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When glucose is fermented, a variety of products are possible. Some bacteria continue to produce more acid in ‘mixed acid fermentation.’ Some genera use the glucose but produce end products with a more neutral pH like those that result from ‘butanediol fermentation.’
- Obtain one tube of MR-VP broth.
- Using an inoculating loop, swish some of your assigned organism in the broth
- Incubate the tube for at least 48 hours.
- After the incubation period, vortex the tube and pour half of the broth into another clean test tube.
- Complete the tests on the two individual tubes as follows:
MR: to one tube add a drop or two of the pH indicator methyl red and swirl.
VP: TO the other tube add three drops of Barritt's 1/A reagent (5% alpha-napthol solution) and a drop of Barritt's 2/B reagent (40% potassium hydroxide solution). Shake vigorously and watch the tube over the next hour or two. The alternative, if not shaken only the air-broth interface might indicate a change in color.
MR: If the tube turns red, the test is positive for mixed acid fermentation (one or more organic acids formed during the fermentation of glucose).
VP: If the tube (or interface) turns pink or red, the test is positive for acetoin, a precursor of 2,3-butanediol.
Left: MR Positive
Right: VP Positive
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