Phylogenomics characterization of a highly virulent Edwardsiella strain ET080813T encoding two distinct T3SS and three T6SS gene clusters: Propose a novel species as Edwardsiella anguillarum sp. nov

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.

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.

Expanding the Spectrum of Diseases and Disease Associations Caused by Edwardsiella tarda and Related Species

The genus Edwardsiella, previously residing in the family Enterobacteriaceae and now a member of the family Hafniaceae, is currently composed of five species, although the taxonomy of this genus is still unsettled. The genus can primarily be divided into two pathogenic groups: E. tarda strains are responsible for almost all human infections, and two other species (E. ictaluri, E. piscicida) cause diseases in fish. Human infections predominate in subtropical habitats of the world and in specific geospatial regions with gastrointestinal disease, bloodborne infections, and wound infections, the most common clinical presentations in decreasing order. Gastroenteritis can present in many different forms and mimic other intestinal disturbances. Chronic gastroenteritis is not uncommon. Septicemia is primarily found in persons with comorbid conditions including malignancies and liver disease. Mortality rates range from 9% to 28%. Most human infections are linked to one of several risk factors associated with freshwater or marine environments such as seafood consumption. In contrast, edwardsiellosis in fish is caused by two other species, in particular E. ictaluri. Both E. ictaluri and E. piscicida can cause massive outbreaks of disease in aquaculture systems worldwide, including enteric septicemia in channel catfish and tilapia. Collectively, these species are increasingly being recognized as important pathogens in clinical and veterinary medicine. This article highlights and provides a current perspective on the taxonomy, microbiology, epidemiology, and pathogenicity of this increasingly important group.

Transcriptional regulation of the Japanese flounder Cu,Zn-SOD (Jfsod1) gene in RAW264.7 cells during oxidative stress caused by causative bacteria of edwardsiellosis

Edwardsiellosis is one of the most important bacterial diseases in fish, sometimes causing extensive economic losses in the aquaculture industry. Our previous studies demonstrated that the Cu,Zn-SOD (sod1) activity has significantly increased in Japanese flounder, Paralichthys olivaceus, hepatopancreas infected by causative bacteria of edwardsiellosis Edwardsiella tarda NUF251. In this study, NUF251 stimulated intracellular superoxide radical production in mouse macrophage RAW264.7 cells, which was reduced by N-acetylcysteine. This result suggests that NUF251 infection causes oxidative stress. To evaluate the regulatory mechanism of Jfsod1 at transcriptional levels under oxidative stress induced by NUF251 infection, we cloned and determined the nucleotide sequence (1124 bp) of the 5'-flanking region of the Jfsod1 gene. The sequence analysis demonstrated that the binding sites for the transcription factors C/EBPα and NF-IL6 involved in the transcriptional regulation of the mammalian sod1 gene existed. We constructed a luciferase reporter system with the 5'-flanking region (-1124/-1) of the Jfsod1 gene, and a highly increased transcriptional activity of the region was observed in NUF251-infected RAW264.7 cells. Further studies using several mutants indicated that deletion of the recognition region of NF-IL6 (-272/-132) resulted in a significant decrease in the transcriptional activity of the Jfsod1 gene in NUF251-infected RAW264.7 cells. In particular, the binding site (-202/-194) for NF-IL6 might play a major role in upregulating the transcriptional activity of the 5'-flanking region of the Jfsod1 gene in response to oxidative stress induced by NUF251 infection. These results could be provided a new insight to understand the pathogenic mechanism of causative bacteria of edwardsiellosis.

The pathway of Edwardsiella piscicida infecting Lateolabrax maculatus via the immersion bath

Edwardsiella piscicida, an infectious bacterium, causes great economic losses to the aquaculture industry. Immersion bath which is the closest way to how the fish infect bacterial pathogens in the natural environment is an effective route of artificial infection. In this study, the dynamic process of E. piscicida infection, in the spotted sea bass (Lateolabrax maculatus) was evaluated via the immersion bath. The results showed that soaking the spotted sea bass with 3 × 106  CFU mL-1 E. piscicida for 30 min could artificially induce edwardsiellosis. The higher culture temperature (28.5 ± 0.5°C) or the longer bath time (30 min) would lead to higher mortality of fish. E.piscicida first invaded the gill, then entered the blood circulation to infect the spleen and kidney, where it is colonized, and gradually multiplied in the liver and brain. Meanwhile, the fluorescence in situ hybridization showed that the localization of E. piscicida in the gill and foregut after the immersion challenge proceeded from the exterior to the interior. The invasion of pathogens triggers the immune response of fish and causes tissue damage to the host. The quantitative real-time PCR results displayed an increase in the relative expression level of immune genes (NK-lysin, LZM, IgM and IgD). Otherwise, the most notable histopathological changes of the infected spotted sea bass were multifocal necrosis. Findings in this study broaden our understanding of the infection conditions of E. piscicida and its pathogenicity to the spotted sea bass.

The immune response of fairy shrimp Branchinella kugenumaensis against Edwardsiella anguillarum infections by de novo transcriptome analysis

To explore the immune defense mechanisms of the ancient crustacean fairy shrimp (B.kugenumaensis) and uncover antibacterial-related gene resources, the present study analyzed the pathological changes in B. kugenumaensis infected with E. anguillarum. Differential gene expression changes between the infected and uninfected groups were investigated through comparative transcriptome sequencing to elucidate the molecular responses to the infection. Under transmission electron microscopy, the intestinal mucosal structure of B. kugenumaensis was damaged, the microvilli disappeared, the number of mitochondria and endoplasmic reticulum increased, mitochondria vacuolated and arranged disordered. The transcriptome data indicated that a total of 250,520,580 clean reads were assembled into 66,502 unigenes, with an average length of 789 bp and an N50 length of 1326 bp. Following bacterial infection, approximately 2678 differentially expressed genes (DEGs) were identified, with 1732 genes upregulated and 946 genes downregulated. The detected DEGs related to immune responses, particularly involving apoptosis, lysosome, autophagy, phagosome, and MAPK signaling pathways. Moreover, 9 immunity-related genes with different expressions were confirmed by using real-time quantitative PCR (RT-qPCR). This study first reports the pathogenicity of E. anguillarum on B. kugenumaensis and speculates that immune effectors such as lysozyme and lectin, as well as apoptosis, lysosome, and the MAPK signaling pathway, play crucial roles in the innate immunity of fairy shrimp. These findings deepen our understanding of fairy shrimp immune regulatory mechanisms and provide a theoretical foundation for disease prevention and control.

