Simple Carbohydrate Fermentations

Media for testing carbohydrate fermentation are often prepared as tubed broths, each tube containing a small inverted “fermentation” (or Durham) tube for trapping any gas formed when the broth is inoculated and incubated (see colorplate 18). Each broth contains essential nutrients, a specific carbohydrate, and a color reagent to indicate a change in pH if acid is produced in the culture (the broth is adjusted to a neutral pH when prepared). Organisms that grow in the broth but do not ferment the carbohydrate produce no change in the color of the medium, and no gas is formed. Some organisms may produce acid products in fermenting the sugar, but no gas, whereas others may form both acid and gas. In some cases, organisms that do not ferment the carbohydrate use the protein nutrients in the broth, thereby producing alkaline end products, a result that is also evidenced by a change in indicator color (see colorplate 19).

Purpose	To distinguish bacterial species on the basis of simple carbohydrate fermentation
Materials	Tubed phenol red glucose broth Tubed phenol red lactose broth Tubed phenol red sucrose broth Slant cultures of Escherichia coli, Serratia marcescens, Pseudomonas aeruginosa, and Proteus vulgaris

Procedures

1. Inoculate growth from each of the four cultures into separate tubes of each of the three carbohydrate broths. Be certain no bubbles are inside the Durham tubes before inoculation.
2. Label each of the 12 inoculated tubes with the name of the carbohydrate it contains and the name of the bacterial culture.
3. Incubate at 35°C for 24 hours.

Results
Record your results in the following table. Use these symbols to indicate specific changes observed in the broths.

• A = acid production
• K = alkaline color change
• N = neutral (no change in color)
• G = gas formation