Question

Lab name: Phenol Red Broth. (please be detailed with. your answer)

2. Early formulations of this medium used a smaller amount of carbohydrate and occasionally produced false alkaline (pink) results after 48 hours. Why do you think this happened? List at least 2 steps, as a microbiologist, you could take to prevent the problem.

3. Suppose you inoculate a PR broth with a slow-growing fermenter. After 48 hours, you see slight turbidity but score (-/-). Is this a false positive or a false negative. Is this failure a result of a lack of specificity or sensitivity of the test system?

Answer 1

Answer 1)

Phenol red broth is a test used to distinguish gram negative enteric bacteria like E.coli. This test indicate the acid production by changing the colour of liquid after incubation to yellow, thus indicating a pH drop because of acid formation. Acid is formed by the carbohydrates fermentation in the liquid.
You have used small amounts of carbohydrates in the early formulations of this medium that's the reason that it produces false alkaline (pink) results after 48 hours. Pink colour comes when the pH reaches above 7.4, making it alkaline because of ammonia production due to deamination of amino acids supplied by peptones means degradation of peptones causes this.
2 steps to prevent this are:
1. As you have added small amount of carbohydrates, they are degraded quickly therefore more amount of carbohydrates should be added. Almost equal to the protein.
2. Maintaining anaerobic conditions to avoid fermentation of carbohydrates.
3. Take proper care to avoid contamination of tubes.

Answer 2) This is false negative. Because when you inoculated a PR broth with a slow-growing fermenter but after 48 hours, you saw slight turbidity but score (-/-). This means that the organism is growing in the medium as turbidity can be seen but it is not producing any products ( acid or alkaline) which will change the colour of phenol red broth. The organism with such characteristics show poor sensitivity by not producing any products, thus giving false negative results.

Hope this helps. A thumbs up would be highly appreciated. Thanks in advance. Have a great day ahead.