Construction and characterization of a live, attenuated esrB mutant of Edwardsiella tarda and its potential as a vaccine against the haemorrhagic septicaemia in turbot, Scophthamus maximus (L.)

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.

Efficacy of bivalent vaccine against Aeromonas salmonicida and Edwardsiella tarda infections in turbot

In recent years, more than one pathogenic organism has usually been isolated from diseased turbot Scophthalmus maximus, creating a pressing need for the development of combination vaccines to prevent fish diseases brought on simultaneously by various infections. In this study, the inactivated bivalent vaccine of Aeromonas salmonicida and Edwardsiella tarda was prepared by the formalin inactivation method. After challenge with A. salmonicida and E. tarda at 4 weeks post-vaccination in turbot, the relative percentage survival (RPS) of the inactivated bivalent vaccine was 77.1%. In addition, we assessed the effects of the inactivated bivalent vaccine and evaluated the immunological processes after immunization in a turbot model. Serum antibody titer and lysozyme activity of the vaccinated group were both upregulated and higher than that in control group after vaccination. The expression levels of genes (TLR2, IL-1β, CD4, MHCI, MHCⅡ) that related to antigen recognition, processing and presentation were also studied in the liver, spleen and kidney tissues of vaccinated turbot. All the detected genes in the vaccinated group had a significant upward trend, and most of them reached the maximum value at 3-4 weeks, which had significant differences from the control group, suggesting that antigen recognition, processing and presentation pathway was activated by the inactivated bivalent vaccine. Our study provides a basis for further application of the killed bivalent vaccine against A. salmonicida and E. tarda in turbot, making it good potential that can be applied in aquaculture.

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.

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.

Characterization of a novel live attenuated Edwardsiella piscicida vaccine based on the overexpressed type III secretion system and systematic deletion of the associated effectors

Edwardsiella piscicida causes edwardsiellosis in a variety of fish species and leads to tremendous economic losses in the global aquaculture industries. Thus, effective and safe prevention and control of this bacterium are urgently needed to combat the related infections. Live attenuated vaccines (LAVs) effectively prevent infectious diseases. However, most of the existing E. piscicida LAVs are based on the deletion of genes encoding the translocon components of the type III secretion system (T3SS), the core virulence system, which is the most prominent protective bacterial antigen with the strongest immunogenicity. In this study, we systematically deleted all of the 9 established T3SS effectors in E. piscicida (aka 9Δ) and the rpoS gene encoding the alternative sigma factor, the esrB repressor (10Δ), then we overexpressed esrB and T3SS in E. piscicida to obtain the recombinant strain 10Δ/esrBOE. The modified strains 10Δ and 10Δ/esrBOE exhibited severe attenuation and in vivo colonization defects. Additionally, vaccination by intraperitoneal injection with 10Δ and 10Δ/esrBOE could significantly upregulate the expression of the antigen recognition related gene (TLR5) and the adaptive immune response-related gene (MHC II) in the spleen/kidney of turbot fish, and it also enhanced the hosts' serum bactericidal capacity. Finally, vaccination with 10Δ/esrBOE led to increased immune protection against the challenge of wild type E. piscicida EIB202 in turbot fish. Collectively, these findings demonstrated that 10Δ/esrBOE was a novel LAV strain and therefore a potential novel strategy for the construction of LAVs against bacterial pathogens.

MviN mediates the regulation of environmental osmotic pressure on esrB to control the virulence in the marine fish pathogen Edwardsiella piscicida

Edwardsiella piscicida is a notorious pathogen infecting diverse kinds of fish and causes substantial economic losses in the global aquaculture industries. The EsrA-EsrB two-component system plays a critical role in the regulation of virulence genes expression, including type III and type VI secretion systems (T3/T6SSs). In this study, the putative regulators of esrB were screened by the transposon insertion sequencing (TIS) technology. As a result, MviN, a lipid II flippase, was identified as a modulator to upregulate esrB and downstream T3/T6SS gene expression in the earlier growth phases while downregulate esrB at the later stages. Complement or overexpression of the mviN restored the esrB as well as T3/T6SS expression in the ΔmviN mutant strain. Moreover, MviN also mediated the regulation of environmental osmotic pressure on the expression of esrB. MviN was also found to significantly influence the in vivo colonization of E. piscicida in turbot. Collectively, this study enhanced our understanding of pathogenesis and virulence regulatory network of E. piscicida.

