During a total coliform multiple tube fermentation, when is the sample dilution considered positive?

In a total coliform multiple tube fermentation test, the presence of gas bubbles in the Durham tube is a crucial indicator of a positive result. The test is based on the principle that if coliform bacteria are present in the sample, they ferment lactose and produce gas as a by-product. The Durham tube, which is an inverted tube placed in the culture medium, captures this gas. When gas bubbles form in the Durham tube, it signifies that coliform bacteria have metabolized lactose, indicating fecal contamination.

The other aspects of the test also relate to microbial activity but are not definitive indicators of a positive result. For instance, a color change can occur for various reasons not directly tied to coliform presence, and the absence of color change or sample sedimentation does not provide evidence of coliform activity. Therefore, the observation of gas bubbles in the Durham tube serves as the most reliable confirmation of a positive result in this testing method.