceptor in respiration. This does NOT mean that they are strict aerobes.

The reagent is a dark-blue to maroon color when oxidized, and colorless when reduced. Oxidase-positive bacteria possess cytochrome oxidase or indophenol oxidase (an iron-containing hemoprotein). These both catalyze the transport of electrons from donor compounds (NADH) to electron acceptors (usually oxygen).