page_title: Mixed acid fermentation
section_title: Methyl red test
hierarchical_section_title: Mixed acid fermentation / Methyl red test
caption_attribution_description: English: Methyl red test: Escherichia coli (left), showing a positive result (the microorganism has produced a large amount of acid during fermentation of glucose, which is detected by the pH indicator), and Enterobacter cloacae (right) showing a negative result (the microorganism has produced a small amount of acid or no acid during fermentation of glucose).Deutsch: Methylrot-Probe: Escherichia coli (links), mit positivem Ergebnis (der Mikroorganismus hat eine große Menge an Säure während der Fermentation von Glucose gebildet, die durch den pH-Indikator angezeigt wird), und Enterobacter cloacae (rechts) mit negativem Ergebnis (der Mikroorganismus hat nur geringe Säuremengen oder gar keine Säure in der Fermentation von Glucose gebildet).
context_page_description: Mixed acid fermentation is the biological process by which a six-carbon sugar e.g. glucose is converted into a complex and variable mixture of acids. It is an anaerobic fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.
  The mixture of end products produced by mixed acid fermentation includes lactate, acetate, succinate, formate, ethanol and the gases H₂ and CO₂. The formation of these end products depends on the presence of certain key enzymes in the bacterium. The proportion in which they are formed varies between different bacterial species. The mixed acid fermentation pathway differs from other fermentation pathways, which produce fewer end products in fixed amounts. The end products of mixed acid fermentation can have many useful applications in biotechnology and industry. For instance, ethanol is widely used as a biofuel. Therefore, multiple bacterial strains have been metabolically engineered in the laboratory to increase the individual yields of certain end products. This research has been carried out primarily in E. coli and is ongoing.
context_section_description: The methyl red (MR) test can detect whether the mixed acid fermentation pathway occurs in microbes when given glucose. A pH indicator is used that turns the test solution red if the pH drops below 4.4. If the fermentation pathway has taken place, the mixture of acids it has produced will make the solution very acidic and cause a red colour change.
  The methyl red test belongs to a group known as the IMViC tests.
caption: Methyl red test: The test tube on the left shows a positive result as acidic end products are formed by mixed acid fermentation in E. coli. The test tube on the right shows a negative result as no acidic products are formed by fermentation.
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