Defining the role of aerobic respiration in the metabolism and bioenergetics of Enterococcus faecalis

Enterococcus faecalis is an opportunistic pathogen that can thrive in both aerobic and anaerobic environments. The bacterium primarily relies on a process called fermentative metabolism to survive in these different conditions. When exposed to a substance called heme, E. faecalis can assemble a minimal electron transport chain, which generates energy by transferring electrons through a series of protein complexes.

This process produces a proton motive force, which drives the synthesis of ATP, the energy currency of the cell.

Additionally, a protein called the cytosolic NADH oxidase (Nox) consumes NADH and oxygen, potentially competing with the electron transport chain for reducing equivalents and terminal electron acceptors. However, it is unclear how these two oxygen-reducing pathways contribute to the overall metabolism and energy production of E. faecalis.

To better understand the roles of the electron transport chain and Nox, researchers created genetic mutants that lacked these proteins. They found that the {Delta}nox mutant had significantly reduced oxygen utilization compared to the wild type, indicating that Nox was the major consumer of oxygen under both normoxic and hypoxic conditions. The {Delta}cydAB mutant, on the other hand, did not show any significant effect on oxygen utilization.

Further analysis revealed that the {Delta}nox mutant caused significant alterations in the central metabolism, leading to pronounced shifts in the ATP and NADH ratios. This suggests that Nox plays a crucial role in maintaining intracellular redox balance and energy homeostasis.

Single-cell fluorescence microscopy showed that the membrane potential, a component of the proton motive force generated by the F-type ATP synthase, was substantially diminished when both CydAB and Nox were absent, or when the F-type ATP synthase was not functioning. This finding indicates that the F-type ATP synthase is a major generator of proton motive force, even under aerobic growth conditions.

The results also demonstrate that both the electron transport chain and Nox play complementary roles in the bioenergetics of E. faecalis.