Edwardsiella tarda – Virulence mechanisms of an emerging gastroenteritis pathogen

MicroRNA-regulated flounder CLDN4 functions in anti-bacterial immunity

CLDN4 belongs to a multi-transmembrane protein family of claudins, which mainly functions in cell-cell adhesion and migration. MicroRNAs (miRNAs) are important post-transcriptional regulating factors that participate in broad biological process including immunity. Through high-throughput RNA sequencing strategy, a flounder miRNA, miR-29-x, was identified to be responsible to both bacteria and virus. In this study, we explored the regulatory mechanism and function of miR-29-x and its target gene of flounder CLDN4 (named PoCLDN4). We proved that miR-29-x could interact with the 3'UTR of PoCLDN4 and negatively regulate its expression. PoCLDN4 located on cell membrane, while the depletion of extracellular loop E2 abolished the membrane localization of this protein. E3 could bind different bacteria, and mutation of the amino acids of 13E and 18E enhanced this capacity, while mutation of 10L abolish this capacity. Further study revealed the bacteria killing effect of E3 and verified 10L as a key factor. These results identified the interaction between miR-29-x and PoCLDN4, and unraveled the function as well as the molecular basis of flounder CLDN4 in anti-bacterial immunity.

A case of septic shock caused by drug-resistant Edwardsiella tarda and literature review

BACKGROUND

Edwardsiella tarda (E. tarda) causes highly mortality, which is rare in septic patients. We herein reported a case of septic shock caused by drug-resistant E. tarda.

CASE PRESENTATION

We herein describe a 32-year-old female with septic shock who had the medical history of abortion 1 month ago and "systemic lupus erythematosus and rheumatoid arthritis" presented abdominal pain, diarrhea, and dyspnea as the primary symptoms and rapidly deteriorated to MODS following breakfast (undercooked fish porridge) in the ICU. Sepsis surviving bundle was initiated by collecting pathogen culture (sputum, urine and blood samples), empirically broad-spectrum antibiotics administration (Meropenem), along with fluid resuscitation, vasopressor use. E. tarda was confirmed both in blood culture and mNGS (metagenomics next generation sequencing). Thus, the antibiotics were switched to piperacillin-tazobactam according to the susceptibility test that was susceptible to piperacillin-tazobactam and resistant to ampicillin, quinolones and gentamicin. The patient finally recovered and discharged after 18 days of ICU treatment.

CONCLUSIONS

Empiric antibiotics should be selected with piperacillin-tazobactam and amikacin, and avoid ampicillin, quinolones and gentamicin for suspecting E. tarda infection in southern China. Bacteremia complicated with septic shock caused by E. tarda requires intensive care to improve survival rates.

Fulminant Edwardsiella tarda bacteremia following near-drowning episode in a patient without cirrhosis: A case report

Edwardsiella tarda is a Gram-negative intracellular pathogen within the Enterobacterales order, recognized as a causative agent of hemorrhagic septicemia in fish but also pathogenic to humans. However, the clinical course and prognostic factors of E. tarda bacteremia are not fully understood. We present a case of fulminant E. tarda bacteremia in a 90-year-old Japanese man without significant immunosuppression, developing after exposure to freshwater. The patient was admitted to the intensive care unit with septic shock following an accidental fall into an irrigation ditch. Despite appropriate empiric treatment, he passed away within 18 hours of admission. Blood cultures identified E. tarda, while Aeromonas hydrophila was detected in sputum. This case highlights the rapid progression and high mortality risk associated with E. tarda bacteremia; prolonged exposure to contaminated water and delayed transport to medical care were likely contributory factors to the fatal outcome. Furthermore, the detection of waterborne pathogens like A. hydrophila in respiratory samples may serve as a clue to consider possible water exposure in diagnostic reasoning.

T3SS translocon induces pyroptosis by direct interaction with NLRC4/NAIP inflammasome

Type III secretion system (T3SS) is a virulence apparatus existing in many bacterial pathogens. Structurally, T3SS consists of the base, needle, tip, and translocon. The NLRC4 inflammasome is the major receptor for T3SS needle and basal rod proteins. Whether other T3SS components are recognized by NLRC4 is unclear. In this study, using Edwardsiella tarda as a model intracellular pathogen, we examined T3SS-inflammasome interaction and its effect on cell death. E. tarda induced pyroptosis in a manner that required the bacterial translocon and the host inflammasome proteins of NLRC4, NLRP3, ASC, and caspase 1/4. The translocon protein EseB triggered NLRC4/NAIP-mediated pyroptosis by binding NAIP via its C-terminal region, particularly the terminal 6 residues (T6R). EseB homologs exist widely in T3SS-positive bacteria and share high identities in T6R. Like E. tarda EseB, all of the representatives of the EseB homologs exhibited T6R-dependent NLRC4 activation ability. Together these results revealed the function and molecular mechanism of EseB to induce host cell pyroptosis and suggested a highly conserved inflammasome-activation mechanism of T3SS translocon in bacterial pathogens.

Host-intestinal microbiota interactions in Edwardsiella piscicida-induced lethal enteritis in big-belly seahorses: Novel insights into the role of Carbohydrate-Active enzymes and host transcriptional responses

Edwardsiella piscicida-induced lethal enteritis is a major threat to the sustainable development of seahorse aquaculture. The roles of Carbohydrate-Active enzymes (CAZymes) in interactions between the pathogen and the host are poorly understood. In this study, we found that 22 key CAZymes encoded by E. piscicida might involve in the coordination of five key stages of infection. Specifically, during the motility, adherence, and invasion stages, 10 CAZymes, including CE4, PL8, and CBM48, may significantly increase the activities of Lipid metabolism-associated pathways of the intestinal microbiota (P < 0.01), facilitating pathogen invasion of the host intestinal epithelium. During the replication stage, 11 CAZymes, including GH20, GT4, and GH3, may significantly increase activities of pathways associated with Carbohydrate metabolism (P < 0.01) to promote replication and proliferation of the pathogen. And for avoiding host defenses, GH2 and GH1 may enhance activities of both Carbohydrate and Amino acid metabolic pathways (P < 0.01), facilitating infection and immune evasion. Conjoint analysis showed that E. piscicida might mainly rely on Carbohydrate metabolism for infection, while the host might activate Amino acid metabolic pathways for self-defense. In addition, expressions of 10 key genes, Aldh9a1b, Aoc1, Tpi1b, PCK1, Ldha, Me1, Gla, Cel.2, Ugdh, and Mao, were significantly altered (P < 0.01) and may be used for characterizing host responses to E. piscicida infection. Activities of both Glycolysis/Gluconeogenesis and Tryptophan metabolism were found oppositely changed (P < 0.01) between pathogen and host, respectively, representing the primary focuses of the competition. Overall, this study provides new insights into E. piscicida-mediated intestinal enteritis in fish for the first time from the perspective of CAZymes, as well as a theoretical reference for the prevention and control of these diseases in the aquaculture of seahorses and other fish.

An intracellular bacterial pathogen triggers RIG-I/MDA5-dependent necroptosis

RIG-I and MDA5 are members of RIG-I-like receptors (RLRs) that detect viral RNA within the cytoplasm and subsequently initiate antiviral immune responses. Necroptosis is a form of programmed cell death (PCD) executed by mixed lineage kinase domain-like (MLKL), which, upon phosphorylation by receptor-interacting protein kinase 3 (RIPK3), causes necrotic cell death. To date, no link between RLRs and necroptosis has been observed during bacterial infection. Edwardsiella tarda is a zoonotic bacterial pathogen that can thrive in host macrophages. In a previous study, we identified RIG-I and MDA5 as two hub factors of RAW264.7 cells responsive to E. tarda infection. The present study aimed to determine the specific form of cell death triggered by E. tarda and explore the association between RIG-I/MDA5 and PCD in the context of bacterial infection. Our results showed that E. tarda infection induced RIPK3-MLKL-mediated necroptosis, rather than pyroptosis or apoptosis, in RAW264.7 cells. Meanwhile, E. tarda promoted RIG-I/MDA5 production and activated the RIG-I/MDA5 pathways that led to IRF3 phosphorylation, IFN-β secretion, and interferon-stimulated gene (ISG) and cytokine expression. Both RIG-I and MDA5 were essential for E. tarda-triggered necroptosis and required for effective inhibition of intracellular bacterial replication. Furthermore, the regulatory effect of RIG-I/MDA5 on necroptosis was not affected by type I IFN or TNF-α signaling blockage. Together these results revealed that necroptosis could be triggered by intracellular bacterial infection through the RIG-I/MDA5 pathways, and that there existed intricate interplays between PCD and RLRs induced by bacterial pathogen.

mNGS-identified cellulitis due to quinolone-resistant Edwardsiella tarda: a case report

Edwardsiella tarda is frequently isolated from aquatic animals and environments. While human infections caused by E. tarda are rare, some extraintestinal infections can be severe. This case report describes a patient with cellulitis of the right upper extremity of unknown origin. Metagenomic next-generation sequencing (mNGS) indicated that the patient was infected with E. tarda. Antimicrobial susceptibility testing revealed that the isolate was resistant to quinolones and trimethoprim/sulfamethoxazole. The isolate, positive for four virulence genes (fimA, gadB, mukF, and sodB), was confirmed to be virulent using the Galleria mellonella larvae model. Following early pus drainage and a 9-day course of imipenem, the patient ultimately recovered. This case report aimed to illustrate the presentation, diagnosis, and management of uncommon cellulitis caused by drug-resistant, virulent E. tarda.

