oxidasepositive

Oxidase-positive is a term used in microbiology to describe organisms that produce an active cytochrome c oxidase enzyme and yield a positive result on the oxidase test. This enzyme participates in the organism’s respiratory electron transport chain, transferring electrons to oxygen. The standard oxidase test uses a redox dye, such as tetramethyl-p-phenylenediamine, which changes color when oxidized by cytochrome c oxidase. A positive result is typically seen as a rapid color change to purple within seconds.

In clinical microbiology, the oxidase test helps differentiate certain groups of Gram-negative bacteria. Most Enterobacterales are oxidase negative, whereas many other genera, including Pseudomonas species, Neisseria species, Vibrio species, Helicobacter species, and some Burkholderia and Campylobacter strains, are oxidase positive. Because the test is quick and inexpensive, it is commonly used as an initial step in identifying cultured isolates and guiding further testing.

Procedure and interpretation are straightforward but require caution. A fresh bacterial colony is tested by applying the reagent to a small amount of the culture on a swab or filter paper and observing promptly, usually within 10 to 30 seconds. A rapid purple color indicates an oxidase-positive organism, while no color change or a color change after a prolonged delay suggests oxidase negativity or an inconclusive result. Factors such as reagent age, exposure to air, and sample handling can affect outcomes, leading to false positives or negatives if not performed properly.

Limitations include that oxidase status is only one part of organism identification and does not distinguish species within oxidase-positive groups. It should be interpreted alongside morphology, growth requirements, and other biochemical tests.