Fermentation Media Visual Guide
An original broth-tube visual for reading carbohydrate fermentation as acid production, with gas captured separately in a Durham tube.
Fermentation broths A-D
Read color for acid, then inspect the Durham tube for gas. The two observations answer different questions.
Carbohydrate is fermented and gas is trapped in the Durham tube.
Fermentation produced acid, but gas is not detected.
Growth without acid shift or gas. Interpret only after the validated incubation time.
Use the baseline tube to judge true color movement.
Carbohydrate is fermented and gas is trapped in the Durham tube.
Fermentation produced acid, but gas is not detected.
Growth without acid shift or gas. Interpret only after the validated incubation time.
Use the baseline tube to judge true color movement.
Read acid and gas separately
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Pink/yellow acid shift | Carbohydrate fermentation produced acid | Fermentation positive |
| Bubble in Durham tube | Gas was produced during metabolism | Gas positive |
| No color shift | No acid detected from that carbohydrate | Fermentation negative or delayed |
| Unchanged control | Baseline appearance for the medium and indicator | Do not report as an isolate reaction |
Fermentation study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Acid + gas | Fermentation positive, gas positive | Report acid production and gas production for the tested carbohydrate. |
| Acid only | Fermentation positive, gas negative | The carbohydrate was fermented, but gas was not produced or detected. |
| No acid, no gas | Fermentation negative or delayed | Interpret with incubation time, growth, and organism group. |
| Late alkaline shift | Possible peptone use after carbohydrate is exhausted | Do not let late reversion erase an earlier valid acid result unless your method says to. |
- Fermentation media answer whether a specific carbohydrate produced acid.
- The Durham tube answers whether gas was produced.
- Indicator systems differ, so learn the color logic for the medium in front of you.
Best mental model: color is acid; Durham bubble is gas; timing decides whether the read is valid.