What does the presence of bubbles in a Durham tube during a total coliform multiple tube fermentation indicate?

The presence of bubbles in a Durham tube during a total coliform multiple tube fermentation indicates that there are coliforms present in the sample. The Durham tube is specifically designed to capture gas that is produced during fermentation, which occurs when coliform bacteria metabolize the lactose present in the medium. This gas production is a sign of biological activity from coliforms, confirming their presence.

When coliforms ferment lactose, they typically produce acid and gas. The formation of gas bubbles in the inverted Durham tube is a direct indication that fermentation is occurring, thereby providing evidence that coliform bacteria, which can indicate fecal contamination and other health concerns, are present in the water sample being tested. Thus, this observation is crucial in assessing the microbiological quality of the water, especially in the context of determining potability and safety standards.