The pH of a phenol red broth __ when sugar fermentation occurs.
1) Increases
2) Stays the same
3) Decreases
4) Can increase or decrease
The Correct Answer and Explanation is:
The correct answer is 3) Decreases.
Phenol red broth is a common differential medium used to determine carbohydrate fermentation and the associated changes in pH. This broth contains phenol red, a pH indicator that changes color in response to changes in acidity. The base pH of phenol red broth is typically around 7.4 (neutral).
When a microorganism ferments sugar in the phenol red broth, it metabolizes the sugar and produces organic acids as byproducts. These acids lead to a decrease in the pH of the medium. As the pH drops below 6.8, the phenol red indicator changes from red (neutral) to yellow, signaling acid production due to fermentation. The yellow coloration indicates that fermentation has occurred, and the organism is capable of utilizing the sugar present in the broth.
The decrease in pH is significant in differentiating between organisms that can ferment sugars and those that cannot. For example, in clinical microbiology, phenol red broth can help identify certain bacteria based on their ability to ferment specific carbohydrates. If a bacterium ferments the sugar present in the broth, the production of acids results in a yellow color change, indicating a positive result for fermentation. If the bacterium does not ferment the sugar, the broth remains red, indicating a negative result for fermentation and suggesting that the organism does not utilize that sugar as a carbon source.
In summary, the pH of phenol red broth decreases when sugar fermentation occurs due to the production of organic acids by the fermenting organism, leading to a color change from red to yellow. This characteristic change is essential for microbiologists to identify and differentiate microbial species based on their metabolic capabilities.