Question

Next, several biochemical tests are run to further identify the microorganism. Lactose fermentation is run for 24 hours at 37⁰C in lactose broth containing a Durham tube and phenol red indicator. The result shows a red broth with no air bubbles in the Durham tube.

What do these results mean in terms of this microorganism’s ability to ferment lactose?

Answer 1

The organism is not a lactose fermenter.

Step-by-step explanation:

The result means that the microorganism was not able to ferment lactose.

The lactose fermentation test is conducted to test the ability of a bacterium to utilize the carbohydrate, lactose, as its carbon source. A bacterium that is positive for this test would be able to ferment lactose to produce lactic acid.

In order to conduct the test, a pure culture of the microorganism is inoculated in phenol red lactose brought and incubated at the optimum temperature for about 24 hours. If the organism is able to ferment lactose, lactic acid and carbon dioxide gas would be produced. This means that the phenol red indicator would color to yellow from red and the Durham tube would contain air bubbles.

In this case, there was no change in color of the indicator nor production of gas in the Durham tube. Hence, the microorganism is concluded to be a non-lactose fermenting one.