Development and Validation of the MAST ISOPLEX®VTEC Kit for Simultaneous Detection of Shiga Toxin/Verotoxin 1 and 2 (stx1/vt1 and stx2/vt2) with Inhibition Control (IC) in a Rapid Loop-Mediated Isothermal Amplification (LAMP) Multiplex Assay

Loop-mediated isothermal amplification (LAMP) is a cost-effective, rapid, and highly specific method of replicating nucleic acids. Adding multiple targets into a single LAMP assay to create a multiplex format is highly desirable for clinical applications but has been challenging due to a need to develop specific detection techniques and strict primer design criteria. This study describes the evaluation of a rapid triplex LAMP assay, MAST ISOPLEX®VTEC, for the simultaneous detection of Shiga toxin/verotoxin 1 and 2 (stx1/vt1 and stx2/vt2) genes in verotoxigenic Escherichia coli (E. coli) (VTEC) isolates with inhibition control (IC) synthetic DNA using a single fluorophore-oligonucleotide probe, MAST ISOPLEX®Probes, integrated into the primer set of each target. MAST ISOPLEX®Probes used in the MAST ISOPLEX®VTEC kit produce fluorescent signals as they integrate with reaction products specific to each target, allowing tracking of multiple amplifications in real time using a real-time analyzer. Initial validation on DNA extracts from fecal cultures and synthetic DNA sequences (gBlocks) showed that the MAST ISOPLEX®VTEC kit provides a method for sensitive simultaneous triplex detection in a single assay with a limit of detection (LOD) of less than 100 target copies/assay and 96% and 100% sensitivity and specificity, respectively.

Establishment and clinical application of a droplet digital PCR method for the detection of Edwardsiella tarda

Edwardsiella tarda (E. tarda) can infect humans and a variety of animals, including fish, amphibians, reptiles, birds, and mammals. However, a more highly sensitive, specific, and repeatable test for its detection is lacking. The objective of this study was to develop a highly sensitive, specific, and repeatable droplet digital polymerase chain reaction (ddPCR)-based method for the quantitative detection of E. tarda. The gyrB gene was selected as the target gene, and primers and probe were designed and synthesized. Using E. tarda genomic DNA as templates, the reaction method was optimized to establish a linear relationship with real-time PCR detection methods. The sensitivity, specificity, and repeatability of the method were analyzed, and clinical samples were tested. When the primer and probe concentrations were 900 and 300 nM, respectively, and the annealing temperature was 57°C, the efficiency of the ddPCR amplification reaction was highest and the boundary between positive and negative droplet distribution was clearest. The sensitivity was high, with detection limit being as low as 0.56 copies·μL-1; additionally, and a good linear relationship (R 2 = 0.9962) between ddPCR and real-time PCR detection, within the range of 1-25,000 copies·μL-1, was evident. The repeatability was good, with a detection coefficient of variation of 2.74%. There was no cross-reactivity with 15 other common pathogenic microorganisms in aquatic animals (Streptococcus agalactiae, Streptococcus iniae, Streptococcus suis type 2, Nocardia seriolae, Vibrio parahaemolyticus, Aeromonas sobria, red sea bream iridovirus, decapod iridescent virus 1, enterocytozoon hepatopenaei, carp edema virus, Koi herpesvirus, goldfish hematopoietic necrosis virus, tilapia lake virus, viral nervous necrosis virus, or grass carp reovirus) in positive samples. Among the 48 clinical samples, including Bahaba taipingensis and its live food fish, pond water samples, and routine monitoring samples (Koi), 21 were positive for E. tarda, consistent with the bacterial isolation and identification results. The E. tarda ddPCR detection method has high specificity, sensitivity, and repeatability, can more accurately quantify E. tarda, and provides a useful reference for research related to this bacterium.

Comparative pathogenicity and histopathological analysis of Edwardsiella anguillarum intraperitoneal infection in milkfish (Chanos chanos), Nile tilapia (Oreochromis niloticus) and Asian seabass (Lates calcarifer)

Edwardsiella anguillarum, a highly virulent species within the Edwardsiella genus, causes significant mortality in milkfish farms in Taiwan. This study aimed to investigate the comparison of milkfish susceptibility, a newly identified host species in Taiwanese aquaculture, with other species Nile tilapia (Oreochromis niloticus) and Asian seabass (Lates calcarifer), to E. anguillarum, elucidating its pathogenicity across both seawater and freshwater aquaculture environments. The results showed milkfish exhibited the highest mortality rate of 85% within 48 h of infection, whereas Nile tilapia exhibited a mortality rate of 70% between the second- and tenth-day post challenge, and seabass exhibited a mortality rate of 25% between the second- and sixth-day post challenge. Gross lesions observed in milkfish included splenomegaly and haemorrhage, whereas Nile tilapia exhibited signs of ascites, exophthalmia and brain haemorrhage. Seabass displayed spleen granulomas and haemorrhage at the injection site. Histopathological analysis revealed common features across all three species, including multifocal necrosis, bacterial presence in the necrotic areas, serositis and oedema. Asian seabass also exhibited chronic lesions in the form of splenic granulomas. This study highlights the high susceptibility of milkfish and Nile tilapia to E. anguillarum, emphasizing the urgent need for further investigation into targeted vaccine development for these fish species. These results not only deepen our understanding of the differing levels of pathogenicity among the three species but also offer valuable insights for improving disease prevention and management strategies in aquaculture, including those applied within polyculture systems and for the maintenance of aquaculture water environments.

Accumulation and Phagocytosis of Fluorescently Visualized Macrophages Against Edwardsiella piscicida Infection in Established mpeg1.1-Transgenic Japanese Medaka Oryzias latipes

Intracellular bacteria such as those belonging to the genus Edwardsiella can survive and proliferate within macrophages. However, the detailed mechanisms underlying the host macrophage immune response and pathogen evasion strategies remain unknown. To advance the field of host macrophage research, we successfully established transgenic (Tg) Japanese medaka Oryzias latipes that possesses fluorescently visualized macrophages. As a macrophage marker, the macrophage-expressed gene 1.1 (mpeg1.1) was selected because of its predominant expression across various tissues in medaka. To validate the macrophage characteristics of the fluorescently labeled cells, May-Grünwald Giemsa staining and peroxidase staining were conducted. The labeled cells exhibited morphological features consistent with those of monocyte/macrophage-like cells and tested negative for peroxidase activity. Through co-localization studies, the fluorescently labeled cells co-localized with E. piscicida in the intestines and kidneys of infected medaka larvae, confirming the ingestion of bacteria through phagocytosis. In addition, the labeled cells expressed macrophage markers but lacked a neutrophil marker. These results suggested that the fluorescently labeled cells of Tg[mpeg1.1:mCherry/mAG] medaka were monocytes/macrophages, which will be useful for future studies aimed at understanding the mechanisms of macrophage-mediated bacterial infections.

Long-chain unsaturated fatty acids sensor controlling the type III/VI secretion system is essential for Edwardsiella piscicida infection

Edwardsiella piscicida is an acute marine pathogen that causes severe damage to the aquaculture industry worldwide. The pathogenesis of E. piscicida is dependent mainly on the type III secretion system (T3SS) and type VI secretion system (T6SS), both of which are critically regulated by EsrB and EsrC. In this study, we revealed that fatty acids influence T3SS expression. Unsaturated fatty acids (UFAs), but not saturated fatty acids (SFAs), directly interact with EsrC, which abolishes the function of EsrC and results in the turn-off of T3/T6SS. Moreover, during the in vivo colonization of E. piscicida, host fatty acids were observed to be transported into E. piscicida through FadL and to modulate the expression of T3/T6SS. Furthermore, the esrCR38G mutant blocked the interaction between EsrC and UFAs, leading to dramatic growth defects in DMEM and impaired colonization in HeLa cells and zebrafish. In conclusion, this study revealed that the interaction between UFAs and EsrC to turn off T3/T6SS expression is essential for E. piscicida infection.