An Edwardsiella piscicida esaS mutant reveals contribution to virulence and vaccine potential

Edwardsiella piscicida is a Gram-negative pathogen that causes disease in diverse aquatic organisms. The disease leads to extensive losses in commercial aquaculture species, including farmed U.S. catfish. The type III secretion system (T3SS) often contributes to virulence of Gram-negative bacteria. The E. piscicida esaS gene encodes a predicted T3SS export apparatus protein. In the current study, an E. piscicida esaS mutant was constructed and characterized to increase our understanding of the role of T3SS in E. piscicida virulence. Deletion of esaS did not significantly affect biofilm formation and hemolytic activity of E. piscicida, but it had significant effects on expression of hemolysis and T3SS effector genes during biofilm growth. EpΔesaS showed significantly (P < 0.05) reduced virulence in catfish compared to the parent strain. No mortalities occurred in fish infected with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU/fish compared to 26% mortality in fish infected with wild-type E. piscicida at 7.5 × 10⁵ CFU/fish. Bioluminescence imaging indicated that EpΔesaS invades catfish and colonizes for a short period in the organs. Furthermore, catfish immunized with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU provided 47% and 87% relative percent survival, respectively. These findings demonstrated that esaS plays a role in E. piscicida virulence, and the deletion mutant has vaccine potential for protection against wild-type E. piscicida infection.

PepA binds to and negatively regulates esrB to control virulence in the fish pathogen Edwardsiella piscicida

As an important marine fish pathogen, Edwardsiella piscicida infects a broad range of fish species and causes substantial economic losses. The EsrA-EsrB two-component system is essential for the expression of type III and type VI secretion systems (T3/T6SSs), the key virulence determinants in the bacterium. In this study, a pull-down assay with the esrB promoter as bait was performed to identify the upstream regulators of esrB. As a result, PepA, a leucyl aminopeptidase, was identified as a repressor of EsrB and T3/T6SS expression. PepA bound to the esrB promoter region and negatively regulated the production of T3/T6SS proteins in early stages. Moreover, PepA was found to affect the in vivo colonization of E. piscicida in turbot livers through the regulation of EsrB expression. Collectively, our results enhance the understanding of the virulence regulatory network and in vivo colonization mechanism of E. piscicida. One sentence summary: PepA regulates EsrB expression in Edwardsiella piscicida.

EsrB negatively regulates expression of the glutamine sythetase GlnA in the fish pathogen Edwardsiella piscicida

Edwardsiella piscicida is a gram-negative bacterial pathogen invading a wide range of fish species. Response regulator EsrB is essential for the activation of type III and type VI secretion systems (T3/T6SS). In this study, proteomes of the wild-type E. piscicida EIB202 and ΔesrB mutant strains were compared to identify the regulon components of EsrB cultured in DMEM allowing T3/T6SS expression. As a result, 19 proteins showed different expression, which were identified to be associated with T3/T6SS, related to amino acid transport and metabolism, and energy production. Particularly, GlnA, a glutamine synthetase essential for ammonia assimilation and glutamine biosynthesis from glutamate, was found to be regulated negatively by EsrB. Moreover, GlnA affected bacterial growth in vitro and bacterial colonization in vivo. Collectively, our results indicated that EsrB plays important roles in regulating the expression of metabolic pathways and virulence genes, including glutamine biosynthesis in E. piscicida during infection.

Pattern analysis of conditional essentiality (PACE)-based heuristic identification of an in vivo colonization determinant as a novel target for the construction of a live attenuated vaccine against Edwardsiella piscicida

Edwardsiella piscicida is the aetiological agent of fish edwardsiellosis, causing huge economic losses in aquaculture industries. The use of a live attenuated vaccine (LAV) will be an effective strategy to control the disease in farmed fish. Thus, methods facilitating exploration of targets used for construction of an LAV will be of great significance. Previously, we devised an algorithm termed pattern analysis of conditional essentiality (PACE) to perform genome-wide analysis of the temporal dynamic behaviour of E. piscicida mutants colonizing turbot. Here, we correlated the conditional essentiality patterns of the PACE-derived colonization determinants with that of the aroC gene encoding chorismate synthase, the established target for LAV construction in E. piscicida, and identified ETAE_0023 as a novel valuable LAV target. ETAE_0023 encodes an uncharacterized DcrB family protein. Deletion of ETAE_0023 dramatically impaired E. piscicida invasion capability in ZF4 cells as well as colonization in fish and resulted in in vivo clearance at ∼30 days post-infection. ΔETAE_0023 showed an ∼2500-fold higher 50% lethal dose (LD₅₀) than that of the wild type strain. Vaccination with ΔETAE_0023 by intraperitoneal (i.p.) injection upregulated expression of immune factors, i.e., IL-1β, IgM, MHC-I and MHC-II, and produced significantly high levels of E. piscicida-specific IgM as well as serum bactericidal capacities in turbot. Moreover, a single i.p. inoculation with ΔETAE_0023 generated significant protection comparable to the established WED LAV strain in turbot against challenge with the wild type strain after 5 weeks of vaccination. Taken together, we demonstrated a PACE-based method for heuristic identification of targets for LAV construction and presented ΔETAE_0023 as a new LAV candidate against edwardsiellosis.

