E. coli is a common bacterium found in the intestines of warm-blooded animals and in diverse environments like water and soil. This widespread distribution is possible because E. coli is a highly adaptable facultative anaerobe. This means the bacterium can thrive in the presence of oxygen or continue to grow when oxygen is completely absent. Its survival relies on metabolic flexibility—the ability to rapidly switch energy generation pathways in response to environmental changes. This article explores the two primary metabolic modes, aerobic and anaerobic, and the control systems governing the switch.
Energy Production in Oxygen-Rich Environments
When oxygen is readily available, E. coli relies on aerobic respiration, its most efficient method for generating energy. This process involves the complete oxidation of carbon sources, such as glucose, through glycolysis and the tricarboxylic acid (TCA) cycle. This breakdown generates substantial quantities of electron-carrying molecules, specifically NADH and FADH₂.
These electrons are funneled into the membrane-bound electron transport chain (ETC), which includes NADH dehydrogenases and terminal oxidases. As electrons move, protons are pumped out of the cell, establishing the proton motive force. Oxygen serves as the final electron acceptor, combining with protons and electrons to form water. The resulting proton gradient is harnessed by ATP synthase to produce adenosine triphosphate (ATP) via oxidative phosphorylation. Aerobic respiration is highly productive, yielding up to 26 molecules of ATP per glucose molecule, allowing for the fastest growth rate.
Adapting to Oxygen Deprivation
When E. coli encounters environments with low or no oxygen, it must immediately shift its metabolism. The bacterium uses two primary, less energy-efficient strategies. The preferred alternative is anaerobic respiration, which utilizes a modified electron transport chain.
Instead of oxygen, E. coli employs alternative terminal electron acceptors like nitrate or fumarate. These compounds are reduced by specific terminal reductases, expressed only under anoxic conditions. This pathway still generates a proton motive force, allowing for some ATP synthesis via oxidative phosphorylation. However, the energy yield is lower because these alternative acceptors have a smaller reduction potential than oxygen.
If no suitable electron acceptors are available, the bacterium switches to fermentation, the least efficient pathway. Fermentation relies entirely on substrate-level phosphorylation, generating only two net molecules of ATP per glucose molecule. To regenerate the NAD⁺ needed for glycolysis, E. coli performs a mixed-acid fermentation, converting pyruvate into acidic byproducts. These end products include lactate, acetate, succinate, and ethanol, which are then excreted.
Cellular Sensing and Metabolic Control
The metabolic switch is tightly controlled by a sophisticated sensing mechanism that monitors the cell’s oxygen status. Two major regulatory systems, FNR and ArcA/B, act as molecular switches to execute this change.
FNR System
The FNR (Fumarate and Nitrate Reduction) protein is a primary oxygen sensor, active only when oxygen levels drop. FNR contains an oxygen-sensitive iron-sulfur cluster that becomes inactive when bound to oxygen. In the absence of oxygen, active FNR binds to DNA sites, turning on genes required for anaerobic respiration, such as those encoding nitrate and fumarate reductases. FNR also represses genes for aerobic functions, including terminal oxidases in the ETC.
ArcA/B System
The ArcA/B (Aerobic Respiration Control) two-component system provides additional control by repressing aerobic metabolism in anoxic conditions. The ArcB protein, a membrane-bound sensor kinase, detects a change in the cell’s redox state, signaling oxygen depletion. ArcB then phosphorylates its partner, the ArcA response regulator.
Phosphorylated ArcA represses the expression of genes involved in aerobic energy generation, including those for the TCA cycle. This coordinated action by FNR and ArcA ensures resources are immediately redirected toward anaerobic machinery while shutting down aerobic components. This rapid genetic reprogramming enables E. coli to adapt effectively to changing oxygen concentrations.
Why E. coli’s Flexibility Matters
The metabolic versatility of E. coli is fundamental to its success in diverse ecological niches and its utility in biotechnology. In the mammalian gut, oxygen concentration varies dramatically, creating steep gradients. E. coli rapidly colonizes this environment by consuming trace oxygen, creating an anoxic environment suitable for its strictly anaerobic neighbors.
The ability to use alternative electron acceptors, such as nitrate and fumarate, is a significant competitive advantage within the host. This flexibility allows the bacterium to thrive in different zones of the gut and outcompete other microbes.
From an industrial perspective, researchers exploit this metabolic adaptability to use E. coli as a workhorse organism. By controlling oxygen levels in large bioreactors, scientists can force E. coli into specific metabolic states to maximize the production of desired chemicals. Maintaining an anaerobic state is often necessary to maximize the output of valuable organic acids, such as lactate and succinic acid, which are precursors for biofuels and pharmaceuticals.