PR-Carbohydrate Broth

(Glucose, Lactose, Sucrose, Mannitol, Inulin, or other Carb)

Introduction Results Methods Photos


Fermentation ability is often used to help characterize bacterial species. Bacteria are diverse in the types of carbohydrates they can ferment and diverse in the types of by-products they produce. The production of many kinds of foods and beverages relies on this diversity of fermentation processes.

PR-Carbohydrate broths test fermentation ability. Each PR-Carbohydrate broth has the following ingredients:

Phenol-Red is red at pH > 7. When a microbe is inoculated into a PR-Carb broth, that organism will either ferment the carbohydrate or not. If fermentation occurs, the by-products are almost always acidic. Thus, if fermentation occurs, the broth should change from red to yellow.

Some organisms produce gaseous compounds like carbon dioxide or hydrogen gas as fermentation by-products. If gas is produced, some gas will be trapped in the Durham Tube and a gas bubble will form. The presence of a bubble in the Durham Tube indicates two things: (1) fermentation has occured and (2) it was a gas-producing kind of fermentation.


AG = Acid and Gas production (Yellow coloration with a gas bubble within 24 hours to 48 hours)
A = Acid but not gas production (Yellow coloration after 24 to 48 hours)
- = No evidence of fermentation (Red)
V = Variable

CAUTION: Some organisms may further metabolize fermentation by-products (acids) and convert them back to neutral or alkaline pH. Thus, PR-Carb broths should be incubated for no more than 48 to 72 hours.


1. Obtain two PR-Carb broths containing the desired carbohydrate from the back shelf.

2. Inoculate one broth using aseptic technique. Leave the other broth uninoculated (this will be a control).

3. Incubate at appropriate temperature (whatever temperature your organism grows well at). Incubate for 24 to 48 hours (do not exceed 48 to 72 hours for this test).

4. Obtain your broths from the incubator and observe the color.