Escherichia coli (E. coli) readily ferments glucose when conditions require it. This bacterium is a facultative anaerobe, an organism that can switch between metabolic strategies based on the availability of oxygen. This metabolic flexibility allows it to generate energy to survive and multiply in diverse environments, whether oxygen is present or not. The ability to ferment sugars like glucose is an aspect of its adaptability.
The Biochemical Process of Glucose Fermentation
The initial stage for breaking down glucose is a process called glycolysis, which occurs under both oxygen-rich and oxygen-poor conditions. During glycolysis, one molecule of glucose is split into two smaller molecules of pyruvate. This conversion also produces a small amount of ATP, the cell’s main energy currency, and reduces a molecule called NAD+ to NADH.
In the absence of an external electron acceptor like oxygen, the cell cannot continue producing NADH from glycolysis. The limited supply of NAD+ would quickly be depleted, halting the glycolytic pathway and energy production. To solve this, E. coli employs mixed-acid fermentation to regenerate NAD+ from the NADH produced during glycolysis.
This specific fermentation pathway involves converting the pyruvate from glycolysis into a variety of acidic compounds. The chemical reactions that form these acids also oxidize NADH back into NAD+, replenishing the pool of available NAD+ and allowing glycolysis to continue generating ATP.
Byproducts of Mixed-Acid Fermentation
The mixed-acid fermentation pathway results in a characteristic blend of end products. The primary substances produced from the breakdown of pyruvate are the stable acids: lactic acid, acetic acid, and succinic acid. In addition to these acids, the pathway also yields formic acid.
Formic acid is further broken down by an enzyme complex called formate hydrogenlyase. This reaction splits formic acid into hydrogen gas (H₂) and carbon dioxide gas (CO₂). The production of these gases is a distinct hallmark of E. coli’s fermentation of glucose.
The accumulation of lactic, acetic, and succinic acids leads to a sharp decrease in the pH of the surrounding medium. This acidification is a defining outcome of mixed-acid fermentation and serves as a reliable indicator of this metabolic process.
Laboratory Identification of Glucose Fermentation
Scientists use the distinct byproducts of glucose fermentation to identify E. coli in the laboratory. A common method is the Phenol Red (PR) Glucose Broth test. This test uses a liquid growth medium that contains glucose, a pH indicator called phenol red, and a small, inverted glass tube known as a Durham tube. When E. coli is introduced into this broth and incubated, it begins to ferment the glucose.
The production of stable acids lowers the pH of the broth. Phenol red indicator is red at a neutral pH but turns yellow in acidic conditions (below a pH of 6.8). A color change from red to yellow signals that the organism has produced acid from glucose fermentation. Simultaneously, the hydrogen and carbon dioxide gases are captured as a bubble inside the inverted Durham tube, confirming gas production.
Another diagnostic tool is the Methyl Red (MR) test, which detects the high acid production of the mixed-acid pathway. The MR-VP broth contains glucose and a phosphate buffer. While many bacteria produce some acid from glucose, E. coli produces such a large quantity of stable acids that it overwhelms the buffer’s capacity, causing a significant and lasting drop in pH. Adding the methyl red indicator, which turns red at a pH of 4.4 or lower, reveals this strong acidic environment, confirming a positive result for E. coli.
Fermentation vs. Aerobic Respiration in E. coli
While fermentation is an effective survival strategy, it is not E. coli’s most efficient method for generating energy. When oxygen is present, the bacterium will preferentially use aerobic respiration. This process is more productive, yielding substantially more ATP from a single molecule of glucose compared to the small amount produced through fermentation. Aerobic respiration fully oxidizes glucose, resulting in simple, neutral end products: carbon dioxide and water.
In contrast, fermentation only partially breaks down glucose, leaving behind the energy-rich organic acids and gases as byproducts. The choice between these two pathways is a matter of environmental circumstance. Fermentation, though less energy-efficient, allows E. coli to thrive in anaerobic environments where oxygen is scarce or absent, such as the lower gastrointestinal tract of mammals. This metabolic duality is fundamental to its success as a species, providing the versatility needed to colonize a wide range of habitats.