Explain how to interpret a carbohydrate fermentation test that uses a pH indicator and a Durham tube.

Explain how to interpret a carbohydrate fermentation test that uses a pH indicator and a Durham tube.

• A. A color change indicates acid production; gas in the Durham tube indicates gas production during fermentation.
• B. A color change indicates alkaline production; no gas indicates fermentation.
• C. No color change indicates fermentation.
• D. Gas production indicates oxygen consumption.

Answer: (A)

When you run a carbohydrate fermentation test with a pH indicator and a Durham tube, you’re detecting two things about what the microbe does to the sugar. If the organism ferments the sugar to organic acids, the pH drops and the indicator changes color (typically from red to yellow). If the organism also produces gas during fermentation, that gas is captured in the inverted Durham tube, seen as a bubble. So the color change tells you acid production, and a bubble in the Durham tube tells you gas production. If neither signal appears, no fermentation occurred. If you see a color change without gas, fermentation happened without gas production; if you see gas without a color change, that would be unusual for this setup and may indicate a different metabolic outcome or an experimental issue.

When you run a carbohydrate fermentation test with a pH indicator and a Durham tube, you’re detecting two things about what the microbe does to the sugar. If the organism ferments the sugar to organic acids, the pH drops and the indicator changes color (typically from red to yellow). If the organism also produces gas during fermentation, that gas is captured in the inverted Durham tube, seen as a bubble. So the color change tells you acid production, and a bubble in the Durham tube tells you gas production. If neither signal appears, no fermentation occurred. If you see a color change without gas, fermentation happened without gas production; if you see gas without a color change, that would be unusual for this setup and may indicate a different metabolic outcome or an experimental issue.