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.

Complete genome sequence of Edwardsiella sp. NBRC12716 isolated in 1962 from the liver of diseased eel

We report the complete genome sequence of Edwardsiella sp. NBRC12716 isolated from a diseased eel in 1962. The genome consists of a single, circular chromosome 3,771,060 bp in length with 59.74% GC content and encodes 25 rRNA, 96 tRNA, and 3,182 protein-coding genes.

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.

Edwardsiella tarda in Tambaqui (Colossoma macropomum): A Pathogenicity, Antimicrobial Susceptibility, and Genetic Analysis of Brazilian Isolates

Edwardsiella tarda is a crucial pathogenic bacterium in tropical aquaculture. This bacterium was recently isolated from tambaqui (Colossoma macropomum), a commercially important fish species in Brazil. This study assessed the antimicrobial susceptibility, pathogenicity, and genetic diversity of the tambaqui-derived E. tarda isolates. Fourteen bacterial isolates isolated from tambaqui were identified as E. tarda by using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry and dnaJ gene sequencing. Antimicrobial susceptibility tests were conducted against seven drugs using the disc diffusion assay. The pathogenicity test conducted by intraperitoneal injection of 2.4 × 107 colony-forming units (CFU) fish-1 of E. tarda (ED38-17) into tambaqui juveniles eventually revealed that neither clinical signs nor death were present. However, splenomegaly and whitish areas in the spleen and kidneys were observed. The histological investigation also revealed granulomatous splenitis, nephritis, and hepatitis occurring internally. Repetitive extragenic palindromic-PCR fingerprinting separated the 14 isolates into three genetic groups. The antibiogram revealed that all E. tarda isolates were wild-type (WT) to florfenicol (FLO), norfloxacin (NOR), neomycin (NEO), erythromycin (ERY), and oxytetracycline (OXY); however, some were non-wild-type to sulfamethoxazole/trimethoprim (7.1%) and amoxicillin (21.4%). Therefore, through experimental infection, E. tarda ED38-17 could induce pathogenic effects in C. macropomum. Additionally, three distinct genetic types were found, and the E. tarda isolates were WT to FLO, NOR, NEO, ERY, and OXY. These findings raise awareness of a bacteria causing unseen lesions, a pathogen that will potentially impact tambaqui aquaculture in the future.

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.

Analysis of Alternative Splicing and Long Noncoding RNAs After the Edwardsiella anguillarum Infected the Immunized European Eels (Anguilla anguilla) Revealed the Role of Outer Membrane Protein A in OmpA Subunit Vaccine

Edwardsiella anguillarum is a bacterium that commonly infects cultivated eels. Outer membrane protein A (OmpA) emulsified with Freund's adjuvant has been shown to be an effective fishery vaccine against this pathogen. However, the specific roles of OmpA in the vaccine have not been fully explored. In this study, we performed RNA-seq in the liver of a European eel (Anguilla anguilla) after challenge with E. anguillarum in eels previously immunized with an OmpA subunit vaccine. Our aim was to elucidate the differentially alternative splicing (DAS) and differentially expressed long noncoding RNAs (DE-lncRNAs) using a genome-wide transcriptome. The results showed after that at 28 days post-immunization, eels challenged with E. anguillarum (Con_inf) exhibited severe pathological changes in the liver. In contrast, the OmpA infused eels (OmpA_inf group) showed infiltrated lymphocytes, while Freund's adjuvant-inoculated eels (FCIA_inf group) showed edema of hepatocytes and blood coagulation. The relative percent survival (RPS) was 77.7% and 44.4% for OmpA_inf and FCIA_inf compared to the Con_inf group. We identified 37 DE-lncRNAs and 293 DAS genes between OmpA_inf and FCIA_inf. Interactions between DAS gene-expressed proteins indicated that 66 expressed proteins formed 20 networks. Additionally, 33 DE-lncRNAs interacted with 194 target genes formed 246 and 41 networks in co-expression and co-location. Taken together, our findings demonstrate that the OmpA subunit vaccine elicits a higher RPS and provides novel insights into the role of OmpA through DAS genes and DE-lncRNAs perspective. These results are significant for the development of fishery subunit vaccines.

Inoculation of Freund's adjuvant in European eel (Anguilla anguilla) revealed key KEGG pathways and DEGs of host anti-Edwardsiella anguillarum infection

Freund's complete (FCA) and incomplete adjuvants (FIA), generally applied in subunit fishery vaccine, have not been explored on the molecular mechanism of the nonspecific immune enhancement. In this study, we examined the RNA-seq in the spleen of European eel (Anguilla anguilla) inoculated with FCA and FIA (FCIA group) to elucidate the key KEGG pathways and differential expressed genes (DEGs) in the process of Edwardsiella anguillarum infection and A. anguilla anti-E. anguillarum infection using genome-wide transcriptome. After eels were challenged by E. anguillarum at 28 d post the first inoculation (dpi), compared to the control uninfected eels (Con group), the control infected eels (Con_inf group) showed severe pathological changes in the liver, kidney and spleen, although infected eels post the inoculation of FCIA (FCIA_inf group) also formed slight bleeding. Compared to the FCIA_inf group, there was more than 10 times colony forming unit (cfu) in the Con_inf group per 100 μg spleen, kidney or blood, and the relative percent survival (RPS) of eels was 44.4% in FCIA_inf vs Con_inf. Compared to the Con group, the SOD activity in the FCIA group increased significantly in the liver and spleen. Using high-throughput transcriptomics, DEGs were identified and 29 genes were verified using fluorescence real-time polymerase chain reaction (qRT-PCR). The result of DEGs clustering showed 9 samples in 3 groups of Con, FCIA and FCIA_inf were similar, contrast to distinct differences of 3 samples in the Con_inf group. We found 3795 up and 3548 down regulated DEGs in the compare of FCIA_inf vs Con_inf, of which 5 enriched KEGG pathways of "Lysosome", "Autophagy", "Apoptosis", "C-type lectin receptor signaling" and "Insulin signaling" were ascertained, and 26 of 30 top GO terms in the compare were significantly enriched. Finally, protein-protein interactions between the DEGs of the 5 KEGG pathways and other DEGs were explored using Cytoscape 3.9.1. The compare of FCIA_inf vs Con_inf showed 110 DEGs from the 5 pathways and 718 DEGs from other pathways formed total of 9747° in a network, of which 9 hub DEGs play vital roles in anti-infection or apoptosis. Together, the interaction networks revealed that 9 DEGs involved in the 5 pathways underlies the key process of A. anguilla anti-E. anguillarum infection or host cell apoptosis.

