Methyl Red / Voges-Proskauer (MRVP) Reaction Guide
An original four-tube bench visual for reading the Methyl Red and Voges-Proskauer combination test used to separate members of the Enterobacteriaceae family.
MRVP tubes A-D
Read MR first (color with reagent added), then VP separately. Each tube answers a different metabolic question.
Mixed acid fermentation lowered pH below 4.4; methyl red indicator turns bright red.
pH remains above 6.0; indicator stays yellow or red-orange. Not a mixed acid pattern.
Acetoin is oxidized to diacetyl, which reacts with creatine under KOH to produce a red complex.
No acetoin production; the broth remains yellow after both reagents are added.
Mixed acid fermentation lowered pH below 4.4; methyl red indicator turns bright red.
pH remains above 6.0; indicator stays yellow or red-orange. Not a mixed acid pattern.
Acetoin is oxidized to diacetyl, which reacts with creatine under KOH to produce a red complex.
No acetoin production; the broth remains yellow after both reagents are added.
Color-to-result anchors
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Bright red (MR) | Mixed acid fermentation sustained; pH dropped below 4.4 | MR positive — classic E. coli pattern |
| Yellow or orange (MR) | Mixed acid pathway not dominant; buffering kept pH higher | MR negative — classic Klebsiella/Enterobacter pattern |
| Red after KOH/alpha-naphthol (VP) | Acetoin oxidized to diacetyl; creatine forms red complex | VP positive — 2,3-butanediol pathway active |
| Yellow after reagents (VP) | No acetoin produced; no diacetyl complex formed | VP negative |
| Timing | MR should not be read before 48 hours of incubation | Early reads may be falsely MR negative |
MRVP study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| MR (+) VP (-) | E. coli pattern | Mixed acid fermentation organism; classic IMViC profile of (+)(-). |
| MR (-) VP (+) | Klebsiella / Enterobacter pattern | 2,3-Butanediol pathway; classic IMViC profile of (-)(+). |
| MR (+) VP (+) | Unusual or organism-dependent | Some organisms can test positive for both; correlate with colony and Gram stain. |
| MR (-) VP (-) | Non-fermenter or slow fermenter | Repeat if growth is questionable; correlate with the full biochemical profile. |
- MR and VP are always read as a pair and from separate tube aliquots.
- MR detects mixed acid fermentation; VP detects acetoin (2,3-butanediol pathway).
- E. coli is MR positive, VP negative; Klebsiella pneumoniae is MR negative, VP positive.
Best mental model: MR asks if the organism makes and keeps mixed acids; VP asks if it converts those acids to a neutral product (acetoin/2,3-butanediol).