Functional analysis of OmpA and its contribution to pathogenesis of Edwardsiella tarda

Outer membrane protein A (OmpA), a major component of outer membrane proteins in gram-negative bacteria, is considered to be an important virulence factor in various pathogenic bacteria, but its underlying mechanisms involved in pathogenic process of Edwardsiella tarda has not yet been fully elucidated. E. tarda is an important facultative intracellular pathogen with a broad host range. This bacterium could survive and replicate in macrophages as an escape mechanism from the host defense. To address the functions of OmpA and its potential roles in the pathogenesis of E. tarda, ΔompA mutant strain and ΔompA-C complementary strain were constructed by the allelic exchange method in this study. Here, we demonstrate that the abilities of motility, biofilm formation and adherence to RAW264.7 cells of ΔompA were significantly impaired, although there was no difference in growth between wild-type (WT) strain and ΔompA. Moreover, inactivation of ompA rendered E. tarda more sensitive to oxidative, heat shock and osmotic stress, which simulate the in vivo conditions that E. tarda encounters within the intramacrophage environment. Consist with this observation, ΔompA was also found to be markedly attenuated for growth within macrophages. In addition, compared with the WT strain, ΔompA activated macrophages to release more inflammatory mediators, including tumor necrosis factor alpha (TNF-α), reactive oxygen species (ROS) and nitric oxide (NO). However, flow cytometry analysis revealed that ΔompA induced less apoptosis of RAW264.7 cells as compared with WT strain, characterized by decreased Annexin V binding and the activation of caspase-3. Overall, our findings suggest an importance of OmpA to E. tarda and provide the first comprehensive insight into its functions and potential roles in the pathogenesis of E. tarda, including its effect on interaction with macrophages.

Overview on the role of dietary Spirulina platensis on immune responses against Edwardsiellosis among Oreochromis niloticus fish farms

Edwardsiellosis is a bacterial fish disease that mostly occurs in freshwater farms and is characterized by a high mortality rate. Edwardsiella tarda strain was recovered from 17 fish out of 50 Nile tilapia, which were harboring clinical signs of systemic septicemia. The level of un-ionized ammonia (NH3) in the fish farm's water was 0.11-0.15 mg/L, which was stressful for the Nile tilapia.Sequencing of the gyrB1 gene confirmed that the isolate was E. tarda JALO4, and it was submitted to NCBI under the accession number PP449014. The isolated E. tarda harbored the virulence gene edw1 AHL-synthase (quorum sensing). In addition, the isolate was sensitive to trimethoprim and sulfamethoxazole mean while it was intermediate to florfenicol. The median lethal dose (LD50) of E. tarda JALO4 was determined to be 1.7 × 105 CFU/mL in Nile tilapia.In the indoor experiment, Nile tilapia (45.05 ± 0.4 g), which received dietary Spirulina platensis (5 and 10 g/kg fish feed), showed optimum growth and feed utilization. Meanwhile, after receiving dietary S. platensis, the fish's feed conversion ratio (FCR) was significantly enhanced compared to the control, which was 1.94, 1.99, and 2.88, respectively. The expression of immune-related genes interleukin (IL)-1β and tumor necrosis factor (TNF)-α were upsurged in E. tarda-challenged fish with higher intensity in S. platensis groups. Dietary S. platensis at a dose of 10 g/kg fish feed could provide a relative protection level (RPL) of 22.2% Nile tilapia challenged against E. tarda. Nile tilapia experimentally infected E. tarda, drastically altering their behavior: higher operculum movement, low food apprehension, and abnormal swimming dietary S. platensis (10 g/kg fish feed) could rapidly restore normal status.It was concluded that Edwardsiellosis could alter Nile tilapia behavior with a high loss in fish population. Fish received dietary-S. platensis could rapidly restore normal behavior after E. tarda infection. It is recommended the incorporation of S. platensis at doses of 10 g/kg into the Nile tilapia diet to boost their immunity and counteract E. tarda infection.

Intestinal microbiota and gene expression alterations in leopard coral grouper (Plectropomus leopardus) under enteritis

Enteritis poses a significant threat to fish farming, characterized by symptoms of intestinal and hepatic inflammation, physiological dysfunction, and dysbiosis. Focused on the leopard coral grouper (Plectropomus leopardus) with an enteritis outbreak on a South China Sea farm, our prior scrutiny did not find any abnormalities in feeding or conventional water quality factors, nor were any specific pathogen infections related to enteritis identified. This study further elucidates their intestinal flora alterations, host responses, and their interactions to uncover the underlying pathogenetic mechanisms and facilitate effective prevention and management strategies. Enteritis-affected fish exhibited substantial differences in intestinal flora compared to control fish (P = 0.001). Notably, norank_f_Alcaligenaceae, which has a negative impact on fish health, predominated in enteritis-affected fish (91.76 %), while the probiotic genus Lactococcus dominated in controls (93.90 %). Additionally, certain genera with pathogenesis potentials like Achromobacter, Sphingomonas, and Streptococcus were more abundant in diseased fish, whereas Enterococcus and Clostridium_sensu_stricto with probiotic potentials were enriched in control fish. At the transcriptomic level, strong inflammatory responses, accompanied by impaired metabolic functions, tissue damage, and iron death signaling activation were observed in the intestines and liver during enteritis. Furthermore, correlation analysis highlighted that potential pathogen groups were positively associated with inflammation and tissue damage genes while presenting negatively correlated with metabolic function-related genes. In conclusion, dysbiosis in the intestinal microbiome, particularly an aberrantly high abundance of Alcaligenaceae with pathogenic potential may be the main trigger for this enteritis outbreak. Alcaligenaceae alongside Achromobacter, Sphingomonas, and Streptococcus emerged as biomarkers for enteritis, whereas some species of Lactococcus, Clostridium_sensu_stricto, and Enterococcus showed promise as probiotics to alleviate enteritis symptoms. These findings enhance our understanding of enteritis pathogenesis, highlight intestinal microbiota shifts in leopard coral grouper, and propose biomarkers for monitoring, probiotic selection, and enteritis management.

Genomic Insights into Edwardsiella ictaluri: Molecular Epidemiology and Antimicrobial Resistance in Striped Catfish (Pangasianodon hypophthalmus) Aquaculture in Vietnam

Edwardsiella ictaluri is responsible for causing bacillary necrosis (BNP) in striped catfish (Pangasianodon hypophthalmus) in Vietnam. This study offers a comprehensive genomic characterization of E. ictaluri to enhance understanding of the molecular epidemiology, virulence, and antimicrobial resistance. E. ictaluri isolates were collected from diseased striped catfish in the Mekong Delta. The species was confirmed through PCR. Antimicrobial susceptibility testing was conducted using minimum inhibitory concentrations for commonly used antimicrobials. Thirty representative isolates were selected for whole genome sequencing to delineate their genomic profiles and phylogeny. All strains belonged to ST-26 and exhibited genetic relatedness, differing by a maximum of 90 single nucleotide polymorphisms. Most isolates carried multiple antimicrobial resistance genes, with the tet(A) gene present in 63% and floR in 77% of the genomes. The ESBL gene, blaCTX-M-15, was identified in 30% of the genomes. Three plasmid replicon types were identified: IncA, p0111, and IncQ1. The genomes clustered into two clades based on their virulence gene profile, one group with the T3SS genes and one without. The genetic similarity among Vietnamese isolates suggests that disease spread occurs within the Mekong region, underscoring the importance of source tracking, reservoir identification, and implementation of necessary biosecurity measures to mitigate spread of BNP.

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.

A Case of Edwardsiella tarda Infection With Iliopsoas Abscess Following Acute Pyelonephritis

Edwardsiella tarda (E. tarda) is a gram-negative bacillus commonly isolated from aquatic environments and various aquatic animals. It rarely causes infections in humans, but rare human infections occur primarily through ingestion of infected seafood or aquatic animals. Symptoms include fever, gastroenteritis, and diarrhea, but severe extraintestinal infections have also been reported. This report describes a 76-year-old female developing E. tarda infection with iliopsoas abscess following acute pyelonephritis. Her chief complaint was fatigue and difficulty moving. Blood tests showed an increased inflammatory response, but the cause could not be identified from the patient's medical history, physical findings, and imaging findings. We diagnosed it as a urinary tract infection from the results of gram staining and started treatment, but the fever persisted thereafter, and a contrast-enhanced CT scan performed for re-evaluation revealed an iliopsoas abscess. After CT-guided abscess drainage, the patient made good progress and was transferred to a rehabilitation hospital on day 48 of the presentation. To the best of our knowledge, this is the first report of a case of E. tarda infection with iliopsoas abscess following acute pyelonephritis. Iliopsoas abscess is often difficult to diagnose. In this case report, we also present how we diagnosed and treated iliopsoas abscesses.

