Escherichia coli, commonly known as E. coli, is a widespread bacterium found in various environments, including the intestines of mammals. Like all living organisms, E. coli requires energy for life processes such as growth, movement, and reproduction. This energy acquisition involves complex biochemical reactions that extract usable energy from various nutrients.
Fueling E. coli
E. coli demonstrates metabolic flexibility, utilizing a diverse range of organic compounds as fuel sources. Simple sugars, especially glucose, are a preferred and efficient energy source. Glucose is broken down through glycolysis, initiating energy harvesting.
Beyond glucose, E. coli can metabolize other mono- and disaccharides, amino acids, and dicarboxylates, which feed into its central metabolic pathways. When preferred sugars are scarce, E. coli adapts by breaking down less common carbon sources found in its environment. It can even use DNA as a nitrogen source or, in some instances, as a sole source of carbon and energy.
Energy Harvesting with Oxygen
In the presence of oxygen, E. coli employs aerobic respiration, an efficient process for generating adenosine triphosphate (ATP), the cell’s primary energy currency. This process begins with glycolysis, where glucose is broken down into pyruvate, yielding a small amount of ATP and electron carriers like NADH. Pyruvate then enters the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, where it is further oxidized, producing more NADH and FADH2, along with carbon dioxide.
The majority of ATP is generated through oxidative phosphorylation, which involves the electron transport chain. NADH and FADH2 donate their electrons to protein complexes embedded in the bacterial cell membrane. As electrons move through this chain, protons are actively pumped across the membrane, creating an electrochemical gradient known as the proton motive force. Oxygen acts as the final electron acceptor in the chain, combining with electrons and protons to form water. The accumulated protons then flow back across the membrane through an enzyme called ATP synthase, driving the synthesis of a large quantity of ATP.
Energy Harvesting Without Oxygen
When oxygen is scarce or absent, E. coli switches to alternative energy harvesting strategies. One strategy is anaerobic respiration, where E. coli uses alternative molecules instead of oxygen as the final electron acceptor in its electron transport chain. Common alternative electron acceptors include nitrate, nitrite, fumarate, trimethylamine N-oxide (TMAO), and dimethyl sulfoxide (DMSO). This process still generates a proton motive force and ATP, but it is less efficient than aerobic respiration because these alternative acceptors yield less energy.
If suitable external electron acceptors are unavailable, E. coli resorts to fermentation. Fermentation is a less efficient process that primarily relies on glycolysis to produce a small amount of ATP through substrate-level phosphorylation. During fermentation, pyruvate, the end product of glycolysis, is converted into various organic byproducts, such as lactic acid, acetate, ethanol, and succinate. These conversions regenerate NAD+ from NADH, which is necessary to keep glycolysis running and continue producing ATP, even in the absence of an electron transport chain.