Analysis of Six Gene Homologs in Bacteroides fragilis Revealed That is Essential for Survival in Experimental Intestinal Colonization and Intra-Abdominal Infection.

The opportunistic, anaerobic pathogen and commensal of the human large intestinal tract, Bacteroides fragilis strain 638R, contains six predicted TonB proteins, termed TonB1-6, four ExbBs orthologs, ExbB1-4, and five ExbDs orthologs, ExbD1-5. The inner membrane TonB/ExbB/ExbD complex harvests energy from the proton motive force (Δp), and the TonB C-terminal domain interacts with and transduces energy to outer membrane TonB-dependent transporters (TBDTs). However, TonB's role in activating nearly one hundred TBDTs for nutrient acquisition in B. fragilis during intestinal colonization and extraintestinal infection has not been established. In this study, we show that growth was abolished in the mutant when heme, vitamin B, Fe(III)-ferrichrome, starch, mucin-glycans, or N-linked glycans were used as a substrate for growth . Genetic complementation of the mutant with the gene restored growth on these substrates. The , , and single mutants did not show a growth defect. This indicates that there was no functional compensation for the lack of TonB3, and it demonstrates that TonB3, alone, drives the TBDTs involved in the transport of essential nutrients. The mutant had a severe growth defect in a mouse model of intestinal colonization compared to the parent strain. This intestinal growth defect was enhanced in the double mutant strain, which completely lost its ability to colonize the mouse intestinal tract compared to the parent strain. The , , and mutants did not significantly affect intestinal colonization. Moreover, the survival of the mutant strain was completely eradicated in a rat model of intra-abdominal infection. Taken together, these findings show that TonB3 was essential for survival . The genetic organization of , , and gene orthologs indicates that they may interact with periplasmic and nonreceptor outer membrane proteins, but the physiological relevance of this has not been defined. Because anaerobic fermentation metabolism yields a lower Δp than aerobic respiration and B. fragilis has a reduced redox state in its periplasmic space-in contrast to an oxidative environment in aerobes-it remains to be determined if the diverse system of TonB/ExbB/ExbD orthologs encoded by B. fragilis have an increased sensitivity to PMF (relative to aerobic bacteria) to allow for the harvesting of energy under anaerobic conditions.