The dual-functioning fumarate reductase is the sole succinate:quinone reductase in Campylobacter jejuni and is required for full host colonization.

Campylobacter jejuni encodes all the enzymes necessary for a complete oxidative tricarboxylic acid (TCA) cycle. Because of its inability to utilize glucose, C. jejuni relies exclusively on amino acids as the source of reduced carbon, and they are incorporated into central carbon metabolism. The oxidation of succinate to fumarate is a key step in the oxidative TCA cycle. C. jejuni encodes enzymes annotated as a fumarate reductase (Cj0408 to Cj0410) and a succinate dehydrogenase (Cj0437 to Cj0439). Null alleles in the genes encoding each enzyme were constructed. Both enzymes contributed to the total fumarate reductase activity in vitro. The frdA::cat(+) strain was completely deficient in succinate dehydrogenase activity in vitro and was unable to perform whole-cell succinate-dependent respiration. The sdhA::cat(+) strain exhibited wild-type levels of succinate dehydrogenase activity both in vivo and in vitro. These data indicate that Frd is the only succinate dehydrogenase in C. jejuni and that the protein annotated as a succinate dehydrogenase has been misannotated. The frdA::cat(+) strain was also unable to grow with the characteristic wild-type biphasic growth pattern and exhibited only the first growth phase, which is marked by the consumption of aspartate, serine, and associated organic acids. Substrates consumed in the second growth phase (glutamate, proline, and associated organic acids) were not catabolized by the the frdA::cat(+) strain, indicating that the oxidation of succinate is a crucial step in metabolism of these substrates. Chicken colonization trials confirmed the in vivo importance of succinate oxidation, as the frdA::cat(+) strain colonized chickens at significantly lower levels than the wild type, while the sdhA::cat(+) strain colonized chickens at wild-type levels.