Type III secretion system genes of Edwardsiella tarda associated with intracellular replication and virulence in zebrafish

Virulent and Multi-drug-Resistant Edwardsiella tarda Infection in Oscar Fish: Unveiling the Threat of Mass Mortality and AMR Dissemination

The Oscar fish (Astronotus ocellatus) is among the most commonly domesticated and exported ornamental fish species from Kerala. The ornamental fish industry faces a significant challenge with the emergence of diseases caused by multi-drug-resistant bacteria. In the present study, six isolates were resolved from the diseased Oscar fish showing haemorrhages, necrosis, and loss of pigmentation. After phenotypic and genotypic characterization, the bacteria were identified as Edwardsiella tarda, Klebsiella pneumoniae, Enterococcus faecalis, Escherichia coli, Brevibacillus borstelensis, and Staphylococcus hominis. Experimental challenge studies in healthy Oscar fish showed that E. tarda caused 100% mortality within 240 h with 6.99 × 106 CFU/fish as LD50 and histopathology revealed the typical signs of infection. The pathogen was re-recovered from the moribund fish thereby confirming Koch's postulates. E. tarda was confirmed through the positive amplification of tarda-specific gene and virulence genes viz., etfD and escB were also detected using PCR. Antibiotic susceptibility tests using disc diffusion displayed that the pathogen is multi-drug-resistant towards antibiotics belonging to aminoglycosides, tetracyclines, and quinolones categories with a MAR index of 0.32, which implicated the antibiotic pressure in the farm. Plasmid curing studies showed a paradigm shift in the resistance pattern with MAR index of 0.04, highlighting the resistance genes are plasmid-borne except for the chromosome-borne tetracycline resistance gene (tetA). This study is the first of its kind in detecting mass mortality caused by E. tarda in Oscar fish. Vigilant surveillance and strategic actions are crucial for the precise detection of pathogens and AMR in aquaculture.

Secreted in a Type III Secretion System-Dependent Manner, EsaH and EscE Are the Cochaperones of the T3SS Needle Protein EsaG of Edwardsiella piscicida

The intracellular EscE protein tightly controls the secretion of the type III secretion system (T3SS) middle and late substrates in Edwardsiella piscicida. However, the regulation of secretion by EscE is incompletely understood. In this work, we reveal that EscE interacts with EsaH and EsaG. The crystal structures of the EscE-EsaH complex and EscE-EsaG-EsaH complex were resolved at resolutions of 1.4 Å and 1.8 Å, respectively. EscE and EsaH form a hydrophobic groove to engulf the C-terminal region of EsaG (56 to 73 amino acids [aa]), serving as the cochaperones of T3SS needle protein EsaG in E. piscicida. V61, K62, M64, and M65 of EsaG play a pivotal role in maintaining the conformation of the ternary complex of EscE-EsaG-EsaH, thereby maintaining the stability of EsaG. An in vivo experiment revealed that EscE and EsaH stabilize each other, and both of them stabilize EsaG. Meanwhile, either EscE or EsaH can be secreted through the T3SS. The secondary structure of EsaH lacks the fourth and fifth α helices presented in its homologs PscG, YscG, and AscG. Insertion of the α4 and α5 helices of PscG or swapping the N-terminal 25 aa of PscG with those of EsaH starkly decreases the protein level of the chimeric EsaH, resulting in instability of EsaG and deactivation of the T3SS. To the best of our knowledge, these data represent the first reported structure of the T3SS needle complex of pathogens from Enterobacteriaceae and the first evidence for the secretion of T3SS needle chaperones. IMPORTANCE Edwardsiella piscicida causes severe hemorrhagic septicemia in fish. Inactivation of the type III secretion system (T3SS) increases its 50% lethal dose (LD₅₀) by ~10 times. The secretion of T3SS middle and late substrates in E. piscicida is tightly controlled by the intracellular steady-state protein level of EscE, but the mechanism is incompletely understood. In this study, EscE was found to interact with and stabilize EsaH in E. piscicida. The EscE-EsaH complex is structurally analogous to T3SS needle chaperones. Further study revealed that EscE and EsaH form a hydrophobic groove to engulf the C-terminal region of EsaG, serving as the cochaperones stabilizing the T3SS needle protein EsaG. Interestingly, both EscE and EsaH are secreted. Our study reveals that the EscE-EsaH complex controls T3SS protein secretion by stabilizing EsaG, whose secretion in turn leads to the secretion of the middle and late T3SS substrates.