The isolation, identification, whole-genome sequencing of Clostridium butyricum LV1 and its effects on growth performance, immune response, and disease-resistance of Litopenaeus vannamei

In this study, a strain of Clostridium butyricum was isolated from the intestine of Litopenaeus vannamei with the method of anaerobic microbial isolation and culture. Next, the probiotic properties of LV1 were evaluated with susceptibility tests, tolerance tests, and whole genome sequencing in vivo and in vitro, followed by the analysis of the effect of LV1 on the growth performance, immune response, and disease resistance of Litopenaeus vannamei. According to the results, the 16 S rDNA sequence of LV1 was 100% homolofgous to the reference sequence of Clostridium butyricum. Moreover, LV1 was resistant to several antibiotics including amikacin, streptomycin, and gentamicin and highly tolerated artificial gastric and artificial intestinal fluids. The whole genome of LV1 was 4625,068 bp in size and included 4336 coding genes. Among these genes, GO, KEGG, and COG databases exhibited the highest number of genes annotated to metabolic pathway classes and 105 genes annotated as glycoside hydrolases. Meanwhile, 176 virulence genes were predicted. The use of diets supplemented with 1.2 × 10⁹ CFU/kg of LV1 live cells significantly increased the weight gain and specific growth rates of Litopenaeus vannamei and the activity of serum superoxide dismutase, glutathione peroxidase, acid phosphatase, and alkaline phosphatase (P < 0.05). Meanwhile, the use of these diets markedly improved the relative expression of intestinal immunity- and growth-related genes. In conclusion, LV1 has excellent probiotic properties. Specifically, the addition of 1.2 × 10⁹ CFU/kg of LV1 live cells to the diet improved the growth performance, immune response, and disease-resistance of Litopenaeus vannamei.

Comparative genomics of Edwardsiella anguillarum and Edwardsiella piscicida isolated in Taiwan enables the identification of distinctive features and potential virulence factors using Oxford-Nanopore MinION® sequencing

Edwardsiella tarda (ET) and Edwardsiella anguillarum (EA) are the most harmful bacterial fish pathogens in Taiwan. However, there is confusion regarding the genotypic identification of E. tarda and E. piscicida (EP). Therefore, we used a novel Nanopore MinION MK1C platform to sequence and compare the complete genomes of E. piscicida and E. anguillarum. The number of coding genes, rRNA, and tRNA recorded for E. anguillarum and E. piscicida were 8322, 25, and 98, and 5458, 25, and 98, respectively. Ribosomal multilocus sequence typing (rMLST) for E. piscicida indicated 35 rps. The shared clusters between E. anguillarum and E. piscicida indicated several unique clusters for the individual genomes. The phylogenetic tree analysis for all complete genomes indicated that E. anguillarum and E. piscicida were placed into two species-specific genotypes. Distribution of subsystems for annotated genomes found that genes related to virulence, defence, and disease for E. anguillarum were 103 and those for E. piscicida were 60 and pathogenic islands (PI) were 498 and 225, respectively. Vaccine candidates were identified in silico from the core genes using high antigenic, solubility, and secretion probabilities. Altogether, the genome data revealed distinctive features between E. anguillarum and E. piscicida, which suggest different pathogenicity and thus the need for separate preventive strategies.

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.

Structural Diversity among Edwardsiellaceae Core Oligosaccharides

The Edwardsiella genus presents five different pathogenic species: Edwardsiella tarda, E. anguillarum, E. piscicida, E. hoshinae and E. ictaluri. These species cause infections mainly in fish, but they can also infect reptiles, birds or humans. Lipopolysaccharide (endotoxin) plays an important role in the pathogenesis of these bacteria. For the first time, the chemical structure and genomics of the lipopolysaccharide (LPS) core oligosaccharides of E. piscicida, E. anguillarum, E. hoshinae and E. ictaluri were studied. The complete gene assignments for all core biosynthesis gene functions were acquired. The structure of core oligosaccharides was investigated by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The structures of E. piscicida and E. anguillarum core oligosaccharides show the presence of →3,4)-L-glycero-α-D-manno-Hepp, two terminal β-D-Glcp, →2,3,7)-L-glycero-α-D-manno-Hepp, →7)-L-glycero-α-D-manno-Hepp, terminal α-D-GlcpN, two →4)-α-D-GalpA, → 3)-α-D-GlcpNAc, terminal β-D-Galp and →5-substituted Kdo. E. hoshinare core oligosaccharide shows only one terminal β-D-Glcp, and instead of terminal β-D-Galp a terminal α-D-GlcpNAc. E. ictaluri core oligosaccharide shows only one terminal β-D-Glcp, one →4)-α-D-GalpA and do not have terminal α-D-GlcpN (see complementary figure).

Bullfrogs (Lithobates catesbeianus) as a Potential Source of Foodborne Disease

In Mexico, bullfrogs (Lithobates catesbeianus) are produced as gourmet food. However, bullfrogs can be carriers of pathogens because the frogs' preferred living conditions occur in stagnant water. The present study aimed to identify bacteria that cause foodborne diseases or are associated with human diseases. For molecular identification, based on the sequential analysis by 16S rRNA or rpoD was conducted on all isolates obtained from bullfrog. A total of 91 bacterial isolates were obtained from bullfrogs; 14 genera and 23 species were identified, including Acinetobacter johnsonii 16.5%; Aeromonas media 14.3%; Aeromonas veronii 13.2%; Providencia rettgeri 7.7%; Citrobacter freundii 6.6%; Aeromonas caviae 4.4%; Aeromonas hydrophila and Elizabethkingia ursingii 3.3%; Pseudomonas stutzeri, Raoultella ornithinolytica, and Shewanella putrefaciens 2.2%; Acinetobacter guillouiae, Acinetobacter pseudolwoffii, Citrobacter portucalensis, Citrobacter werkmanii, Edwardsiella anguillarum, Klebsiella michiganensis, Kluyvera intermedia, Kocuria rosea, Myroides odoratimimus, Myroides odoratus, Proteus sp., and Proteus hauseri 1.1%. In this study, 49.4% of the isolates obtained cause foodborne disease, 19.8% are bacteria that play an important role in the spoilage of food, 5.5% of isolates have nosocomial significance, 13.2% of bacteria are considered to be pollutants of the ecosystem, and in the case of A. salmonicida and Edwardsiella anguillarum (12.1%) to have a negative impact on aquaculture. Acinetobacter pseudolwoffii and Citrobacter portucalensis have not been reported to cause disease. Lastly of these isolates, 97.8% (89/91) can cause disease by food consumption or by direct contact for immunocompromised persons. The presence of these bacteria in bullfrogs represents a significant problem for human health. There is evidence that these microorganisms are pathogenic and frogs may also be reservoirs.

