Oxidase Test
The oxidase test detects cytochrome c oxidase and is a key early branch for many Gram-negative organisms.
Core idea
Oxidase helps separate Enterobacterales from oxidase-positive nonfermenters, curved rods, Neisseria, and other important groups.
Principle
A positive reaction turns the oxidase reagent purple within the expected reading time.
Visual reactions
Oxidase Branch Points
| Result | Common branch | What to do next |
|---|---|---|
| Negative | Enterobacterales or Acinetobacter-like organisms | Use MacConkey pattern and biochemical reactions |
| Positive | Pseudomonas-like, Vibrio/Aeromonas-like, Neisseria-like, or other groups | Use morphology, media, and specimen source |
| Delayed color | May be invalid depending on timing | Repeat and read within the approved window |
Basic workflow
- Use fresh growth and the correct reagent.
- Avoid media or tools that can interfere with the reaction.
- Apply organism to reagent paper or approved test system.
- Read within the required time window.
- Use the result with Gram stain, colony, and media behavior.
How to read it
- Oxidase negative supports Enterobacterales or Acinetobacter-like branches in the right setting.
- Oxidase positive supports Pseudomonas-like, Vibrio/Aeromonas-like, Neisseria-like, or other branches.
- Delayed color change may be invalid depending on lab rules.
High-yield trapCommon mistakes
- Reading the test too late.
- Using metal loops or interfering media.
- Assuming oxidase positive means Pseudomonas.
ShortcutStudent shortcut
For Gram-negative rods, oxidase tells you which road you are on before the details start.