Fulminant necrotizing fasciitis by Edwardsiella tarda in a patient with alcoholic liver cirrhosis: A case report

We herein present a unique and extremely rare fulminant case of Edwardsiella tarda infection-related necrotizing fasciitis. The patient had alcoholic cirrhosis and preferred to consume raw fish. He experienced painful swelling of the right forearm one day after he got a minor injury when falling from the ladder, and visited our hospital. His accompanied symptoms were diarrhea and general fatigue. His consciousness got deteriorated after the admission. The lesion of the right forearm had spread and the color had deteriorated with epidermolysis in a few hours. Necrotizing soft-tissue infection was suspected, and emergency debridement of the swollen forearm was performed 4 hours after the admission. However, unfortunately, he died of sepsis approximately 5 hours later. Histological examination of the biopsy specimen revealed features consistent with those of necrotizing fasciitis. The bacterial cultures of blood and the wound identified E. tarda. Since this microorganism is usually isolated from aquatic environments and can cause intestinal infection, sometimes followed by bacteremia especially in immunocompromised hosts, two possible infection routes were suspected. One route was from the skin injury, leading to bacteremia. Another possible route was per oral: orally taken E. tarda invaded deeper tissues from the intestine and reach the bloodstream, leading to extraintestinal infections, although direct evidence remains elusive. Raw fish eaten 1 week prior is considered to be the most possible contaminated food. Overall mortality rate of E. tarda bacteremia is very high and the clinician should pay attention on characteristic clinical findings of E. tarda infection on cirrhotic patients.

Edwardsiella piscicida causes iron storage disorders by an autophagy pathway in fish monocytes/macrophages

Edwardsiella piscicida (E. piscicida) is a gram-negative pathogen that survives in intracellular environment. Currently, the interplay between E. piscicida and host cells has not been completely explored. In this study, we found that E. piscicida disturbed iron homeostasis in grass carp monocytes/macrophages to maintain its own growth. Further investigation revealed the bacteria induced an increase of intracellular iron, which was subjected to the degradation of ferritin. Moreover, the autophagy inhibitor impeded the degradation of ferritin and increase of intracellular iron in E. piscicida-infected monocytes/macrophages, implying possible involvement of autophagy response in the process of E. piscicida-broken iron homeostasis. Along this line, confocal microscopy observed that E. piscicida elicited the colocalization of ferritin with LC3-positive autophagosome in the monocytes/macrophages, indicating that E. piscicida mediated the degradation of ferritin possibly through the autophagic pathway. These results deepened our understanding of the interaction between E. piscicida and fish cells, hinting that the disruption of iron homeostasis was an important factor for pathogenicity of E. piscicida. They also indicated that autophagy was a possible mechanism governing intracellular iron metabolism in response to E. piscicida infection and might offer a new avenue for anti-E. piscicida strategies in the future.

Type III secretion system effector YfiD inhibits the activation of host poly(ADP-ribose) polymerase-1 to promote bacterial infection

Modulation of cell death is a powerful strategy employed by pathogenic bacteria to evade host immune clearance and occupy profitable replication niches during infection. Intracellular pathogens employ the type III secretion system (T3SS) to deliver effectors, which interfere with regulated cell death pathways to evade immune defenses. Here, we reveal that poly(ADP-ribose) polymerase-1 (PARP1)-dependent cell death restrains Edwardsiella piscicida's proliferation in mouse monocyte macrophages J774A.1, of which PARP1 activation results in the accumulation of poly(ADP-ribose) (PAR) and enhanced inflammatory response. Moreover, E. piscicida, an important intracellular pathogen, leverages a T3SS effector YfiD to impair PARP1's activity and inhibit PAR accumulation. Once translocated into the host nucleus, YfiD binds to the ADP-ribosyl transferase (ART) domain of PARP1 to suppress its PARylation ability as the pharmacological inhibitor of PARP1 behaves. Furthermore, the interaction between YfiD and ART mainly relies on the complete unfolding of the helical domain, which releases the inhibitory effect on ART. In addition, YfiD impairs the inflammatory response and cell death in macrophages and promotes in vivo colonization and virulence of E. piscicida. Collectively, our results establish the functional mechanism of YfiD as a potential PARP1 inhibitor and provide more insights into host defense against bacterial infection.

Recurrent cholangitis and bacteraemia due to Edwardsiella tarda: a case report

Edwardsiella tarda is typically isolated from aquatic environments. It rarely causes infections in humans. Edwardsiella tarda infections in humans result from the consumption of infected or contaminated food. Here, we present a case of recurrent cholangitis and bacteraemia associated with E. tarda. An 82-year-old man with no history of seafood inoculation was admitted to our hospital because of difficulty in moving his body. The patient was diagnosed with cholangitis, and the blood culture revealed the presence of E. tarda. The patient underwent bile duct stenting and received antibiotic therapy for 14 days. Forty-four days after discharge, cholangitis recurred, and blood culture again showed the presence of E. tarda. The patient underwent bile duct stenting and antibiotic therapy for 11 days. No cholangitis or bacteraemia associated with E. tarda was observed in the following 3 years. Our case strongly suggests that colonization with E. tarda results in recurrent cholangitis and bacteraemia.

Case Report: Disseminated Edwardsiella tarda infection in an immunocompromised patient

Human infection caused by bacteria of the Edwardsiella genus is rare and most often presents with gastroenteritis that rarely requires antibiotics. Our case report describes a medically complex patient with chronic steroid use contributing to an immunocompromised state, who presented with fever and abdominal pain. The patient was later found to have Edwardsiella tarda (E. tarda) bacteremia and underwent paracentesis confirming E. tarda bacterial peritonitis requiring a prolonged antibiotic course. This case report aims to illustrate the presentation, diagnosis, and management of an uncommon infection that can have severe complications especially among immunocompromised patients.

Unraveling and characterization of novel T3SS effectors in Edwardsiella piscicida

Type III secretion system (T3SS) facilitates survival and replication of Edwardsiella piscicida in vivo. Identifying novel T3SS effectors and elucidating their functions are critical in understanding the pathogenesis of E. piscicida. E. piscicida T3SS effector EseG and EseJ was highly secreted when T3SS gatekeeper-containing protein complex EsaB-EsaL-EsaM was disrupted by EsaB deficiency. Based on this observation, concentrated secretomes of ΔesaB strain and ΔesaBΔesaN strain were purified by loading them into SDS-PAGE gel for a short electrophoresis to remove impurities prior to the in-the gel digestion and mass spectrometry. Four reported T3SS effectors and two novel T3SS effector candidates EseQ (ETAE_2009) and Trx2 (ETAE_0559) were unraveled by quantitative comparison of the identified peptides. EseQ and Trx2 were revealed to be secreted and translocated in a T3SS-dependent manner through CyaA-based translocation assay and immunofluorescent staining, demonstrating that EseQ and Trx2 are the novel T3SS effectors of E. piscicida. Trx2 was found to suppress macrophage apoptosis as revealed by TUNEL staining and cleaved caspase-3 of infected J774A.1 monolayers. Moreover, Trx2 has been shown to inhibit the p65 phosphorylation and p65 translocation into the nucleus, thus blocking the NF-κB pathway. Furthermore, depletion of Trx2 slightly but significantly attenuates E. piscicida virulence in a fish infection model. Taken together, an efficient method was established in unraveling T3SS effectors in E. piscicida, and Trx2, one of the novel T3SS effectors identified in this study, was demonstrated to suppress apoptosis and block NF- κB pathway during E. piscicida infection. IMPORTANCE Edwardsiella piscicida is an intracellular bacterial pathogen that causes intestinal inflammation and hemorrhagic sepsis in fish and human. Virulence depends on the Edwardsiella type III secretion system (T3SS). Identifying the bacterial effector proteins secreted by T3SS and defining their role is key to understanding Edwardsiella pathogenesis. EsaB depletion disrupts the T3SS gatekeeper-containing protein complex, resulting in increased secretion of T3SS effectors EseG and EseJ. EseQ and Trx2 were shown to be the novel T3SS effectors of E. piscicida by a secretome comparison between ∆esaB strain and ∆esaB∆esaN strain (T3SS mutant), together with CyaA-based translocation assay. In addition, Trx2 has been shown to suppress macrophage apoptosis and block the NF-κB pathway. Together, this work expands the known repertoire of T3SS effectors and sheds light on the pathogenic mechanism of E. piscicida.

