Genetic and Biochemical Analysis of Anaerobic Respiration in Bacteroides fragilis and Its Importance .

In bacteria, the respiratory pathways that drive molecular transport and ATP synthesis include a variety of enzyme complexes that utilize different electron donors and acceptors. This property allows them to vary the efficiency of energy conservation and to generate different types of electrochemical gradients (H or Na). We know little about the respiratory pathways in species, which are abundant in the human gut, and whether they have a simple or a branched pathway. Here, we combined genetics, enzyme activity measurements, and mammalian gut colonization assays to better understand the first committed step in respiration, the transfer of electrons from NADH to quinone. We found that a model gut species, , has all three types of putative NADH dehydrogenases that typically transfer electrons from the highly reducing molecule NADH to quinone. Analyses of NADH oxidation and quinone reduction in wild-type and deletion mutants showed that two of these enzymes, Na-pumping ADH:uinone oxidoeductase (NQR) and ADH eydrogenase (NDH2), have NADH dehydrogenase activity, whereas H-pumping ADH:biquinone xidoreductase (NUO) does not. Under anaerobic conditions, NQR contributes more than 65% of the NADH:quinone oxidoreductase activity. When grown in rich medium, none of the single deletion mutants had a significant growth defect; however, the double Δ Δ mutant, which lacked almost all NADH:quinone oxidoreductase activity, had a significantly increased doubling time. Despite unaltered growth, the single deletion mutant was unable to competitively colonize the gnotobiotic mouse gut, confirming the importance of NQR to respiration in and the overall importance of respiration to this abundant gut symbiont. species are abundant in the human intestine and provide numerous beneficial properties to their hosts. The ability of species to convert host and dietary glycans and polysaccharides to energy is paramount to their success in the human gut. We know a great deal about the molecules that these bacteria extract from the human gut but much less about how they convert those molecules into energy. Here, we show that has a complex respiratory pathway with two different enzymes that transfer electrons from NADH to quinone and a third enzyme complex that may use an electron donor other than NADH. Although fermentation has generally been believed to be the main mechanism of energy generation in , we found that a mutant lacking one of the NADH:quinone oxidoreductases was unable to compete with the wild type in the mammalian gut, revealing the importance of respiration to these abundant gut symbionts.