Edwardsiella piscicida Interferes with Classical Endocytic Trafficking and Replicates in a Specialized Replication-Permissive Niche in Nonphagocytic Cells

Edwardsiella piscicida is an intracellular pathogen within a broad spectrum of hosts. Essential to E. piscicida's virulence is its ability to invade and replicate inside host cells, yet the survival mechanisms and the nature of the replicative compartment remain unknown. Here, we characterized its intracellular lifestyle in nonphagocytic cells and showed that the intracellular replication of E. piscicida in nonphagocytic cells is dependent on its type III secretion system (T3SS) but not its type VI secretion system. Following internalization, E. piscicida is contained in vacuoles that transiently mature into early endosomes but subsequently bypasses the classical endosome pathway and fusion with lysosomes, which depend on its T3SS. Following rapid escape from the degradative pathway, E. piscicida was found to create a specialized replication-permissive niche characterized by endoplasmic reticulum (ER) markers. Furthermore, we found that a T3SS effector, EseJ, is responsible for the intracellular replication of E. piscicida by preventing endosome/lysosome fusion. In vivo experiments also confirmed that EseJ is necessary for bacterial colonization by E. piscicida in the epithelial layer, followed by systemic dissemination in both zebrafish and mice. Thus, this work elucidates the tactics used by E. piscicida to survive and proliferate within host nonphagocytic cells. IMPORTANCE E. piscicida is a facultative intracellular bacterium associated with septicemia and fatal infections in many animals, including fish and humans. However, little is known about its intracellular life, which is important for successful invasion of the host. The present study is the first comprehensive characterization of E. piscicida's intracellular lifestyle in host cells. Upon internalization, E. piscicida is transiently contained in Rab5-positive vacuoles, but the pathogen prevents further endosome maturation and fusion with lysosomes by utilizing a T3SS effector, EseJ. In addition, the bacterium creates a specialized replication niche for rapid growth via an interaction with the ER. Our study provides new insights into the strategies used by E. piscicida to successfully establish an intracellular lifestyle that contributes to its survival and dissemination during infection.

The zebrafish as a model for gastrointestinal tract-microbe interactions

The zebrafish (Danio rerio) has become a widely used vertebrate model for bacterial, fungal, viral, and protozoan infections. Due to its genetic tractability, large clutch sizes, ease of manipulation, and optical transparency during early life stages, it is a particularly useful model to address questions about the cellular microbiology of host-microbe interactions. Although its use as a model for systemic infections, as well as infections localised to the hindbrain and swimbladder having been thoroughly reviewed, studies focusing on host-microbe interactions in the zebrafish gastrointestinal tract have been neglected. Here, we summarise recent findings regarding the developmental and immune biology of the gastrointestinal tract, drawing parallels to mammalian systems. We discuss the use of adult and larval zebrafish as models for gastrointestinal infections, and more generally, for studies of host-microbe interactions in the gut.

Isolation and characterization of phage (ETP-1) specific to multidrug resistant pathogenic Edwardsiella tarda and its in vivo biocontrol efficacy in zebrafish (Danio rerio)