Pathogenicity of Edwardsiella anguillarum to American eels (Anguilla rostrata) and RNA-seq analysis of host immune response to the E. anguillarum infection

Edwardsiella anguillarum is a commonly pathogenic bacterium in cultivated eels, but its pathogenicity to American eel (Anguilla rostrata) and the molecular mechanism of host anti-E. anguillarum infection remains uncertain. In this study, LD50 of E. anguillarum to American eels was determined and bacterial load in the liver and kidney of eels was assessed post the LD50 of E. anguillarum infection. The results showed that LD50 of E. anguillarum to American eels was determined to be 2.5 × 105 cfu/g body weight, and the bacterial load peaked at 36 and 72 h post the infection (hpi) in the kidney and liver, respectively. Then, the histopathology was highlighted by congestion in splenic blood vessels, atrophied glomeruli, and necrotic hepatocytes, as well as ultrastructural pathology in the kidney were charactered by acute nephritis, showing necrosis of the renal tubular epithelial cells, glomerular capillaries dilate, mitochondria swelling and ribosomes separate from the endoplasmic reticulum. Furthermore, the results of qRT-PCR revealed that 12 host immune-related genes showed significantly up or downregulated post-infection compare to that of pre-infection. Finally, results of the RNA-seq revealed 6 hub DEGs play essential role to the anti-E. anguillarum infection in American eels. Pathogenicity of E. anguillarum to American eels and hub genes related host anti- E. anguillarum infection were firstly reported in this study, shedding new light on our understanding of the E. anguillarum pathogenesis and the host immune response to the E. anguillarum infection strategies in gene transcript.

PD-L1/BTLA Checkpoint Axis Exploited for Bacterial Immune Escape by Restraining CD8+ T Cell-Initiated Adaptive Immunity in Zebrafish

Programmed death-ligand 1/programmed cell death 1 (PD-L1/PD-1) is one of the most important immune checkpoints in humans and other mammalian species. However, the occurrence of the PD-L1/PD-1 checkpoint in evolutionarily ancient vertebrates remains elusive because of the absence of a PD-1 homolog before its appearance in tetrapods. In this article, we identified, to our knowledge, a novel PD-L1/B and T lymphocyte attenuator (BTLA) checkpoint in zebrafish by using an Edwardsiella tarda-induced bacterial infection model. Results showed that zebrafish (Danio rerio) PD-L1 (DrPD-L1) and BTLA (DrBTLA) were differentially upregulated on MHC class II+ macrophages (Mϕs) and CD8+ T cells in response to E. tarda infection. DrPD-L1 has a strong ability to interact with DrBTLA, as shown by the high affinity (KD = 5.68 nM) between DrPD-L1/DrBTLA proteins. Functionally, the breakdown of DrPD-L1/DrBTLA interaction significantly increased the cytotoxicity of CD8+BTLA+ T cells to E. tarda-infected PD-L1+ Mϕ cells and reduced the immune escape of E. tarda from the target Mϕ cells, thereby enhancing the antibacterial immunity of zebrafish against E. tarda infection. Similarly, the engagement of DrPD-L1 by soluble DrBTLA protein diminished the tolerization of CD8+ T cells to E. tarda infection. By contrast, DrBTLA engagement by a soluble DrPD-L1 protein drives aberrant CD8+ T cell responses. These results were finally corroborated in a DrPD-L1-deficient (PD-L1-/-) zebrafish model. This study highlighted a primordial PD-L1/BTLA coinhibitory axis that regulates CD8+ T cell activation in teleost fish and may act as an alternative to the PD-L1/PD-1 axis in mammals. It also revealed a previously unrecognized strategy for E. tarda immune evasion by inducing CD8+ T cell tolerance to target Mϕ cells through eliciting the PD-L1/BTLA checkpoint pathway.

Exploring the roles of phytobiotics in relieving the impacts of Edwardsiella tarda infection on fish: a mini-review

Edwardsiellosis caused by Edwardsiella tarda resulted in significant economic losses in aquaculture operations worldwide. This disease could infect a wide range of hosts, including freshwater, brackish water, and marine aquatic animals. Currently, antibiotics and vaccines are being used as prophylactic agents to overcome Edwardsiellosis in aquaculture. However, application of antibiotics has led to antibiotic resistance among pathogenic bacteria, and the antibiotic residues pose a threat to public health. Meanwhile, the use of vaccines to combat Edwardsiellosis requires intensive labor work and high costs. Thus, phytobiotics were attempted to be used as antimicrobial agents to minimize the impact of Edwardsiellosis in aquaculture. These phytobiotics may also provide farmers with new options to manage aquaculture species' health. The impact of Edwardsiellosis in aquaculture worldwide was elaborated on and highlighted in this review study, as well as the recent application of phytobiotics in aquaculture and the status of vaccines to combat Edwardsiellosis. This review also focuses on the potential of phytobiotics in improving aquatic animal growth performance, enhancing immune system function, and stimulating disease resistance.

Butylparaben disordered intestinal homeostasis in Chinese striped-necked turtles (Mauremys sinensis)

Butylparaben (BuP) is regarded as a widespread pollutant, which has potential risk to aquatic organisms. Turtle species are an important part of aquatic ecosystems, however, the effect of BuP on aquatic turtles is not known. In this study, we evaluated the effect of BuP on intestinal homeostasis of Chinese striped-necked turtle (Mauremys sinensis). We exposed turtles to concentrations of BuP (0, 5, 50, and 500 μg/L) for 20 weeks, then investigated the composition of gut microbiota, the structure of intestine, and the inflammatory and immune status. We found BuP exposure significantly changed the composition of gut microbiota. Specially, the unique genus in three concentrations of BuP-treated groups mainly was Edwardsiella, which was not present in control group (0 μg/L of BuP). In addition, the height of intestinal villus was shortened, and the thickness of muscularis was thinned in BuP-exposed groups. Particularly, the number of goblet cells obviously decreased, the transcription of mucin2 and zonulae occluden-1 (ZO-1) significantly downregulated in BuP-exposed turtles. Meanwhile, neutrophils and natural killer cells in lamina propria of intestinal mucosa increased in BuP-treated groups, especially in high concentration of BuP (500 μg/L). Moreover, the mRNA expression of pro-inflammatory cytokines, especially IL-1β showed a significant upregulation with BuP concentrations. Correlation analysis indicated the abundance of Edwardsiella was positively correlated with IL-1β and IFN-γ expression, whereas its abundance was negatively correlative with the number of goblet cells. Taken together, the present study demonstrated BuP exposure disordered intestinal homeostasis through inducing dysbiosis of gut microbiota, causing inflammatory response and impairing gut physical barrier in turtles, which emphasized the hazard of BuP to health of aquatic organism.

Edwardsiella piscicida infection reshapes the intestinal microbiome and metabolome of big-belly seahorses: mechanistic insights of synergistic actions of virulence factors

Uncovering the mechanism underlying the pathogenesis of Edwardsiella piscicida-induced enteritis is essential for global aquaculture. In the present study, we identified E. piscicida as a lethal pathogen of the big-belly seahorse (Hippocampus abdominalis) and revealed its pathogenic pattern and characteristics by updating our established bacterial enteritis model and evaluation system. Conjoint analysis of metagenomic and metabolomic data showed that 15 core virulence factors could mutually coordinate the remodeling of intestinal microorganisms and host metabolism and induce enteritis in the big-belly seahorse. Specifically, the Flagella, Type IV pili, and Lap could significantly increase the activities of the representative functional pathways of both flagella assembly and bacterial chemotaxis in the intestinal microbiota (P < 0.01) to promote pathogen motility, adherence, and invasion. Legiobactin, IraAB, and Hpt could increase ABC transporter activity (P < 0.01) to compete for host nutrition and promote self-replication. Capsule1, HP-NAP, and FarAB could help the pathogen to avoid phagocytosis. Upon entering epithelial cells and phagocytes, Bsa T3SS and Dot/Icm could significantly increase bacterial secretion system activity (P < 0.01) to promote the intracellular survival and replication of the pathogen and the subsequent invasion of the neighboring tissues. Finally, LPS3 could significantly increase lipopolysaccharide biosynthesis (P < 0.01) to release toxins and kill the host. Throughout the pathogenic process, BopD, PhoP, and BfmRS significantly activated the two-component system (P < 0.01) to coordinate with other VFs to promote deep invasion. In addition, the levels of seven key metabolic biomarkers, Taurine, L-Proline, Uridine, L-Glutamate, Glutathione, Xanthosine, and L-Malic acid, significantly decreased (P < 0.01), and they can be used for characterizing E. piscicida infection. Overall, the present study systematically revealed how a combination of virulence factors mediate E. piscicida-induced enteritis in fish for the first time, providing a theoretical reference for preventing and controlling this disease in the aquaculture of seahorses and other fishes.

A novel teleost microRNA regulates autophagy and NF-κB activation during bacterial infection

MicroRNAs (miRNAs) are a class of non-coding RNAs with regulatory functions in many cellular processes, including immune defense. In this study, we identified novel-m0089-3p, a novel miRNA with unknown function, in the teleost fish Japanese flounder (Paralichthys olivaceus) and investigated its immune function. Novel-m0089-3p was found to target the autophagy-associated gene ATG7 and negatively regulate ATG7 expression via interaction with the 3' UTR of ATG7. During the infection of the bacterial pathogen Edwardsiella tarda, novel-m0089-3p expression was induced in flounder, which in turn repressed ATG7 expression. Overexpression of novel-m0089-3p or blocking ATG7 expression inhibited autophagy and promoted the intracellular replication of E. tarda. Novel-m0089-3p overexpression, as well as E. tarda infection, activated NF-κB and stimulated the expression of inflammatory cytokines. Together these results revealed an important role of novel-m0089-3p in response to bacterial infection.

