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 TrxBEp, TrxAEp, and TrxCEp, respectively) by constructing a correspondingly markerless in-frame mutant strain: ΔtrxB, ΔtrxA, and ΔtrxC, respectively. We found that (i) TrxBEp 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 trxBEp, trxAEp, and trxCEp damage E. piscicida's flagella formation and motility, and trxBEp plays a decisive role; (iv) deletions of trxBEp, trxAEp, and trxCEp substantially abate bacterial resistance against host serum, especially trxBEp deletion; (v) trxAEp and trxCEp, but not trxBEp, 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.

Metabacillus arenae sp. nov., isolated from seashore sand.

A Gram-stain-positive, non-motile, rod-shaped, facultatively anaerobic bacterium, designated as IB182487T, was isolated from a seashore sand sample collected from Zhaoshu Island, PR China. Strain IB182487T grew at pH 6.0-10.0 (optimum, pH 8.0), 4-45 °C (optimum, 25-30 °C) and with 0-17 % (w/v) NaCl (optimum, 2-10 %). Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain IB182487T belonged to the genus Metabacillus and was closely related to Metabacillus idriensis SMC 4352-2T, (96.6 %), Metabacillus indicus LMG 22858T (96.5 %), Metabacillus niabensis DSM 17723T (96.3 %) and Metabacillus halosaccharovorans DSM 25387T (96.1 %). Strain IB182487T had meso-diaminopimelic acid as the diagnostic diamino acid in the cell-wall peptidoglycan and contained menaquinone MK-7 as the predominant isoprenoid quinone. Its polar lipids consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, two unidentified phospholipids and three unidentified glycolipids. The major cellular fatty acids of strain IB182487T were iso-C15 : 0 and anteiso-C15 : 0. The whole genome average nucleotide identity and digital DNA-DNA hybridization analysis between the isolate and its closely related type strains demonstrated that the strain significantly differed from other Metabacillus species. The genomic DNA G+C content of strain IB182487T was 37.4 mol%. On the basis of phenotypic and chemotaxonomic properties, phylogenetic relatedness as well as genomic characteristics, strain IB182487T represents a novel species of the genus Metabacillus, for which the name Metabacillus arenae sp. nov. is proposed. The type strain of M. arenae is IB182487T (=MCCC 1K04629T=JCM 34523T).

Universal stress proteins contribute Edwardsiella piscicida adversity resistance and pathogenicity and promote blocking host immune response.

Universal stress proteins (Usps) exist ubiquitously in bacteria and other organisms. Usps play an important role in adaptation of bacteria to a variety of environmental stresses. There is increasing evidence that Usps facilitate pathogens to adapt host environment and are involved in pathogenicity. Edwardsiella piscicida (formerly included in E. tarda) is a severe fish pathogen and infects various important economic fish including tilapia (Oreochromis niloticus). In E. piscicida, a number of systems and factors that are involved in stress resistance and pathogenesis were identified. However, the function of Usps in E. piscicida is totally unknown. In this study, we examined the expressions of 13 usp genes in E. piscicida and found that most of these usp genes were up-regulated expression under high temperature, oxidative stress, acid stress, and host serum stress. Particularly, among these usp genes, usp13, exhibited dramatically high expression level upon several stress conditions. To investigate the biological role of usp13, a markerless usp13 in-frame mutant strain, TX01Δusp13, was constructed. Compared to the wild type TX01, TX01Δusp13 exhibited markedly compromised tolerance to high temperature, hydrogen peroxide, and low pH. Deletion of usp13 significantly retarded bacterial biofilm growth and decreased resistance against serum killing. Pathogenicity analysis showed that the inactivation of usp13 significantly impaired the ability of E. piscicida to invade into host cell and infect host tissue. Introduction of a trans-expressed usp13 gene restored the lost virulence of TX01Δusp13. In support of these results, host immune response induced by TX01 and TX01Δusp13 was examined, and the results showed reactive oxygen species (ROS) levels in TX01Δusp13-infected macrophages were significantly higher than those in TX01-infected cells. The expression level of several cytokines (IL-6, IL-8, IL-10, TNF-α, and CC2) in TX01Δusp13-infected fish was significantly higher than that in TX01-infected fish. These results suggested that the deletion of usp13 attenuated the ability of bacteria to overcome the host immune response to pathogen infection. Taken together, our study indicated Usp13 of E. piscicida was not only important participant in adversity resistance, but also was essential for E. piscicida pathogenicity and contributed to block host immune response to pathogen infection.

HutZ is required for biofilm formation and contributes to the pathogenicity of Edwardsiella piscicida.

Edwardsiella piscicida is a severe fish pathogen. Haem utilization systems play an important role in bacterial adversity adaptation and pathogenicity. In this study, a speculative haem utilization protein, HutZEp, was characterized in E. piscicida. hutZEp is encoded with two other genes, hutW and hutX, in an operon that is similar to the haem utilization operon hutWXZ identified in V. cholerae. However, protein activity analysis showed that HutZEp is probably not related to hemin utilization. To explore the biological role of HutZEp, a markerless hutZEp in-frame mutant strain, TX01ΔhutZ, was constructed. Deletion of hutZEp did not significantly affect bacterial growth in normal medium, in iron-deficient conditions, or in the presence of haem but significantly retarded bacterial biofilm growth. The expression of known genes related to biofilm growth was not affected by hutZEp deletion, which indicated that HutZEp was probably a novel factor promoting biofilm formation in E. piscicida. Compared to the wild-type TX01, TX01ΔhutZ exhibited markedly compromised tolerance to acid stress and host serum stress. Pathogenicity analysis showed that inactivation of hutZEp significantly impaired the ability of E. piscicida to invade and reproduce in host cells and to infect host tissue. In contrast to TX01, TX01ΔhutZ was defective in blocking host macrophage activation. The expression of hutZEp was directly regulated by the ferric uptake regulator Fur. This study is the first functional characterization of HutZ in a fish pathogen, and these findings suggested that HutZEp is essential for E. piscicida biofilm formation and contributes to host infection.