The role of TonB-dependent receptor TbdR1 in Riemerella anatipestifer in iron acquisition and virulence.

Riemerella anatipestifer is an important duck pathogen and causes serious economic losses to the duck industry worldwide. To date, four full R. anatipestifer genomic sequences have been submitted to the GenBank database and 31 TonB-dependent outer membrane receptors, which may play critical roles in host-bacteria interactions, were predicted for R. anatipestifer strain GSM15868. In our previous study, we reported that the TonB-dependent receptor TbdR1 was a cross immunogenic antigen among R. anatipestifer serotypes 1, 2, and 10. However, the biological functions of TbdR1 in R. anatipestifer remain unclear. In the present study, a tbdR1 (Riean_1607) deletion mutant CH3ΔtbdR1 of R. anatipestifer strain CH3 was constructed and characterized for iron-limited growth, biofilm formation, and pathogenicity to ducklings. Our results showed that TbdR1 was involved in hemin iron acquisition and the tbdR1 deletion significantly reduced biofilm formation and adhesion to and invasion of Vero cells. Animal experiments indicated that the median lethal dose of the CH3ΔtbdR1 mutant in ducklings was about 45-fold higher than that of the wild-type CH3 strain. Additional analysis indicated that bacterial loads in blood, liver, and brain tissues in CH3ΔtbdR1-infected ducklings were decreased significantly compared to those in wild-type CH3-infected ducklings. Thus, our results demonstrated that TbdR1 was involved in hemin iron acquisition and necessary for optimal bacterial virulence.