Japanese Flounder pol-miR-155 Is Involved in Edwardsiella tarda Infection via ATG3

MicroRNAs (miRNAs) are small RNA molecules that function in the post-transcriptionally regulation of the expression of diverse genes, including those involved in immune defense. Edwardsiella tarda can infect a broad range of hosts and cause severe disease in aquatic species, including Japanese flounder (Paralichthys olivaceus). In this study, we examined the regulation mechanism of a flounder miRNA, pol-miR-155, during the infection of E. tarda. Pol-miR-155 was identified to target flounder ATG3. Overexpression of pol-miR-155 or knockdown of ATG3 expression suppressed autophagy and promoted the intracellular replication of E. tarda in flounder cells. Overexpression of pol-miR-155 activated the NF-κB signaling pathway and further promoted the expression of downstream immune related genes of interleukin (IL)-6 and IL-8. These results unraveled the regulatory effect of pol-miR-155 in autophagy and in E. tarda infection.

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.

Complete genome sequence analysis of Edwardsiella tarda SC002 from hatchlings of Siamese crocodile

Edwardsiella tarda is a Gram-negative, facultative anaerobic rod-shaped bacterium and the causative agent of the systemic disease “Edwardsiellosis”. It is commonly prevalent in aquatic organisms with subsequent economic loss and hence has attracted increasing attention from researchers. In this study, we investigated the complete genome sequence of a highly virulent isolate Edwardsiella tarda SC002 isolated from hatchlings of the Siamese crocodile. The genome of SC002 consisted of one circular chromosome of length 3,662,469 bp with a 57.29% G+C content and four novel plasmids. A total of 3,734 protein-coding genes, 12 genomic islands (GIs), 7 prophages, 48 interspersed repeat sequences, 248 tandem repeat sequences, a CRISPR component with a total length of 175 bp, and 171 ncRNAs (tRNA = 106, sRNA = 37, and rRNA = 28) were predicted. In addition, the coding genes of assembled genome were successfully annotated against eight general databases (NR = 3,618/3,734, COG = 2,947/3,734, KEGG = 3,485/3,734, SWISS-PROT = 2,787/3,734, GO = 2,648/3,734, Pfam = 2,648/3,734, CAZy = 130/3,734, and TCDB = 637/3,734) and four pathogenicity-related databases (ARDB = 11/3,734, CARD = 142/3,734, PHI = 538/3,734, and VFDB = 315/3,734). Pan-genome and comparative genome analyses of the complete sequenced genomes confirmed their evolutionary relationships. The present study confirmed that E. tarda SC002 is a potential pathogen bearing a bulk amount of antibiotic resistance, virulence, and pathogenic genes and its open pan-genome may enhance its host range in the future.

Cloning, DNA sequence, and expression of flagellins from high and low virulence strains of Edwardsiella tarda and their macrophage-stimulating activities

Edwardsiella tarda is a causative pathogen of edwardsiellosis in fish. Our previous studies on high (NUF251) and low (NUF194) virulent strains of E. tarda demonstrated that NUF251 strain induced significantly higher levels of NO and TNF-α from fish and mouse macrophages than NUF194 strain. Subsequent studies suggested that a flagellin-like protein secreted from E. tarda might be a responsible factor for the macrophage-stimulating activities. To evaluate the activities of flagellins of E. tarda, in this study, the flagellin genes of NUF251 and NUF194 strains were isolated by PCR and cloned into pQE-30 and pCold I expression vectors, and then the recombinant flagellins of two strains were overexpressed in E. coli JM109 and pG-Tf/BL21, respectively. The molecular weight of the purified recombinant flagellins of NUF251 and NUF194 strains were estimated to be 45 kDa and 37 kDa, respectively on SDS-PAGE analysis. Referring the three-dimensional structure of Salmonella flagellin, which has been reported to have 4 domains (D0, D1, D2, and D3), high sequence homology between two flagellins of E. tarda was observed at conservative domain (D0 and D1) regions, whereas the sequences equivalent to D2 and D3 domains were different, and even equivalent to 57 amino acids were deleted in NUF194. Both recombinant flagellins induced NO production, mRNA expression level of inducible NO synthase (iNOS), and intercellular ROS production in mouse macrophage cell line RAW264.7 cells. Also, the secretion of TNF-α and its mRNA expression level were increased by treatment of both recombinant flagellins. These results indicate that the recombinant flagellins from different virulent E. tarda strains can stimulate macrophages with nearly equal levels as judged by the parameters tested, even though they are differences in the structure and molecular weight, suggesting that conservative D0 and D1 domains are sufficient structural elements for the recombinant flagellins to induce a certain level of macrophage-stimulation in vitro. Further studies are necessary focusing on the role of D2 and D3 domain regions of the recombinant flagellins as macrophage-stimulating agent as well as their influence on host immune system in vivo.

Versatile lifestyles of Edwardsiella: Free-living, pathogen, and core bacterium of the aquatic resistome

Edwardsiella species in aquatic environments exist either as individual planktonic cells or in communal biofilms. These organisms encounter multiple stresses, include changes in salinity, pH, temperature, and nutrients. Pathogenic species such as E. piscicida, can multiply within the fish hosts. Additionally, Edwardsiella species (E. tarda), can carry antibiotic resistance genes (ARGs) on chromosomes and/or plasmids, that can be transmitted to the microbiome via horizontal gene transfer. E. tarda serves as a core in the aquatic resistome. Edwardsiela uses molecular switches (RpoS and EsrB) to control gene expression for survival in different environments. We speculate that free-living Edwardsiella can transition to host-living and vice versa, using similar molecular switches. Understanding such transitions can help us understand how other similar aquatic bacteria switch from free-living to become pathogens. This knowledge can be used to devise ways to slow down the spread of ARGs and prevent disease outbreaks in aquaculture and clinical settings.

The role and mechanisms of the two-component system EnvZ/OmpR on the intracellular survival of Aeromonas hydrophila

Aeromonas hydrophila infections are common in aquaculture. Our previous studies found that the A. hydrophila B11 strain can survive in fish macrophages for at least 24 h and the two-component system EnvZ/OmpR may be involved in intracellular survival. To reveal the role and mechanism of the two-component system EnvZ/OmpR in intracellular survival of A. hydrophila, the genes of envZ/ompR were silenced by shRNAi. The results showed that the survival rates of the envZ-RNAi and ompR-RNAi strains were only 2.05% and 3.75%, respectively, which were decreased by 91% and 83.6% compared with that of the wild-type strain. The escape ability of envZ-RNAi and ompR-RNAi was also decreased by 51.4% and 19.7%, respectively. The comparative transcriptome analysis revealed that the functional genes directly related to bacterial intracellular survival mainly included the genes related to anti-stress capacity, and the genes related to Zn²⁺ and Mg²⁺ transport. Further research confirmed that two-component system EnvZ/OmpR can regulate the expression of the important molecular chaperones, such as groEL, htpG, dnaK, clpB and grpE. The expression of these molecular chaperones in wild-type strain was up-regulated with the increase in H₂ O₂ concentrations, while the expression of these molecular chaperones in silent strains did not change significantly. Cells that phagocytosed wild-type strain had higher ROS content than cells that phagocytosed silent strains. Two-component system EnvZ/OmpR could also regulate zinc transporter (znuA, znuB, znuC) and zinc efflux protein (zntA) to maintain zinc homeostasis in cells, thus affecting the ability of bacteria to survive in phagocytes. Moreover, two-component system EnvZ/OmpR could affect the growth and intracellular survival of A. hydrophila by regulating the expression of MgtA, MgtC and MgtE and participating in bacterial Mg²⁺ homeostasis in fish macrophages.

TNFR2 is a regulatory target of pol-miR-194a and promotes the antibacterial immunity of Japanese flounder Paralichthys olivaceus

MicroRNAs (miRNAs) are regulatory RNAs that modulate target gene expression after transcription. Pol-miR-194a had been reported to be a miRNA of Japanese flounder (Paralichthys olivaceus) involved in Edwardsiella tarda infection. Here, we identified tumor necrosis factor receptor 2 (TNFR2) as a target gene of pol-miR-194a. Pol-miR-194a markedly repressed the protein expression of flounder TNFR2 (PoTNFR2) via specific interaction with the 3'UTR of PoTNFR2. PoTNFR2 responded to E. tarda infection in a manner that was opposite to that of pol-miR-194a and inhibited E. tarda invasion by activating the NF-κB pathway. Consistently, dysregulation of PoTNFR2 had a significant impact on E. tarda dissemination in flounder tissues. Together, these results add new insights into the regulation mechanism and immune function of fish TNFR2 and pol-miR-194a.

