Specific TonB-ExbB-ExbD energy transduction systems required for ferric enterobactin acquisition in Campylobacter

Ferric enterobactin (FeEnt) acquisition plays a critical role in the pathophysiology of Campylobacter, the leading bacterial cause of human gastroenteritis in industrialized countries. In Campylobacter, the surface-exposed receptor, CfrA or CfrB, functions as a 'gatekeeper' for initial binding of FeEnt. Subsequent transport across the outer membrane is energized by TonB-ExbB-ExbD energy transduction systems. Although there are up to three TonB-ExbB-ExbD systems in Campylobacter, the cognate components of TonB-ExbB-ExbD for FeEnt acquisition are still largely unknown. In this study, we addressed this issue using complementary molecular approaches: comparative genomic analysis, random transposon mutagenesis and site-directed mutagenesis in two representative C. jejuni strains, NCTC 11168 and 81-176. We demonstrated that CfrB could interact with either TonB2 or TonB3 for efficient Ent-mediated iron acquisition. However, TonB3 is a dominant player in the CfrA-dependent pathway. The ExbB2 and ExbD2 components were essential for both CfrA- and CfrB-dependent FeEnt acquisition. Sequences analysis identified potential TonB boxes in CfrA and CfrB, and the corresponding binding sites in TonB. In conclusion, these findings identify specific TonB-ExbB-ExbD energy transduction components required for FeEnt acquisition, and provide insights into the complex molecular interactions of FeEnt acquisition systems in Campylobacter.