Mortality in farmed European eel (Anguilla anguilla) in Italy due to Streptococcus iniae

BACKGROUND

Streptococcal infections are one of the main causes of fish disease. During the last decade, Streptococcus iniae has become one of the most important aquatic pathogens worldwide, causing high losses in marine and freshwater finfish. Clinical signs in farmed fish include loss of appetite, lethargy and grouping at the bottom of the tank. Gross changes comprise darkening of the skin and haemorrhage at the basis of fins and opercula. To date, S. iniae has been isolated from several wild and farmed fish species but never in the European eel (Anguilla anguilla). In Europe, eel production from aquaculture is around 4500 tonnes and Italy is the third largest producer. This communication represents the first report of an outbreak of S. iniae infection in European eels.

CASE PRESENTATION

The outbreak occurred at an eel farm in northern Italy between May 2021 and September 2021. The outbreak caused about 2% mortality per month, resulting in the loss of about 10% of the farmed fish. The diseased eels showed apathy, lethargy, inactivity and inappetence. In July 2021, three eels were necropsied. Necropsy revealed skin and branchial hyperaemia, a few skin ulcers, and diffuse peritoneal congestion with a few haemorrhagic-like spot lesions. Swab samples for bacteriology were taken from the kidneys, liver, spleen, and brain. Additionally, four eels were opened and swap samples as above were taken. All the investigated eels were found dead. Bacteriological examination revealed growth of Streptococcus spp. from all samples. Identification of S. iniae was done by biochemical characterization, the API20STREP microsystem, 16S rDNA sequencing, and MALDI-TOF. Antimicrobial therapy (oxytetracycline and erythromycin) was ineffective.

CONCLUSIONS

This is the first report of S. iniae infection in the European eel. Although this may be an isolated outbreak, it is of concern due to the losses associated with this pathogen in fish worldwide and because the European eel is an endangered species. Due to the difficulties of controlling the disease with antimicrobials, it is advisable to plan other effective control measures, such as improving water quality and the environmental conditions, reducing fish density, improving biosecurity, and by using immunostimulants and, when possible, vaccines.

Phenotypic differences between Edwardsiella piscicida and Edwardsiella anguillarum isolates in Japan

OBJECTIVE

Edwardsiella tarda has been regarded as the causative agent of edwardsiellosis in cultured marine and freshwater fish species in Japan. Our previous study genetically classified an E. tarda-like isolate from diseased Olive Flounder Paralichthys olivaceus as E. piscicida and that from diseased Red Seabream Pagrus major as E. anguillarum. This study aimed to understand the phenotypic differences between E. piscicida and E. anguillarum.

METHODS

Fourteen E. piscicida and seven E. anguillarum isolates were used in this study. The colonies of each isolate were grown on brain-heart infusion agar plates and then subjected to DNA extraction. The extracted DNA was amplified using PCR. carbohydrate fermentation of the isolates was examined using API 50 CH test kits. Moreover, the growth of the two species was examined in defined media. Also, free amino acids in Olive Flounder and Red Seabream sera were detected and quantified via high-performance liquid chromatography-mass spectrometry. Statistical differences in the concentrations of free amino acids were analyzed using Welch's t-tests.

RESULT

The API 50 CH test revealed that L-arabinose and D-mannitol were fermented by E. anguillarum isolates but not E. piscicida isolates. Furthermore, the growth of E. piscicida and E. anguillarum was reduced in the defined medium without methionine and iron sulfate. The growth of E. piscicida was reduced in the defined medium without phenylalanine, tyrosine, alanine, or nicotinic acid, whereas the growth of E. anguillarum was reduced in the defined medium without serine, cysteine, leucine, threonine, or isoleucine. Tyrosine and alanine were present in higher concentrations in the Olive Flounder serum, whereas threonine and isoleucine were present in higher concentrations in the Red Seabream serum, suggesting favorable growth conditions for E. piscicida and E. anguillarum.

CONCLUSION

This study characterizes a minimal defined medium that can be used for developing vaccines against E. piscicida and E. anguillarum.

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.

Genome Report of Emergent Fish Pathogen Edwardsiella piscicida Recovered from Pseudoplatystoma corruscans in Brazil

Edwardsiella piscicida is a Gram-negative bacteria belonging to the Hafniaceae family which affects several species of marine and freshwater fish. We present the complete genome of E . piscicida strain BEP80 recovered from the Brazilian catfish named Surubim ( Pseudoplatystoma corruscans ), consisting a chromosome of 3,883,256 bp and no plasmids.

Phenotypic and genotypic analysis of Edwardsiella isolates from Taiwan indicates wide variation with a particular reference to Edwardsiella tarda and Edwardsiella anguillarum

Edwardsiella spp. is a gram-negative, facultatively anaerobic, intracellular bacteria threatening the aquaculture industry worldwide. Noticeably, E. tarda is now genotypically classified into three distinct groups (E. tarda, E. piscicida and E. anguillarum), but morphologically, it is unclear due to varying degrees of virulence in different fish hosts. Hence, to reclassify E. tarda, we investigated differences in genotypes, phenotypes and pathogenicity. We collected Edwardsiella isolates from five different counties of Taiwan between 2017 and 2021. At first, gyrB gene was amplified for a phylogenetic tree from 40 isolates from different fish and one reference isolate, BCRC10670, from the human. Thirty-nine strains clustered into E. anguillarum, 1 strain into E. piscicida and 1 strain into E. tarda from human strain. Second, all isolates were characterized using various phenotypic (API 20E biochemical profiles) and genotypic (pulsed-field gel electrophoresis [PFGE], and virulence-related gene detection). SpeI digestion revealed 10 pulsotypes and I-CeuI into 7 pulsotypes. Virulent genes (citC, gadB, katB, mukF and fimA) confirmed in 35, 31, 28, 37 and 38 isolates, respectively. Finally, in vivo challenge test in milkfish (Chanos chanos) indicated the highest mortality from E. anguillarum. Overall, results revealed unique features with Edwardsiella spp. genotypes and pathogenicity, which are relevant to the host and provide useful insights for future vaccine development.