Temperature induces metabolic reprogramming in fish during bacterial infection

Water temperature elevation as a consequence of global warming results in increased incidence of bacterial disease, such as edwardsiellosis, in fish farming. Edwardsiellosis is caused by the bacterial pathogen Edwardsiella tarda and affects many farmed fish including flounder (Paralichthys olivaceus). Currently, the effect of temperature on the metabolic response of flounder to E. tarda infection is unclear. In this study, we found that compared to low temperature (15°C), high temperature (23°C) enhanced E. tarda dissemination in flounder tissues. To examine the impact of temperature on the metabolism of flounder induced by E. tarda, comparative metabolomics were performed, which identified a large number of metabolites responsive to E. tarda invasion and temperature alteration. During E. tarda infection, the metabolic profile induced by elevated temperature was mainly featured by extensively decreased amino acids and TCA intermediates such as succinate, a proven immune regulator. Further, 38 potential metabolite markers of temperature effect (MMTE) in association with bacterial infection were identified. When used as exogenous supplements, two of the MMTE, i.e., L-methionine and UDP-glucose, effectively upregulated the expression of pro-inflammatory cytokines and suppressed E. tarda infection in flounder leukocytes. Taken together, the results of this study indicate an important influence of temperature on the metabolism of flounder during bacterial infection, which eventually affects the survivability of the fish.

A case report and review of acute cholangitis with septic shock induced by Edwardsiella tarda

BACKGROUND

Edwardsiella tarda (E. tarda) is a gram-negative facultative anaerobic bacterium. Gastroenteritis is the most common manifestation of E. tarda infection. However, parenteral infections can occur in immunodeficient hosts, as well as hepatobiliary diseases, malignancies, and/or diabetes. The prognosis of sepsis caused by E. tarda is very worse, with a mortality rate of 38%. We report the occurrence of acute cholecystitis with septic shock and E. tarda bloodstream infection.

CASE PRESENTATION

A 64-year-old male with acute cholecystitis secondary to hepatitis B virus infection showed fever and sudden upper abdominal pain. On arrival, right upper abdominal pain, nausea, vomiting, fever, and jaundice were observed. Computed tomography showed common bile duct stones and gallbladder stones. Choledocholithiasis with acute cholangitis was diagnosed and treated surgically. Due to septic shock, a blood culture was assessed showing E. tarda as the main pathogen. Choledocholithotomy, T-tube drainage, cholecystectomy, and intravenous antibiotic treatment after the operation. The patient recovered smoothly after the operation.

CONCLUSIONS

Although E. tarda infection is extremely rare, it can cause rapid episodes of rapidly progressive and life-threatening disease, as well as intestinal and parenteral infections. If necessary, early surgical treatment of parenteral infection should be considered and antibiotics should be used in time.

Edwardsiella tarda TraT is an anti-complement factor and a cellular infection promoter

Edwardsiella tarda is a well-known bacterial pathogen with a broad range of host, including fish, amphibians, and mammals. One eminent virulence feature of E. tarda is its strong ability to resist the killing of host serum complement, but the involving mechanism is unclear. In this report, we identified E. tarda TraT as a key player in both complement resistance and cellular invasion. TraT, a surface-localized protein, bound and recruited complement factor H onto E. tarda, whereby inhibiting complement activation via the alternative pathway. TraT also interacted with host CD46 in a specific complement control protein domain-dependent manner, whereby facilitating the cellular infection and tissue dissemination of E. tarda. Thus, by acting as an anti-complement factor and a cellular infection promoter, TraT makes an important contribution to the complement evasion and systemic infection of E. tarda. These results add insights into the pathogen-host interaction mechanism during E. tarda infection.

Edwardsiella tarda TraT promotes cellular infection and serves as an anti-complement factor, shedding light on the mechanisms of E. tarda’s strong evasion of killing by the host.

Phenotypic and molecular characterization of the causative agents of edwardsiellosis causing Nile tilapia (Oreochromis niloticus) summer mortalities

Edwardsiellosis is a serious bacterial disease affecting Nile tilapia (Oreochromis niloticus), causing septicemia and mortalities. Edwardsiella tarda and Edwardsiella anguillarum were isolated from Nile tilapia summer mortality events in Egypt. Diseased fish showed hemorrhagic septicemia, skin erosions, and eye opacity. A total of 24 Edwardsiella spp. isolates were retrieved from the investigated fish specimens. Phenotypic and biochemical characteristics grouped isolates into typical Ed. tarda (n = 14 strains) and atypical Ed. tarda (n = 10 strains). The BLAST analysis of sodB gene sequencing confirmed the conventional identification of typical Ed. tarda strains (n = 14) and reidentified all the atypical strains (n = 10) as Ed. anguillarum. Isolates showed a combination of virulence factors, including biofilm formation (66.6%), hemolysis (100%), chondroitinase (50%), and proteolytic activity (20.8%). The major part of isolates showed high resistance to ampicillin, amoxicillin, gentamycin antibiotics and harbored tetA, bla_CTX-M, and aadA1 resistance genes. Pathogenicity testing of isolates in O. niloticus confirmed their virulence. Challenged fish exhibited septicemic signs similar to naturally diseased fish. Infections in naturally infected tilapia triggered acute and chronic histopathological alterations. Degenerative and necrotic changes were noticed in hematopoietic organs. Granulomas were noticed in between the hepatic parenchyma. The data extracted from the study confirm that accurate identification of the causative agents of edwardsiellosis should be reliant on genetic-based approaches. Analysis of the bacterium virulence properties offers insights into establishing novel therapeutics for edwardsiellosis control. The findings refer to the need for antimicrobial sensitivity testing to minimize antimicrobial resistance and increase therapy efficacy.

Monitoring the Starvation–Survival Response of Edwardsiella piscicida and E. tarda in Freshwater Microcosms, at Various Temperatures

Edwardsiella piscicida is an important fish pathogen responsible for economic losses in global aquaculture, and E. tarda is also a human zoonotic pathogen. In this study, the survival of E. piscicida and E. tarda strains kept in filtered and sterilized lake water microcosms was investigated during a 20-week period at 7 °C, 15 °C and 25 °C, as well as its pathogenicity retention during a starvation period. E. tarda V43.2 stayed culturable for 6 weeks at 7 °C, 9 weeks at 25 °C and 12 weeks at 15 °C. Both E. piscicida strains (V12.1 and V57.2) stayed culturable even longer, for at least 12 weeks at 7 °C, 15 °C and 25 °C under the same starvation conditions. After Edwardsiella cells entered into the VBNC state, some became shorter and ”rounded up,” but others aggregated and retained a short rod shape. Aggregates of Edwardsiella cells were common throughout the VBNC period, and a well-formed biofilm was observed for all tested strains at the end of the experiment. The growth capacity of VBNC cells was restored by cultivating microcosm water samples in LB broth at 28 °C. Resuscitated E. piscicida cells were as virulent for the European eel as the controls. Natural waters can be a reservoir for Edwardsiella, and its underestimation in environmental samples poses a risk to public health and aquaculture.

The Involvement of Thiamine Uptake in the Virulence of Edwardsiella piscicida

Edwardsiella piscicida is a pathogenic bacterium, which can infect a number of fish species and cause a disease termed edwardsiellosis, threatening global fish farming with high prevalence and mortality. Thiamine (Vitamin B1), functioning in the form of thiamine pyrophosphate (TPP), is essential for almost all organisms. Bacteria acquire TPP by biosynthesis or by transportation of exogenous thiamine. TPP availability has been associated with bacterial pathogenicity, but the underlying mechanisms remain to be discovered. The role of thiamine in the pathogenicity of E. piscicida is unknown. In this study, we characterized a thiamine transporter (TT) operon in E. piscicida. The deletion of the TT operon resulted in an intracellular TPP lacking situation, which led to attenuated overall pathogenicity, impaired abilities associated with motility and host cell adhesion, as well as decreased expression of certain flagellar and adhesion genes. Moreover, TPP starvation led to intracellular c-di-GMP reduction, and introducing into the TPP-suppressed mutant strain an exogenous diguanylate cyclase for c-di-GMP synthesis restored the virulence loss. Taken together, this work reveals the involvement of thiamine uptake in the virulence regulation of E. piscicida, with c-di-GMP implicated in the process. These finding could be employed to explore potential drug targets against E. piscicida.

