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 yellow broth with air bubbles in the Durham tube.

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

Answer 1

The lactose fermentation test is done to see if the microbe can ferment the carbohydrate (sugar) lactose as a carbon source. When the lactose is fermented it produces acids as one of the end product and resulting in decreased pH .when the pH indicator ie, phenol red is added to the broth the colour change .At neutral pH phenol red is red in colour. As the PH is decreased to 6.8 or less the colour change to yellow .

The yellow colour in the broth indicate bacteria present in the broth can ferment lactose .

The bubble which are produced in the Durham tube, indicate a positive result for gas production. If bubbles are not produced in tube indicate that bacteria does not produce gas from fermentation of lactose .It means the broth contain an aerogenic bacteria.some example of bacteria which give positive results in lactose fermentation test.

Escherichia coli

Enterobacter aerogenes

Klebsiella pneumoniae

Positive result indicate the ability of microorganisms to ferment glucose to lactose,lactose to acid and gas end products.The positive test also help to identify the sulfur reducers.