Pathology and virulence of Edwardsiella tarda, Edwardsiella piscicida, and Edwardsiella anguillarum in channel (Ictalurus punctatus), blue (Ictalurus furcatus), and channel × blue hybrid catfish

In the mid-2010s, Edwardsiella tarda was reaffiliated into three discrete taxa (E. anguillarum, E. piscicida, and E. tarda), obscuring previous descriptions of E. tarda-induced pathology in fish. To clarify ambiguity regarding the pathology of E. tarda, E. piscicida, and E. anguillarum infections in US farm-raised catfish, channel catfish (Ictalurus punctatus), blue catfish (I. furcatus), and channel × blue catfish hybrids were challenged with comparable doses of each bacterium. The most severe pathology and mortality occurred in fish challenged with E. piscicida, supporting previous reports of increased pathogenicity in commercially important ictalurids, while E. anguillarum and E. tarda warrant only minimal concern. Acute pathologic lesions among bacterial species were predominantly necrotizing and characteristic of gram-negative sepsis but became progressively granulomatous over time. After 100 days, survivors were exposed to the approximate median lethal doses of E. piscicida and E. ictaluri, revealing some cross-protective effects among E. piscicida, E. anguillarum, and E. ictaluri. In contrast, no fish that survived E. tarda challenge demonstrated any protection against E. piscicida or E. ictaluri. This work supports reports of increased susceptibility of channel, blue, and hybrid catfish to E. piscicida, while highlighting potential cross-protective affects among fish associated Edwardsiella spp.

Comprehensive Relationship Analysis of the Long Noncoding RNAs (lncRNAs) and the Target mRNAs in Response to the Infection of Edwardsiella anguillarum in European eel (Anguilla anguilla) Inoculated with Freund's Adjuvant

Freund's complete adjuvant (FCA) and incomplete adjuvant (FIA), generally applied in subunit fishery vaccine, have not been explored on the molecular mechanism of the non-specific immune enhancement. As long noncoding RNAs (lncRNAs) play vital regulating roles in various biological activities, in this study, we examined the genome-wide expression of transcripts in the liver of European eel (Anguilla anguilla, Aa) inoculated with FCA and FIA (FCIA) to elucidate the regulators of lncRNAs in the process of Edwardsiella anguillarum (Ea) infection and Aa anti-Ea infection using strand-specific RNA-seq. After eels were challenged by Ea at 28 days post the first inoculation (dpi), compared to the control uninfected eels (Li group), the control infected eels (Con_Li group) showed severe bleeding, hepatocyte atrophy, and thrombi formed in the hepatic vessels of the liver, although eels inoculated with FCIA (FCIA_Li group) also formed slight thrombi in the hepatic vessels. Compared to the FCIA_Li group, there was about 10 times colony-forming unit (cfu) in the Con_Li group per 100 μg liver tissue, and the relative percent survival (RPS) of eels was 50% in FCIA_Li vs Con_Li. Using high-throughput transcriptomics, differential expressed genes (DEGs) and transcripts were identified and the results were verified using fluorescence real-time polymerase chain reaction (qRT-PCR). Interactions between the differential expressed lncRNAs (DE-lncRNAs) and the target DEGs were explored using Cytoscape according to their co-expression and co-location relationship. We found 13,499 lncRNAs (10,176 annotated and 3423 novel lncRNAs) between 3 comparisons of Con_Li vs Li, FCIA_Li vs Li, and FCIA_Li vs Con_Li, of which 111, 110, and 129 DE-lncRNAs were ascertained. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DEGs targeted by DE-lncRNAs revealed these DEGs mainly involved in single-organism cellular process in BP, membrane in CC and binding in MF, and KEGG pathways showed that the target DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 118 DE-lncRNAs target 1161 DEGs were involved in an interaction network of 8474 co-expression and 333 co-location-related links, of which 16 DE-lncRNAs play vital roles in anti-Ea infection. Taken together, the interaction networks revealed that DE-lncRNAs underlies the process of Ea infection and Aa anti-Ea infection.

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.

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.

Cross-protective efficacy of a live-attenuated Edwardsiella ictaluri vaccine against heterologous Edwardsiella piscicida isolates in channel and channel × blue catfish hybrids

Edwardsiella piscicida is a growing problem for catfish aquaculture in the southeastern United States, particularly in channel (Ictalurus punctatus) x blue (I. furcatus) catfish hybrids. Research has shown E. piscicida isolates recovered from farmed catfish in Mississippi form at least five discrete phyletic groups, with no apparent differences in virulence in channel and hybrid catfish. Laboratory trials have shown a live-attenuated E. ictaluri vaccine (340X2) cross-protects against at least one E. piscicida isolate (S11-285) in channel and hybrid catfish, although it is unknown if this protection exists for other E. piscicida variants. To this end, channel and hybrid catfish were immunized by immersion with E. ictaluri 340X2. Thirty days later, fish were challenged by intracoelomic injection with representative E. piscicida variants from each phyletic group. Relative percent survival (RPS) for hybrids ranged from 54.7% to 77.8%, while RPS in channels ranged from 80.5% to 100%. A second study investigated whether channel and hybrid catfish exposed to heterologous E. piscicida isolates were similarly protected against wild-type E. ictaluri. Fish were exposed by bath immersion to representative E. piscicida isolates from each phyletic group. Thirty days post-immunization, fish were challenged by immersion with wild-type E. ictaluri isolate S97-773. Regardless of variant, previous exposure to heterologous E. piscicida isolates significantly improved survival following E. ictaluri challenge. These findings suggest the presence of shared and conserved antigens among E. piscicida and E. ictaluri that could be exploited by application of polyvalent or cross-protective vaccines.

Orf1B controls secretion of T3SS proteins and contributes to Edwardsiella piscicida adhesion to epithelial cells

Edwardsiella piscicida is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish. The type III secretion system (T3SS) is one of its two most important virulence islands. T3SS protein EseJ inhibits E. piscicida adhesion to epithelioma papillosum cyprini (EPC) cells by negatively regulating type 1 fimbria. Type 1 fimbria helps E. piscicida to adhere to fish epithelial cells. In this study, we characterized a functional unknown protein (Orf1B) encoded within the T3SS gene cluster of E. piscicida. This protein consists of 122 amino acids, sharing structural similarity with YscO in Vibrio parahaemolyticus. Orf1B controls secretion of T3SS translocon and effectors in E. piscicida. By immunoprecipitation, Orf1B was shown to interact with T3SS ATPase EsaN. This interaction may contribute to the assembly of the ATPase complex, which energizes the secretion of T3SS proteins. Moreover, disruption of Orf1B dramatically decreased E. piscicida adhesion to EPC cells due to the increased steady-state protein level of EseJ within E. piscicida. Taken together, this study partially unraveled the mechanisms through which Orf1B promotes secretion of T3SS proteins and contributes to E. piscicida adhesion. This study helps to improve our understanding on molecular mechanism of E. piscicida pathogenesis.

