Which pair of tests best differentiates organisms by mixed-acid fermentation versus acetoin production via the butanediol pathway?

• A. Indole and Oxidase
• B. Citrate Utilization and TSI
• C. Methyl Red and Voges-Proskauer
• D. Nitrate Reduction and Urease

Answer: (C)

Distinguishing how bacteria dispose of glucose by looking at their fermentation end products is best done with the MRVP set of tests. The methyl red test detects strong, stable acid production characteristic of mixed-acid fermentation. If the organism produces enough acid to push the pH below about 4.4, the methyl red indicator stays red, signaling a positive result for mixed-acid fermentation.

The Voges-Proskauer test, on the other hand, looks for acetoin, a neutral intermediate produced when some bacteria use the butanediol pathway. If acetoin is present, the reagents react to produce a red color, indicating a positive VP result. Organisms that route glucose through the butanediol pathway tend to be MR negative (not strongly acidic) but VP positive.

Using these two tests together lets you clearly separate organisms that favor mixed-acid fermentation from those that accumulate acetoin via the butanediol pathway. For example, many Enterobacteriaceae show this pattern: MR positive and VP negative for some, while others are MR negative and VP positive.

Other test pairs don’t specifically assess the end products of glucose fermentation. Indole production and oxidase activity relate to other metabolic traits; citrate utilization and TSI provide different growth and fermentation profiles; nitrate reduction and urease test distinct enzymatic capabilities.