The oxidative fumarase FumC is a key contributor for E. coli fitness under iron-limitation and during UTI

The energy required for a bacterium to grow and colonize the host is generated by metabolic and respiratory functions of the cell. Proton motive force, produced by these processes, drives cellular mechanisms including redox balance, membrane potential, motility, acid resistance, and the import and export of substrates. Previously, disruption of succinate dehydrogenase (sdhB) and fumarate reductase (frdA) within the oxidative and reductive tricarboxylic acid (TCA) pathways in uropathogenic E. coli (UPEC) CFT073 indicated that the oxidative, but not the reductive TCA pathway, is required for fitness in the urinary tract. Those findings led to the hypothesis that fumA and fumC encoding fumarase enzymes of the oxidative TCA cycle would be required for UPEC colonization, while fumB of the reductive TCA pathway would be dispensable. However, only UPEC strains lacking fumC had a fitness defect during experimental urinary tract infection (UTI). To further characterize the role of respiration in UPEC during UTI, additional mutants disrupting both the oxidative and reductive TCA pathways were constructed. We found that knock-out of frdA in the sdhB mutant strain background ameliorated the fitness defect observed in the bladder and kidneys for the sdhB mutant strain and results in a fitness advantage in the bladder during experimental UTI. The fitness defect was restored in the sdhBfrdA double mutant by complementation with frdABCD. Taken together, we demonstrate that it is not the oxidative or reductive pathway that is important for UPEC fitness per se, but rather only the oxidative TCA enzyme FumC. This fumarase lacks an iron-sulfur cluster and is required for UPEC fitness during UTI, most likely acting as a counter measure against exogenous stressors, especially in the iron-limited bladder niche. Author summary All living organisms have the ability to harvest energy from their environment. Many organisms, including microbes such as E. coli, use cellular respiration to release energy from the chemical bonds in sugars and proteins. The tricarboxylic acid (TCA) cycle and the electron transport system are intimately linked to each other and to the process of converting the released energy into the ability to do work. In this case, the work being explored in this study demonstrates how this process of cellular respiration contributes to the ability for uropathogenic E. coli to colonize the urinary tract and cause infection. Using a genetic approach, we found that certain TCA cycle enzymes together, such as succinate dehydrogenase and fumarate reductase, are dispensable for E. coli during UTI, while the oxidative fumarase FumC is absolutely required for UTI. These findings suggest that modularity in respiratory or TCA cycle components are important for E. coli fitness during infection and could reflect an important strategy to subvert host defenses rather than replication in an aerobic or anaerobic environment.