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.

Whole-Genome Sequences of Three Edwardsiella piscicida Isolates from Diseased Fish in South Korea

Edwardsiella piscicida is a Gram-negative pathogen that is associated with edwardsiellosis in aquaculture systems worldwide. Here, we report the whole-genome sequences of three E. piscicida isolates derived from cultured fish in South Korea.

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.

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.

Multiplex PCR assay for correct identification of the fish pathogenic species of Edwardsiella genus reveals the presence of E. anguillarum in South America in strains previously characterized as E. tarda

AIMS

Develop a species-specific multiplex PCR to correctly identify Edwardsiella species in routine diagnostic for fish bacterial diseases.

METHODS AND RESULTS

The genomes of 62 Edwardsiella spp. isolates available from the National Center for Biotechnology Information (NCBI) database were subjected to taxonomic and pan-genomic analyses to identify unique regions that could be exploited by species-specific PCR. The designed primers were tested against isolated Edwardsiella spp. strains, revealing errors in commercial biochemical tests for bacterial classification regarding Edwardsiella species.

CONCLUSION

Some of the genomes of Edwardsiella spp. in the NCBI platform were incorrectly classified, which can lead to errors in some research. A functional mPCR was developed to differentiate between phenotypically and genetically ambiguous Edwardsiella, with which, we detected the presence of Edwardsiella anguillarum affecting fish in Brazil.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that the misclassification of Edwardsiella spp in Brazil concealed the presence of E. anguillarum in South America. Also, this review of the taxonomic classification of the Edwardsiella genus is a contribution to the field to help researchers with their sequencing and identification of genomes, showing some misclassifications in online databases that must be corrected, as well as developing an easy assay to characterize Edwardsiella species in an end-point mPCR.

Coordinate regulation of carbohydrate metabolism and virulence by PtsH in pathogen Edwardsiella piscicida

Carbohydrate metabolism of bacterial pathogens conducts crucial roles in regulating pathogenesis but the molecular mechanisms by which metabolisms and virulence are been modulated and coordinated remain to be illuminated. Here, we investigated in this regard Edwardsiella piscicida, a notorious zoonotic pathogen previously named E. tarda that could ferment very few PTS sugars including glucose, fructose, mannose, N-acetylglucosamine, and N-acetylgalactosamine. We systematically characterized the roles of each of the predicted 23 components of phosphotransferase system (PTS) with the respective in-frame deletion mutants and defined medium containing specific PTS sugar. In addition, PtsH was identified as the crucial PTS component potentiating the utilization of all the tested PTS sugars. Intriguingly, we also found that PtsH while not Fpr was involved in T3SS gene expression and was essential for the pathogenesis of E. piscicida. To corroborate this, His15 and Ser46, the two established PtsH residues involved in phosphorylation cascade, showed redundant roles in regulating T3SS yields. Moreover, PtsH was shown to facilitate mannose uptake and transform it into mannose-6-phosphate, an allosteric substrate established to activate EvrA to augment bacterial virulence. Collectively, our observations provide new insights into the roles of PTS reciprocally regulating carbohydrate metabolism and virulence gene expression. KEY POINTS: • PTS components' roles for sugar uptake are systematically determined in Edwardsiella piscicida. • PtsH is involved in saccharides uptake and in the regulation of E. piscicida's T3SS expression. • PtsH phosphorylation at His15 and Ser46 is essential for the T3SS expression and virulence.

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.

Genetic characterization of heterologous Edwardsiella piscicida isolates from diverse fish hosts and virulence assessment in a Chinook salmon Oncorhynchus tshawytscha model

Edwardsiella piscicida is an emergent global fish pathogen with a wide host range, although host associations driving genetic diversity remain unclear. This study investigated the genetic and virulence diversity of 37 E. piscicida isolates recovered from 10 fish species in North America. Multilocus sequence analysis (MLSA) was conducted using concatenated alignments of the gyrB, pgi and phoU sequences. MLSA clustered the tested isolates into six discrete clades. In light of recent disease outbreaks in cultured salmonids, the virulence of each clade was evaluated in Chinook salmon Oncorhynchus tshawytscha fingerlings following intracoelomic challenge of ~10⁶  CFU/fish. Challenged and control fish were monitored for 21d, and microbiological and histological examination was performed on dead and surviving fish. Peak mortality occurred 3-5 days post-challenge (dpc) regardless of isolate or genetic group. Edwardsiella piscicida was recovered from all moribund and dead animals. At 21 dpc, fish challenged with isolates from clades II, III and IV presented cumulative mortality ≥83.3%, whereas isolates from clade I, V and VI resulted in cumulative mortality ≤71.4%. This study suggests an underlying genetic basis for strain virulence and potential host associations. Further investigations using other fish models and variable challenge conditions are warranted.

Interplay between ferric uptake regulator Fur and horizontally acquired virulence regulator EsrB coordinates virulence gene expression in Edwardsiella piscicida

Edwardsiella piscicida mediates hemorrhagic septicemia and is a leading pathogen of fish. E. piscicida invades and colonizes macrophages using type III and VI secretion systems (T3/T6SS) that are controlled by a two-component system (TCS) EsrA-EsrB. Iron acquisition is essential for E. piscicida pathogenesis and coordination between iron and TCS signaling in modulating bacterial virulence is not well understood. Here, we show that iron uptake systems are co-regulated by ferric uptake regulator (Fur) in E. piscicida. Fur bound to 98 genes that harbored conserved Fur-box to globally control the expression of ∼755 genes, including those encoding iron uptake systems, T3/T6SS, and Icc, cAMP phosphodiesterase that represses biofilm formation. Additionally, Fur, in complex with iron, bound to the esrB promoter to repress expression and ultimately attenuated virulence. Conversely, EsrB activated the expression of T3/T6SS and iron uptake systems to mitigate a shortage of intracellular iron during iron scarcity. Furthermore, EsrB directly bound to and activated the fur promoter, leading to Fur-ferrous ion-dependent esrB repression in the presence of iron. Finally, Fur-EsrB interplay was essential for bacterial fitness during in vivo infection and survival in seawater environments. Collectively, we highlight the mechanisms that underlie the reciprocal regulatory networks of iron homeostasis and virulence systems in E. piscicida.

