Nitrite Reduction Test
A visual guide to the nitrite reduction test, used to determine whether an organism can reduce nitrites to gaseous nitrogen or other nitrogenous compounds.
Nitrite Reduction Broths A-B
Differentiate reduction of nitrite to nitrogen gas (colorless, gas positive) from a negative reaction (red) after adding reagents A & B.
No color change after adding Reagents A and B (and zinc dust); gas bubble present in the Durham tube. Nitrite was reduced to gaseous nitrogen.
Broth becomes red after addition of Reagents A and B; no gas production. Nitrite remains in the medium and is unreduced.
No color change after adding Reagents A and B (and zinc dust); gas bubble present in the Durham tube. Nitrite was reduced to gaseous nitrogen.
Broth becomes red after addition of Reagents A and B; no gas production. Nitrite remains in the medium and is unreduced.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| No color change to red | Nitrite has been reduced to gaseous nitrogen or other compounds | Positive — nitrite reduced |
| Gas in Durham tube | Gaseous nitrogen produced from nitrite reduction | Positive gas indicator |
| Red color | Nitrite is still present and reacted with reagents A and B | Negative — nitrite not reduced |
| Zinc dust addition | Added if no color and no gas to check for oxidation to nitrate; red after zinc indicates oxidation occurred | Confirms negative if red |
Nitrite Reduction Interpretive Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| Colorless, gas present | Positive | Nitrite reduced to gaseous nitrogen (e.g., Proteus mirabilis). |
| Colorless, no gas (stays colorless after zinc) | Positive | Nitrite reduced to nongaseous nitrogen compounds. |
| Red after Reagents A & B | Negative | Nitrite remains unreduced (e.g., Acinetobacter baumannii). |
| Red after Zinc dust | Invalid/Negative | Nitrite was oxidized to nitrate during incubation. |
- This test specifically asks if an organism can reduce nitrite (NO2-) further down the denitrification pathway.
- A red color means nitrite is still present (Negative).
- A colorless broth with gas means nitrite was completely reduced to nitrogen gas (Positive).
Nitrite test logic: Red means the starting ingredient (nitrite) is still there, so the organism didn't use it. Red = Negative.