Methyl red sodium salt (CAS: 845-10-3)

Methyl red (2-(N,N-Dimethyl-4-aminophenyl)azobenzenecarboxylic acid), also called C.I.

Acid Red 2, is anindicator dye that turns red in acidic solutions. It is an azo dye, and is a dark red

crystalline powder. Methyl red is apH indicator; it is red in pH under 4.4, yellow in pH over 6.2,

and orange in between, with a pKa of 5.1. Murexide and methyl red are investigated as promising

enhancers of sonochemical destruction of chlorinated hydrocarbon pollutants.Methyl red is classed

by the IARC in group 3 – unclassified as to carcinogenic potential in humans.

Methyl red test

In microbiology, methyl red is used in the Methyl Red (MR) Test, used to identify bacteria producing

stable acids by mechanisms of mixed acid fermentation of glucose (cf. Voges–Proskauer (VP) test).

The methyl red test is the “M” portion of the four IMViC tests used to characterize enteric bacteria. The

methyl red test is used to identify enteric bacteria based on their pattern of glucose metabolism. All enterics

initially produce pyruvic acid from glucose metabolism. Some enteric subsequently use the mixed acid

pathway to metabolize pyruvic acid to other acids, such as lactic, acetic, and formic acids. These bacteria

are called methyl-red positive and include Escherichia coli and Proteus vulgaris. Other enterics subsequently

use the butylene glycol pathway to metabolize pyruvic acid to neutral end-products. These bacteria are

called methyl-red-negative and include Serratia marcescens and Enterobacter aerogenes.

Process

An isolate is inoculated into a tube with a sterile transfer loop. The tube is incubated at 35°C for 2–5 days.

After incubation, 2.5ml of the medium is transferred to another tube. Five drops of the pH indicator methyl

red is added to this tube. The tube is gently rolled between the palms of the hands to disperse the methyl red.

Expected results

Enterics that subsequently metabolize pyruvic acid to other acids lower the pH of the medium to 4.2. At this pH,

methyl red turns red. A red color represents a positive test. Enterics that subsequently metabolize pyruvic acid

to neutral end-products lower the pH of the medium to only 6.0. At this pH, methyl red is yellow. A yellow

color represents a negative test.