Genetic variability of Edwardsiella piscicida isolates from Mississippi catfish aquaculture with an assessment of virulence in channel and channel × blue hybrid catfish

The bacterium Edwardsiella piscicida causes significant losses in global aquaculture, particularly channel (Ictalurus punctatus) × blue (I. furcatus) hybrid catfish cultured in the south-eastern United States. Emergence of E. piscicida in hybrid catfish is worrisome given current industry trends towards increased hybrid production. The project objectives were to assess intraspecific genetic variability of E. piscicida isolates recovered from diseased channel and hybrid catfish in Mississippi; and determine virulence associations among genetic variants. Repetitive extragenic palindromic sequence-based PCR (rep-PCR) using ERIC I and II primers was used to screen 158 E. piscicida diagnostic case isolates. A subsample of 39 E. piscicida isolates, representing predominant rep-PCR profiles, was further characterized using BOX and (GTG)₅ rep-PCR primers, virulence gene assessment and multilocus sequence analysis (MLSA) targeting housekeeping genes gyrb, pgi and phoU. The MLSA provided greater resolution than rep-PCR, revealing 5 discrete phylogroups that correlated similarly with virulence gene profiles. Virulence assessments using E. piscicida representatives from each MLSA group resulted in 14-day cumulative mortality ranging from 22% to 54% and 63 to 72% in channel and hybrid fingerlings, respectively. Across all phylogroups, mortality was higher in hybrid catfish (p < .05), supporting previous work indicating E. piscicida is an emerging threat to hybrid catfish aquaculture in the south-eastern United States.

Transcriptome RNA-seq revealed lncRNAs activated by Edwardsiella anguillarum post the immunization of OmpA protecting European eel (Anguilla anguilla) from being infected

Long noncoding RNAs (lncRNAs) play important roles in various biological activities as vital regulators. However, no study has focused on the lncRNA regulation of Outer membrane protein (OMP) immunization against aquatic bacterial infection. In this study, we examined the genome-wide expression of lncRNAs in the liver of European eel (Anguilla anguilla, Aa) administrated by a recombinant OmpA (rOmpA) from Edwardsiella anguillarum (Ea) to elucidate the functions of lncRNAs in the process of Ea infection and Aa anti-Ea infection using strand specific RNA-seq. Eels were challenged by Ea at 28 d post the immunization (dpi) of OmpA, and the result showed, compared to uninfected livers in the PBS group (Con group), the infected livers in the PBS group (Con_inf group) showed severe bleeding, hepatocyte atrophy and thrombi formed in the hepatic vessels; livers in the OmpA group (OmpA_inf) also formed slight thrombi in the hepatic vessels. The relative percent survival of eels in OmpA_inf vs Con_inf was 78.6%. Using high-throughput transcriptomics, we found 13405 lncRNAs in 3 compares of Con_inf vs Con, OmpA_inf vs Con and OmpA_inf vs Con_inf, of which 111, 129 and 158 DE-lncRNAs were ascertained. GO analysis of the DE-lncRNAs revealed the targeting DEGs were mainly involved in single-organism process, signaling, biological process and response to stimulus in BP, component of membrane in CC and binding in MF; KEGG pathways showed that the targeting DEGs in co-expression and co-location enriched in cell adhesion molecules. Finally, 54 DE-lncRNAs targeting 1675 DEGs were involved in an interaction network of 21692 co-expression and 483 co-location related links, of which 18 DE-lncRNAs appear to play crucial roles in anti-Ea infection. Thus, the interaction networks revealed crucial DE-lncRNAs underlying the process of Ea infection and Aa anti-Ea infection pre and post the immunization of OmpA.

Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species

Climate change is expected to have a drastic effect on aquaculture worldwide. As we move forward with the agenda to increase and diversify aquaculture production, rising temperatures will have a progressively relevant impact on fish farming, linked to a multitude of issues associated with fish welfare. Temperature affects the physiology of both fish and pathogens, and has the potential to lead to significant increases in disease outbreaks within aquaculture systems, resulting in severe financial impacts. Significant shifts in future temperature regimes are projected for the Mediterranean Sea. We therefore aim to review and discuss the existing knowledge relating to disease outbreaks in the context of climate change in Mediterranean finfish aquaculture. The objective is to describe the effects of temperature on the physiology of both fish and pathogens, and moreover to list and discuss the principal diseases of the three main fish species farmed in the Mediterranean, namely gilthead seabream (Sparus aurata), European seabass (Dicentrarchus labrax), and meagre (Argyrosomus regius). We will attempt to link the pathology of each disease to a specific temperature range, while discussing potential future disease threats associated with the available climate change trends for the Mediterranean Sea.

Identification and application of T3SS translocation signal in Edwardsiella piscicida attenuated carrier as a bivalent vaccine

Type III secretion system (T3SS)-dependent translocation has been used to deliver heterologous antigens by vaccine carriers into host cells. In this research, we identified the translocation signal of Edwardsiella piscicida T3SS effector EseG and constructed an antibiotic resistance-free balanced-lethal system as attenuated vaccine carrier to present antigens by T3SS. Edwardsiella piscicida LSE40 asd gene deletion mutant was constructed and complemented with pYA3342 harbouring the asd (aspartate β-semialdehyde dehydrogenase) gene from Salmonella. Fusion proteins composed of EseG N-terminal 1-108 amino acids and the TEM1-β-lactamase reporter were inserted in plasmid pYA3342. The fusion protein could secrete into the cell culture, translocate into HeLa cells, and localize in the membrane fraction. Then, the double gene deletion mutant LSE40ΔasdΔpurA was constructed as an attenuated vaccine carrier, and Aeromonas hydrophila GapA (glyceraldehyde-3-phosphate dehydrogenase) was fused with the translocation signal, instead of the TEM1-β-lactamase reporter. The bivalent vaccine could protect blue gourami (Trichogaster trichopterus) against E. piscicida and A. hydrophila, with the relative per cent survival of 80.77% and 63.83%, respectively. These results indicated that EseG N-terminal 1-108 amino acid peptide was the translocation signal of E. piscicida T3SS, which could be used to construct bivalent vaccines based on an attenuated E. piscicida carrier.

