methyl red test

Trait evidence (3)

• https://asm.org/getmedia/40946f85-9357-4563-aa8a-994427efa825/Methyl-Red-and-Voges-Proskauer-Test-Protocols.pdf

  Escherichia coli and other members of the low-ratio organisms described by Clark and Lubs ferment sugars by the mixed acid pathway resulting in a low ratio of CO2 to H2 gas produced by fermentation. The mixed acid pathway gives 4 mol of acidic products (mainly lactic and acetic acid), 1 mol of neutral fermentation product (ethanol), 1 mol of CO2, and 1 mol of H2 per mol of glucose fermented (9). The large quantity of acids produced causes a significant decrease in the pH of the culture medium. In contrast, Enterobacter aerogenes and other members of the high-ratio organisms (those that produce a high ratio of CO2 to H2 from the fermentation of glucose) ferment sugars via the butanediol fermentation pathway, producing only 1 mol of acid per mol of glucose. This pathway results in a lower degree of acidification of the culture medium.

  The ASM protocol contrasts mixed-acid fermentation's 4 mol acidic products per mol glucose with the butanediol pathway's 1 mol acid per mol glucose and lower medium acidification.

• https://asm.org/getmedia/40946f85-9357-4563-aa8a-994427efa825/Methyl-Red-and-Voges-Proskauer-Test-Protocols.pdf

  When the culture medium turns red after addition of methyl red, because of a pH at or below 4.4 from the fermentation of glucose, the culture has a positive result for the MR test (Fig.1A).

  The ASM protocol PDF defines the methyl-red-test positive readout as acidification to pH 4.4 or below after glucose fermentation.

• https://asm.org/getmedia/40946f85-9357-4563-aa8a-994427efa825/Methyl-Red-and-Voges-Proskauer-Test-Protocols.pdf

  A negative MR test is indicated by a yellow color in the culture medium (Fig. 1B), which occurs when less acid is produced (pH is higher) from the fermentation of glucose.

  The same protocol defines the negative readout as a yellow culture reflecting less glucose-derived acid production.