Edwardsiella tarda phage (ETP-1) was isolated from marine fish farm water to characterize its effect against pathogenic multidrug-resistant E. tarda. According to transmission electron microscopy results, ETP-1 is classified as a member of the family Podoviridae. ETP-1 showed MOI dependent E. tarda growth inhibition, a latent period of 60 min, and burst size of 100 PFU per infected cells. In host range tests, five out of eight E. tarda strains were sensitive to ETP-1 which had efficiency of plating index in the range 1-1.28. ETP-1 was stable over a broad range of pH and temperature. The size of the ETP-1 genome was predicted to be approximately 40 kb. Zebrafish exposed to ETP-1 showed no adverse gene responses to the inflammatory mediator cytokines, il1-β, tnf-α, il-6, and il-10, the chemokine, cxcl-8a, and reactive oxygen species, sod-1. When zebrafish were bath exposed to ETP-1 for 12 days and simultaneously challenged with E. tarda (1.08 × 10⁵ CFU fish^-1), the survival rate was higher in phage exposed fish (68%) compared to that of the control (18%) until 4 days post challenge. Our results suggest that ETP-1 can be used as a potential bio-therapeutic candidate to control multi-drug resistant E. tarda infection in aquaculture.

Genome-Wide Identification of Fitness Factors in Seawater for Edwardsiella piscicida

Marine pathogens are transmitted from one host to another through seawater. Therefore, it is important for marine pathogens to maintain survival or growth in seawater. However, little is known about how marine pathogens adapt to living in seawater environments. Here, transposon insertion sequencing was performed to explore the genetic determinants of Edwardsiella piscicida survival in seawater at 16 and 28°C. Seventy-one mutants with mutations mainly in metabolism-, transportation-, and type III secretion system (T3SS)-related genes showed significantly increased or impaired fitness in 16°C water. In 28°C seawater, 63 genes associated with transcription and translation, as well as energy production and conversion, were essential for optimal survival of the bacterium. In particular, 11 T3SS-linked mutants displayed enhanced fitness in 16°C seawater but not in 28°C seawater. In addition, 13 genes associated with oxidative phosphorylation and 4 genes related to ubiquinone synthesis were identified for survival in 28°C seawater but not in 16°C seawater, which suggests that electron transmission and energy-producing aerobic respiration chain factors are indispensable for E. piscicida to maintain survival in higher-temperature seawater. In conclusion, we defined genes and processes related to metabolism and virulence that operate in E. piscicida to facilitate survival in low- and high-temperature seawater, which may underlie the infection outbreak mechanisms of E. piscicida and facilitate the development of improved vaccines against marine pathogens.IMPORTANCE Edwardsiella piscicida is one of the most important marine pathogens and causes serious edwardsiellosis in farmed fish during the summer-autumn seasonal changes, resulting in enormous losses to aquaculture industries worldwide. Survival and transmission of the pathogen in seawater are critical steps that increase the risk of outbreaks. To investigate the mechanism of survival in seawater for this marine pathogen, we used transposon insertion sequencing analysis to explore the fitness determinants in summer and autumn seawater. Approximately 127 genes linked to metabolism and virulence, as well as other processes, were revealed in E. piscicida to contribute to better adaptations to the seasonal alternations of seawater environments; these genes provide important insights into the infection outbreak mechanisms of E. piscicida and potential improved treatments or vaccines against marine pathogens.

The Edwardsiella piscicida Type III Translocon Protein EseC Inhibits Biofilm Formation by Sequestering EseE

