Embedded Files
General Information About Phenol Red Broth
General Information About Phenol Red Broth
Phenol Red Broth is a general-purpose differential test medium typically used to differentiate Gram negative enteric bacteria.
We use three different kinds of phenol red broths. One contains glucose; one contains lactose, and the last contains sucrose. The objective of the exercise is to determine which organisms can utilize each sugar.
The Basic Chemistry of Phenol Red Broth
The Basic Chemistry of Phenol Red Broth
Phenol Red Broth contains peptone, phenol red (a pH indicator), a Durham tube, and one carbohydrate.
Phenol red is a pH indicator which turns yellow below a pH of 6.8 and fuchsia above a pH of 7.4. If the organism is able to utilize the carbohydrate, an acid by-product is created, which turns the media yellow. If the organism is unable to utilize the carbohydrate but does use the peptone, the by-product is ammonia, which raises the pH of the media and turns it fuchsia.
When the organism is able to use the carbohydrate, a gas by-product may be produced. If it is, an air bubble will be trapped inside the Durham tube. If the organism is unable to utilize the carbohydrate, gas will not be produced, and no air bubble will be formed.
Reading the Results
Reading the Results
Important:
When you retrieve the fermentation broths from the racks on the side counter, be sure to label them immediately! If you become confused as to which carbohydrate is in which tube, your results will be useless.
Yellow broth with a bubble in the Durham tube will be read as Acid/Gas or A/G.
Yellow broth with no bubble in the Durham tube will be read as Acid or A/-.
This tube of phenol red broth has not been inoculated.
The Micrococcus luteus grown in this phenol red broth is negative for both fermentation and gas production. It would be read as – / –.
The Streptococcus aureus grown in this phenol red broth fermented the carbohydrate but did not produce gas. It would be read as A/-.
The Escherichia coli grown in this phenol red broth fermented the carbohydrate and produced gas. It would be read as A/G.
Page updated
Report abuse