Account for the fact that E. coli is methyl red-positive and e aerogenes red-negative?

Methyl red test is used to differentiate between bacteria that produce and ferment glucose through different pathways. The test is based on the ability of the bacteria to produce and accumulate acidic end products during glucose fermentation.

Escherichia coli (E. coli)

- It is methyl red-positive because it ferments glucose through the mixed-acid fermentation pathway, producing a mixture of acidic end products, including lactic acid, formic acid, acetic acid, and succinic acid.

- The accumulation of these acids lowers the pH of the culture medium, causing the methyl red indicator to turn red.

Enterobacter aerogenes (E. aerogenes):

- It is methyl red-negative because it ferments glucose almost exclusively through the butanediol fermentation pathway, producing 2,3-butanediol as the major end product.

- Butanediol is a neutral product that does not significantly lower the pH of the culture medium, resulting in a negative methyl red test.

Therefore, the difference in methyl red test results between E. coli and E. aerogenes can be attributed to the different metabolic pathways they utilize during glucose fermentation.