The type III secretion system (T3SS) is one of the most important virulence factors of the fish pathogen Edwardsiella piscicida It contains three translocon proteins, EseB, EseC, and EseD, required for translocation of effector proteins into host cells. We have previously shown that EseB forms filamentous appendages on the surface of E. piscicida, and these filamentous structures mediate bacterial cell-cell interactions promoting autoaggregation and biofilm formation. In the present study, we show that EseC, but not EseD, inhibits the autoaggregation and biofilm formation of E. piscicida At 18 h postsubculture, a ΔeseC strain developed strong autoaggregation and mature biofilm formation, accompanied by enhanced formation of EseB filamentous appendages. This is in contrast to the weak autoaggregation and immature biofilm formation seen in the E. piscicida wild-type strain. EseE, a protein that directly binds to EseC and also positively regulates the transcription of the escC-eseE operon, was liberated and showed increased levels in the absence of EseC. This led to augmented transcription of the escC-eseE operon, thereby increasing the steady-state protein levels of intracellular EseB, EseD, and EseE, as well as biofilm formation. Notably, the levels of intracellular EseB and EseD produced by the ΔeseE and ΔeseC ΔeseE strains were similar but remarkably lower than those produced by the wild-type strain at 18 h postsubculture. Taken together, we have shown that the translocon protein EseC inhibits biofilm formation through sequestering EseE, a positive regulator of the escC-eseE operon.IMPORTANCE Edwardsiella piscicida, previously known as Edwardsiella tarda, is a Gram-negative intracellular pathogen that mainly infects fish. The type III secretion system (T3SS) plays a pivotal role in its pathogenesis. The T3SS translocon protein EseB is required for the assembly of filamentous appendages on the surface of E. piscicida The interactions between the appendages facilitate autoaggregation and biofilm formation. In this study, we explored the role of the other two translocon proteins, EseC and EseD, in biofilm formation. We have demonstrated that EseC, but not EseD, inhibits the autoaggregation and biofilm formation of E. piscicida, providing new insights into the regulatory mechanism involved in E. piscicida biofilm formation.

Neutrophil plays critical role during Edwardsiella piscicida immersion infection in zebrafish larvae

Edwardsiella piscicida is a facultative intracellular pathogen that causes hemorrhagic septicemia and haemolytic ascites disease in aquaculture fish. During bacterial infection, macrophages and neutrophils are the first line of host innate immune system. However, the role of neutrophils in response to E. piscicida infection in vivo remains poorly understood. Here, through developing an immersion infection model in the 5 day-post fertilization (dpf) zebrafish larvae, we found that E. piscicida was mainly colonized in intestine, and resulted into significant pathological changes in paraffin sections. Moreover, a dynamic up-regulation of inflammatory cytokines (TNF-α, IL-1β, GCSFb, CXCL8 and MMP9) was detected in zebrafish larvae during E. piscicida infection. Furthermore, a significant recruitment of neutrophils was observed during the E. piscicida infection in Tg(mpx:eGFP) zebrafish larvae. Thus, we utilized the CRISPR/Cas9 system to generate the neutrophil-knockdown (gcsfr^-/- crispants) larvae, and found a comparative higher mortality and bacterial colonization in gcsfr^-/- crispants, which reveals the critical role of fish neutrophils in bacterial clearance. Taken together, our results developed an effective E. piscicida immersion challenge model in zebrafish larvae to clarify the dynamic of bacterial infection in vivo, which would provide a better understanding of the action about innate immune cells during infection.

The role of regulator FucP in Edwardsiella tarda pathogenesis and the inflammatory cytokine response in tilapia

The animal intestine provides a competitive environment for the microbiota. Successful colonization by pathogens requires sensing chemical signals to regulate the expression of virulence genes. Some bacteria rely on a two-component chemical signal transduction system, named FusKR, to regulate virulence genes expression by intestinal fucose. Here we construct the fucP gene deletion strain prove FucP regulation of the T3SS in E. tarda. The result showed that the mutant strain had down-regulated significantly the gene expression of FusKR and T3SS compared to the wild-type strain (P < 0.05). This mutant strain significantly increased LD₅₀ in zebrafish compared to the wild-type strain (P < 0.05), and significantly decreased penetration and motility in mucin than the wild-type strain (P < 0.05). Meanwhile, tilapia infected with mutant strain show significantly reduced E. tarda adherence and colonization than those infected with the wild-type strain (P < 0.05). Fish infected with EIB202 and ΔfucP showed significantly higher (P < 0.05) gene expression of IL-1β, TNF-α, IFN-γ, TGF-β and HSP-70 in head kidney than fish treated with PBS in the whole observed period; however CPP-3 did not show significant differences (P > 0.05) in all groups. Fish infected with EIB202 showed significantly higher (P < 0.05) gene expression of TGF-β in head kidney than fish treated with ΔfucP in the whole observed period; however other cytokines did not show significant differences (P > 0.05) in the whole observed period. In addition, Fish infected with EIB202 showed significantly higher (P < 0.05) gene expression of IL-1β, TNF-α and TGF-β in spleen than fish treated with ΔfucP in the whole observed period, however IFN-γ, CPP-3, and HSP-70 did not show significant differences (P > 0.05) in the whole observed period. Although the gene expression of cytokines was induced similarly by both strains, all results indicate that the fucP gene deletion down-regulates the key gene expression of FucKR and T3SS, reduces the pathogenicity of E. tarda in fish, particularly decreases inducing the gene expression of TGF-β in the head kidney and IL-1β, TNF-α and TGF-β in the spleen.

