Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
Mixed acid fermentation is a metabolic process by which bacteria convert sugars into a mixture of acids, gases, and other products. It is typical in certain enteric bacteria such as Escherichia coli.
Mixed acid fermentation results in the production of lactic acid, acetic acid, succinic acid, formic acid, ethanol, hydrogen gas (H2), and carbon dioxide (CO2).
This type of fermentation is characteristic of some facultative anaerobes found in the Enterobacteriaceae family.
It typically occurs under anaerobic conditions when oxygen is not available to act as the final electron acceptor.
The presence of mixed acid fermentation can be detected using the Methyl Red (MR) test in microbiology labs.
Formate dehydrogenase plays a key role in converting formate to CO2 and H2 during mixed acid fermentation.
Related terms
Facultative Anaerobe: An organism that can survive with or without oxygen.
Methyl Red Test: A biochemical test used to detect the production of stable acids by means of mixed acid fermentation.
Formate Dehydrogenase: An enzyme involved in the oxidative metabolism of formic acid to carbon dioxide.