If coliforms are present, the gas they produce will begin to form bubbles in each inverted vial within what period?

• A. 24-Hour
• B. 48-Hour
• C. 12-Hour
• D. 72-Hour

Answer: (B)

Gas production here comes from lactose fermentation by coliforms. The inverted vial (Durham tube) traps any gas that forms, so you can see bubbles when fermentation occurs. Not all coliforms ferment quickly—some are slower—so the incubation period is set long enough to catch those slower organisms. Incubating up to 48 hours ensures that, by then, gas bubbles will appear if coliforms are present. If no gas is seen at 48 hours, you’d interpret that as negative for lactose fermentation under these conditions. This is why the period used is 48 hours rather than shorter times.