Intracellular Bacterial Infections: A Challenge for Developing Cellular Mediated Immunity Vaccines for Farmed Fish

Aquaculture is one of the most rapidly expanding farming systems in the world. Its rapid expansion has brought with it several pathogens infecting different fish species. As a result, there has been a corresponding expansion in vaccine development to cope with the increasing number of infectious diseases in aquaculture. The success of vaccine development for bacterial diseases in aquaculture is largely attributed to empirical vaccine designs based on inactivation of whole cell (WCI) bacteria vaccines. However, an upcoming challenge in vaccine design is the increase of intracellular bacterial pathogens that are not responsive to WCI vaccines. Intracellular bacterial vaccines evoke cellular mediated immune (CMI) responses that “kill” and eliminate infected cells, unlike WCI vaccines that induce humoral immune responses whose protective mechanism is neutralization of extracellular replicating pathogens by antibodies. In this synopsis, I provide an overview of the intracellular bacterial pathogens infecting different fish species in aquaculture, outlining their mechanisms of invasion, replication, and survival intracellularly based on existing data. I also bring into perspective the current state of CMI understanding in fish together with its potential application in vaccine development. Further, I highlight the immunological pitfalls that have derailed our ability to produce protective vaccines against intracellular pathogens for finfish. Overall, the synopsis put forth herein advocates for a shift in vaccine design to include CMI-based vaccines against intracellular pathogens currently adversely affecting the aquaculture industry.

ORF13 in the Type III secretion system gene cluster of Edwardsiella tarda binds to the mammalian factor Cugbp2

The Type III secretion system (TTSS) is essential for the intracellular replication of Edwardsiella tarda in phagocytes of fish and mammals, and a hypothetical gene (orf13) located in the TTSS gene cluster is required for intracellular replication and virulence of E. tarda. Here, we show that under TTSS-inducing conditions, the protein ORF13 was secreted into culture supernatant. Then, using a yeast 2-hybrid screen, we show that the mammalian factor Cugbp2, which regulates apoptosis in breast cancer cells, directly interacts with ORF13. A pull-down assay revealed that ORF13 binds to the C-terminal region of Cugbp2. Our results suggest that ORF13 may facilitate E. tarda replication in phagocytes by binding to Cugbp2.

First report of Edwardsiellosis in cage-cultured sharpsnout sea bream, Diplodus puntazzo from the Mediterranean

Background

Edwardsiella tarda, is a serious bacterial pathogen affecting a broad range of aquaculture fish species. The bacterium has also been reported as a human pathogen, however recent studies have dissociated the fish pathogenic Edwardsiella from those isolated from humans by placing them in a new species, E. piscicida. Here we report the first case of Edwardsiellosis in cultured sharpsnout sea breams, Diplodus puntazzo in Greece.

Case presentation

The disease has affected cultured sharpsnout sea breams of a commercial fish farm in a single location in East Greece. Two populations of sharpsnout sea breams stocked in two consecutive years in floating cages presented signs of disease which included nodules and abscesses in spleen and kidney, morbidity and cumulative mortality reaching 5.3 %. Using microbiological, biochemical and molecular tools we have identified Edwardsiella sp. as the main aetiological factor of the disease. Following phylogenetic analysis the bacterial isolates are grouped with the newly described Edwardsiella piscicida species.

Conclusions

This is the first report of Edwardsiellosis in this species but most importantly in sea cage-cultured fish in the Mediterranean which may pose a serious threat for aquaculture fish species in this region.