Question

in the presence/absence test for coliforms, a 100-ml water sample is added to a lactose-containing broth that is selective for gram-negative rods and contains an inverted tube to trap gas. if coliforms are present, ________ .

Answer 1

If coliforms are present in the water sample, they ferment lactose in the selective broth causing gas production, which is seen in the inverted tube, and a color change from red to yellow due to acid produced from lactose fermentation.

Step-by-step explanation:

In the presence/absence test for coliforms, if coliforms are present, they will ferment lactose in the selective broth, leading to the production of gas which accumulates in the inverted tube. Additionally, a pH indicator in the broth will cause the media to change color from red to yellow, indicating acid production from lactose fermentation. The amount of coliform bacteria can then be estimated using the most probable number (MPN) method, where growth in a series of lactose broth tubes at different dilutions suggests a probable number of coliforms per 100 mL of the water sample being tested.