Identification and Characterization of EvpQ, a Novel T6SS Effector Encoded on a Mobile Genetic Element in Edwardsiella piscicida

In this study, a hypothetical protein (ORF02740) secreted by Edwardsiella piscicida was identified. We renamed the ORF02740 protein as EvpQ, which is encoded by a mobile genetic element (MGE) in E. piscicida genome. The evpQ gene is spaced by 513 genes from type VI secretion system (T6SS) gene cluster. Low GC content, three tRNA, and three transposase genes nearby evpQ define this MGE that evpQ localizes as a genomic island. Sequence analysis reveals that EvpQ shares a conserved domain of C70 family cysteine protease and shares 23.91% identity with T3SS effector AvrRpt2 of phytopathogenic Erwinia amylovora. Instead, EvpQ of E. piscicida is proved to be secreted at a T6SS-dependent manner, and it can be translocated into host cells. EvpQ is thereof a novel T6SS effector. Significantly decreased competitive index of ΔevpQ strain in blue gourami fish (0.53 ± 0.27 in head kidney and 0.44 ± 0.19 in spleen) indicates that EvpQ contributes to the pathogenesis of E. piscicida. At 8-, 18-, and 24-h post-subculture into DMEM, the transcription of evpQ was found to be negatively regulated by Fur and positively regulated by EsrC, and the steady-state protein levels of EvpQ are negatively controlled by RpoS. Our study lays a foundation for further understanding the pathogenic role of T6SS in edwardsiellosis.

Transcriptome of Edwardsiella anguillarum in vivo and in vitro revealed two-component system, ABC transporter and flagellar assembly are three pathways pathogenic to European eel (Anguilla anguilla)

Edwardsiella anguillarum is one of the common bacterial pathogens for the cultivated eels in China. The aim of this study was to reveal the cause of E. anguillarum pathogenic to European eel (Anguilla anguilla) from the perspective of the transcriptome. In this study, we first prepared E. anguillarum cultured in vitro and analysed the whole transcriptome after extracting the total RNA. Then, eels were i.p injected with E. anguillarum, and total RNA were extracted from the liver of European eels 48 h after the infection. After sequencing the transcriptome, we obtained average 1.97 × 10⁸ clean reads cultured in vitro and 1.36 × 10⁵ clean reads located in vivo after annotating all reads into the genome of E. anguillarum. The whole transcriptome showed, compared to the E. anguillarum cultured in vitro, 503 significantly up and 657 significantly down-regulated different expressed genes (DEGs) were observed. KEGG analysis showed that 38 DEGs of Two-Component System, 41 DEGs of ABC transporter, and 10 DEGs flagellar assembly pathways were highly upregulated in E. anguillarum located in vivo. Then, we designed primers to analyse the up-regulated DEGs through qRT-PCR and confirmed some up-regulated DEGs. The results of this study provide important reference for the further study of pathogen-host interaction between E. anguillarum and European eel.

Updates on Selected Emerging Infectious Diseases of Ornamental Fish

Emerging infectious diseases of ornamental fish are discussed with special focus on clinical relevance, detection, and treatment, where applicable. Important emerging infectious diseases of fish include goldfish herpesvirus, koi herpesvirus, carp edema virus, Erysipelothrix, Edwardsiella ictaluri, Edwardseilla piscicida, and Francisella. Some diseases are more species or genus specific, but many emerging diseases do not seem to have a species preference and affect a variety of species worldwide. Proper husbandry and biosecurity with a disease detection plan for ornamental fish is essential to monitor and prevent future outbreaks.

Comparative genomics of the fish pathogens Edwardsiella ictaluri 93-146 and Edwardsiella piscicida C07-087

Edwardsiella ictaluri and Edwardsiella piscicida are important fish pathogens affecting cultured and wild fish worldwide. To investigate the genome-level differences and similarities between catfish-adapted strains in these two species, the complete E. ictaluri 93-146 and E. piscicida C07-087 genomes were evaluated by applying comparative genomics analysis. All available complete (10) and non-complete (19) genomes from five Edwardsiella species were also included in a systematic analysis. Average nucleotide identity and core-genome phylogenetic tree analyses indicated that the five Edwardsiella species were separated from each other. Pan-/core-genome analyses for the 29 strains from the five species showed that genus Edwardsiella members have 9474 genes in their pan genome, while the core genome consists of 1421 genes. Orthology cluster analysis showed that E. ictaluri and E. piscicida genomes have the greatest number of shared clusters. However, E. ictaluri and E. piscicida also have unique features; for example, the E. ictaluri genome encodes urease enzymes and cytochrome o ubiquinol oxidase subunits, whereas E. piscicida genomes encode tetrathionate reductase operons, capsular polysaccharide synthesis enzymes and vibrioferrin-related genes. Additionally, we report for what is believed to be the first time that E. ictaluri 93-146 and three other E. ictaluri genomes encode a type IV secretion system (T4SS), whereas none of the E. piscicida genomes encode this system. Additionally, the E. piscicida C07-087 genome encodes two different type VI secretion systems. E. ictaluri genomes tend to encode more insertion elements, phage regions and genomic islands than E. piscicida. We speculate that the T4SS could contribute to the increased number of mobilome elements in E. ictaluri compared to E. piscicida. Two of the E. piscicida genomes encode full CRISPR-Cas regions, whereas none of the E. ictaluri genomes encode Cas proteins. Overall, comparison of the E. ictaluri and E. piscicida genomes reveals unique features and provides new insights on pathogenicity that may reflect the host adaptation of the two species.

The Translocation and Assembly Module (TAM) of Edwardsiella tarda Is Essential for Stress Resistance and Host Infection

Translocation and assembly module (TAM) is a protein channel known to mediate the secretion of virulence factors during pathogen infection. Edwardsiella tarda is a Gram-negative bacterium that is pathogenic to a wide range of farmed fish and other hosts including humans. In this study, we examined the function of the two components of the TAM, TamA and TamB, of E. tarda (named tamA_Et and tamB_Et, respectively). TamA_Et was found to localize on the surface of E. tarda and be recognizable by TamA_Et antibody. Compared to the wild type, the tamA and tamB knockouts, TX01ΔtamA and TX01ΔtamB, respectively, were significantly reduced in motility, flagella formation, invasion into host cells, intracellular replication, dissemination in host tissues, and inducing host mortality. The lost virulence capacities of TX01ΔtamA and TX01ΔtamB were restored by complementation with the tamA_Et and tamB_Et genes, respectively. Furthermore, TX01ΔtamA and TX01ΔtamB were significantly impaired in the ability to survive under low pH and oxidizing conditions, and were unable to maintain their internal pH balance and cellular structures in acidic environments, which led to increased susceptibility to lysozyme destruction. Taken together, these results indicate that TamA_Et and TamB_Et are essential for the virulence of E. tarda and required for E. tarda to survive under stress conditions.