Transcriptome analysis and protein-protein interaction in resistant and susceptible families of Japanese flounder (Paralichthys olivaceus) to understand the mechanism against Edwardsiella tarda

Edwardsiella tarda is one of the most harmful bacterial pathogens for aquaculture flatfish. After artificial infection of 47 Japanese flounder (Paralichthys olivaceus) families, resistant and susceptible families were identified in this study. High-throughput sequencing was performed on the liver transcriptome of uninfected groups (PoRU and PoSU) and infected groups (PoRC and PoSC). Through assembly and annotation, a total of 3012 and 1386 differentially expressed genes (DEGs) were identified in PoRU vs. PoSU and PoRC vs. PoSC. The significant enrichment pathways between PoRU and PoSU were mainly in metabolic and biosynthesis pathways. A total of thirty dominant enrichment pathways between PoRC and PoSC mainly focused on some immune-related pathways, including the hematopoietic cell lineage, cytokine-cytokine receptor interaction, complement and coagulation cascades, antigen processing and presentation, the intestinal immune network for immunoglobulin A (IgA) production and T/B cell receptor signaling pathway. Under the protein-protein interaction (PPI) analysis, hub genes, including CD molecules, complement component factors and chemokines, were identified in the network, and 16 core genes were differentially expressed in resistant and sustainable families in quantitative polymerase chain reaction (qPCR) validation. This study represents the first transcriptome analysis based on resistant and susceptible families and provides resistant genes to understand the potential molecular mechanisms of antibacterial function in marine fish. The results obtained in this study provide crucial information on gene markers for resistant breeding of Japanese flounder.

FabR senses long-chain unsaturated fatty acids to control virulence in pathogen Edwardsiella piscicida

Long-chain unsaturated fatty acids (UFAs) can serve as nutrient sources or building blocks for bacterial membranes. However, little is known about how UFAs may be incorporated into the virulence programs of pathogens. A previous investigation identified FabR as a positive regulator of virulence gene expression in Edwardsiella piscicida. Here, chromatin immunoprecipitation-sequencing coupled with RNA-seq analyses revealed that 10 genes were under the direct control of FabR, including fabA, fabB, and cfa, which modulate the composition of UFAs. The binding of FabR to its target DNA was facilitated by oleoyl-CoA and inhibited by stearoyl-CoA. In addition, analyses of enzyme mobility shift assay and DNase I footprinting with wild-type and a null mutant (F131A) of FabR demonstrated crucial roles of FabR in binding to the promoters of fabA, fabB, and cfa. Moreover, FabR also binds to the promoter region of the virulence regulator esrB for its activation, facilitating the expression of the type III secretion system (T3SS) in response to UFAs. Furthermore, FabR coordinated with RpoS to modulate the expression of T3SS. Collectively, our results elucidate the molecular machinery of FabR regulating bacterial fatty acid composition and virulence in enteric pathogens, further expanding our knowledge of its crucial role in host-pathogen interactions.

Transcriptome Analysis of Immune Responses and Metabolic Regulations of Chinese Soft-Shelled Turtle (Pelodiscus sinensis) against Edwardsiella tarda Infection

The Chinese soft-shelled turtle (Pelodiscus sinensis) is an important aquatic species in southern China that is threatened by many serious diseases. Edwardsiella tarda is one of the highly pathogenic bacteria that cause the white abdominal shell disease. Yet, little is known about the immune and metabolic responses of the Chinese soft-shelled turtle against E. tarda infection. In the paper, gene expression profiles in the turtle liver were obtained to study the immune responses and metabolic regulations induced by E. tarda infection using RNA sequencing. A total of 3908 differentially expressed unigenes between the experimental group and the control group were obtained by transcriptome analysis, among them, were the significantly upregulated unigenes and downregulated unigenes 2065 and 1922, respectively. Further annotation and analysis revealed that the DEGs were mainly enriched in complement and coagulation cascades, phagosome, and steroid hormone biosynthesis pathways, indicating that they were mainly associated with defense mechanisms in the turtle liver against E. tarda four days post infection. For the first time, we reported on the gene profile of anti-E. tarda response in the soft-shelled turtle, and our research might provide valuable data to support further study on anti-E. tarda defense mechanisms in turtles

Dysregulation of Cytosolic c-di-GMP in Edwardsiella piscicida Promotes Cellular Non-Canonical Ferroptosis

Programmed cell death plays an important role in modulating host immune defense and pathogen infection. Ferroptosis is a type of inflammatory cell death induced by intracellular iron-dependent accumulation of toxic lipid peroxides. Although ferroptosis has been associated with cancer and other sterile diseases, very little is known about the role of ferroptosis in modulating host-pathogen interactions. We show that accumulation of the secondary messenger bis-(3′,5′)-cyclic dimeric GMP (c-di-GMP) in the pathogenic bacterium Edwardsiella piscicida (E. piscicida) triggers a non-canonical ferroptosis pathway in infected HeLa cells. Moreover, we observed that the dysregulation of c-di-GMP in E. piscicida promotes iron accumulation, mitochondrial dysfunction, and production of reactive oxygen species, all of which that can be blocked by iron chelator. Importantly, unlike classical ferroptosis that is executed via excess lipid peroxidation, no lipid peroxidation was detected in the infected cells. Furthermore, lipoxygenases inhibitors and lipophilic antioxidants are not able to suppress morphological changes and cell death induced by E. piscicida mutant producing excess c-di-GMP, and this c-di-GMP dysregulation attenuates bacterial virulence in vivo. Collectively, our results reveal a novel non-canonical ferroptosis pathway mediated by bacterial c-di-GMP and provide evidence for a role of ferroptosis in the regulation of pathogen infection.

A peek into mass mortality caused by antimicrobial resistant Edwardsiella tarda in goldfish, Carassius auratus in Kerala

Edwardsiella tarda is one of the serious threats affecting the worldwide aquaculture. In the present study, four isolates were recovered from diseased goldfish, showing hemorrhages, reported with 60% mass mortality in an ornamental fish farm, Ernakulam, Kerala. Based on the phenotypic and genotypic analysis, the bacteria were identified as Edwardsiella tarda, Citrobacter freundii, Acinetobacter junii and Comammonas testosteronii. Experimental challenge studies using healthy goldfish revealed that among the four isolates, E. tarda alone leads to 100% mortality of experimental fish within 175 degree days and the pathogen could be successfully re-isolated from the moribund fish. The LD50 value of E. tarda was calculated as 9.9 × 10⁵ CFU/fish. The histopathology of the infected tissues of goldfish had shown the typical features of E .tarda infection. The pathogen was found positive for the virulence genes viz., hly, etfA, etfD and eseD as detected using PCR. Thus E. tarda was confirmed as the real causative agent of the disease outbreak. Multiple antimicrobial resistance (AMR) exhibited by the pathogen towards 19 tested antibiotics with the MAR index of 0.46 highlighted the exposure of antibiotics to the fish in the farm. The existence of antibiotic resistant genes within the plasmid as revealed through plasmid curing studies pointed out the possibility of rapid dissemination of AMR in aquaculture. Hence proper surveillance and appropriate diagnostic methods need to be implemented at regular intervals to mitigate the menace.

Transcriptomic analysis using dual RNA sequencing revealed a Pathogen-Host interaction after Edwardsiella anguillarum infection in European eel (Anguilla anguilla)

Many studies have explored differentially expressed genes (DEGs) between some pathogens and hosts, but no study has focused on the interaction of DEGs between Edwardsiella anguillarum (Ea) and Anguilla anguilla (Aa). In this study, we examined the interactions of DEGs during Ea infection and Aa anti-infection processes by dual RNA sequencing. Total RNA from in vitro and in vivo (Aa liver) Ea culture was extracted. Using high-throughput transcriptomics, significant DEGs that were expressed between Ea cultured in vitro versus in vivo and those in the liver of the infected group versus control group were identified. Protein-protein interactions between the pathogen and host were explored using Cytoscape according to the HPIDB 3.0 interaction transcription database. The results showed that the liver in the infection group presented with severe bleeding and a large number of thrombi in the hepatic vessels. We found 490 upregulated and 398 downregulated DEGs of Ea in vivo versus Ea cultured in vitro, and 2177 upregulated and 970 downregulated genes in the liver of the infected eels. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of the pathogen DEGs revealed that the upregulated genes were mainly enriched in migration, colonization, biofilm formation, and significantly enriched in ABC transport and quorum sensing; the downregulated genes were mainly involved in metabolism, information transduction, organelle formation, enzyme catalysis, molecular transport, and binding. GO of the host DEGs showed that metabolic process, catalytic activity, single organism metabolic process, small molecule binding, nucleotide binding, nucleotide phosphate binding, and anion binding were markedly enriched. Finally, we found that 79 Ea and 148 Aa proteins encoded by these DEGs were involved in an interaction network, and some pathogen (DegP, gcvP, infC, carB, rpoC, trpD, sthA, and FhuB) and host proteins (MANBA, STAT1, ETS2, ZEP1, TKT1, NMI and RBPMS) appear to play crucial roles in infection. Thus, determining the interaction networks revealed crucial molecular mechanisms underlying the process of pathogenic infection and host anti-infection.