The impairment of methylmenaquinol:fumarate reductase affects hydrogen peroxide susceptibility and accumulation in Campylobacter jejuni

The methylmenaquinol:fumarate reductase (Mfr) of Campylobacter jejuni is a periplasmic respiratory (redox) protein that contributes to the metabolism of fumarate and displays homology to succinate dehydrogenase (Sdh). Since chemically oxidized redox-enzymes, including fumarate reductase and Sdh, contribute to the generation of oxidative stress in Escherichia coli, we assessed the role of Mfr in C. jejuni after exposure to hydrogen peroxide (H2O2). Our results show that a Mfr mutant (Delta mfrA) strain was less susceptible to H2O2 as compared to the wildtype (WT). Furthermore, the H2O2 concentration in the Delta mfrA cultures was significantly higher than that of WT after exposure to the oxidant. In the presence of H2O2, catalase (KatA) activity and katA expression were significantly lower in the Delta mfrA strain as compared to the WT. Exposure to H2O2 resulted in a significant decrease in total intracellular iron in the Delta mfrA strain as compared to WT, while the addition of iron to the growth medium mitigated H2O2 susceptibility and accumulation in the mutant. The Delta mfrA strain was significantly more persistent in RAW macrophages as compared to the WT. Scanning electron microscopy showed that infection with the Delta mfrA strain caused prolonged changes to the macrophages' morphology, mainly resulting in spherical-shaped cells replete with budding structures and craters. Collectively, our results suggest a role for Mfr in maintaining iron homeostasis in H2O2 stressed C. jejuni, probably via affecting the concentrations of intracellular iron.