The Thioredoxin System in Edwardsiella piscicida Contributes to Oxidative Stress Tolerance, Motility, and Virulence

Edwardsiella piscicida is an important fish pathogen that causes substantial economic losses. In order to understand its pathogenic mechanism, additional new virulence factors need to be identified. The bacterial thioredoxin system is a major disulfide reductase system, but its function is largely unknown in E. piscicida. In this study, we investigated the roles of the thioredoxin system in E. piscicida (named TrxB_Ep, TrxA_Ep, and TrxC_Ep, respectively) by constructing a correspondingly markerless in-frame mutant strain: ΔtrxB, ΔtrxA, and ΔtrxC, respectively. We found that (i) TrxB_Ep is confirmed as an intracellular protein, which is different from the prediction made by the Protter illustration; (ii) compared to the wild-type strain, ΔtrxB exhibits resistance against H₂O₂ stress but high sensitivity to thiol-specific diamide stress, while ΔtrxA and ΔtrxC are moderately sensitive to both H₂O₂ and diamide conditions; (iii) the deletions of trxB, trxA, and trxC damage E. piscicida's flagella formation and motility, and trxB plays a decisive role; (iv) deletions of trxB, trxA, and trxC substantially abate bacterial resistance against host serum, especially trxB deletion; (v) trxA and trxC, but not trxB, are involved in bacterial survival and replication in phagocytes; (vi) the thioredoxin system participates in bacterial dissemination in host immune tissues. These findings indicate that the thioredoxin system of E. piscicida plays an important role in stress resistance and virulence, which provides insight into the pathogenic mechanism of E. piscicida.