Dynamic distribution of formalin-inactivated Edwardsiella tarda in flounder (Paralichthys olivaceus) post intraperitoneal vaccination

In order to investigate the dynamic distribution of antigen in different tissues post vaccination, an absolute real-time quantitative PCR was employed to detect the amount of antigen in flounder (Paralichthys olivaceus) post intraperitoneal (i.p.) injection with three concentrations (10⁷, 10⁸, 10⁹ CFU ml^-1) of formalin-inactivated Edwardsiella tarda bacterin. The results showed that the amount of uptaken antigen quickly increased and then decreased in different tissues. The peak occurred first in the spleen and head kidney at 6-9 h after injection, and in the liver and blood at 9-15 h, then in the gill, intestine and skin at 15-24 h, finally in the muscle at 24-36 h. The amount of antigen was highest in the spleen and head kidney, followed by the blood, liver and gill, and lowest in the intestine, skin and muscle. Among the three concentration groups, the amount of antigen increased with the increasing concentration of the vaccine in the blood, liver, gill, intestine, skin and muscle, except for the spleen and head kidney, in which more antigens were found in the 10⁸ CFU ml^-1 group than that in 10⁹ CFU ml^-1 group. Moreover, IIFA and western blotting was performed to examine the tissue distribution of antigen at 9 h after vaccination with 10⁸ CFU ml^-1 formalin-inactivated E. tarda. The bacteria were mainly observed in the spleen and head kidney, then the liver, gill and blood, and least in the intestine, skin and muscle, which was roughly in accordance with the results of absolute qPCR. Furthermore, the expressions of CD4-1, MHC IIα, CD8α and MHC Iα in different tissues were detected by RT-qPCR, and the expression levels of these genes were highest in the spleen and head kidney, then in the blood, gill, liver, and lowest in the intestine, skin and muscle. All these results provided useful information for dynamic transportation of antigen uptake post vaccination, and also deepened the understanding of immune response to the injection vaccination.

Synergistic effect of a combined live Vibrio anguillarum and Edwardsiella piscicida vaccine in turbot

In aquaculture, more than one pathogen usually be isolated from the sick fish, creating an urgent need for developing combined vaccines to control fish disease caused by multiple pathogens simultaneously. In our previous work, two live attenuated vaccines against Vibrio anguillarum and Edwardsiella piscicida were vaccinated in turbot, exhibiting an efficient protection. However, some immunological processes such as antigenic competition, antigenic cross-reaction and antigen induced suppression during combined vaccination are unknown. In this study, we evaluated the effectiveness of the combined live vaccines and explored the immunological processes after vaccination. We found that the combined two live attenuated vaccines for V. anguillarum and E. piscicida induced a stronger immune response without existing antigen competition. Instead, a synergistic effect was observed not only for triggering innate immune response but for stimulation of adaptive immunity. Our study suggested that the two combined live vaccines against V. anguillarum and E. piscicida could be used simultaneously in the future.

Plesiomonas shigelloides sipD mutant, generated by an efficient gene transfer system, is less invasive

Plesiomonas shigelloides is widely associated with human diarrheal disease. Research on this pathogen has been hampered by the absence of an effective genetic manipulation system. In the present study, an efficient and precise conjugation transfer procedure, mediated by suicide vector pRE112 was used to overcome this limitation. The efficiency of generating double recombinants was average 74.3%, and the conjugation protocol may be applied to other P. shigelloides strains. We also identified that the SipD protein of P. shigelloides G5884 (serotype O45) is 65% similar to the SipD in Salmonella pathogenicity island 1 (SPI-1), which is a key element of the type III secretion system related to Salmonella invasion. A P. shigelloides sipD null mutant was generated via the conjugation system, using the suicide vector pRE112. The isogenic mutant strain lacking sipD showed a 50% reduction in its capacity to invade Caco-2 cells.

Critical role for a promoter discriminator in RpoS control of virulence in Edwardsiella piscicida

Edwardsiella piscicida is a leading fish pathogen that causes significant economic loses in the aquaculture industry. The pathogen depends on type III and type VI secretion systems (T3/T6SS) for growth and virulence in fish and the expression of both systems is controlled by the EsrB transcription activator. Here, we performed a Tn-seq-based screen to uncover factors that govern esrB expression. Unexpectedly, we discovered that RpoS antagonizes esrB expression and thereby inhibits production of E. piscicida’s T3/T6SS. Using in vitro transcription assays, we showed that RpoS can block RpoD-mediated transcription of esrB. ChIP-seq- and RNA-seq-based profiling, as well as mutational and biochemical analyses revealed that RpoS-repressed promoters contain a -6G in their respective discriminator sequences; moreover, this -6G proved critical for RpoS to inhibit esrB expression. Mutation of the RpoS R99 residue, an amino acid that molecular modeling predicts interacts with -6G in the esrB discriminator, abolished RpoS’ capacity for repression. In a turbot model, an rpoS deletion mutant was attenuated early but not late in infection, whereas a mutant expressing RpoS^R99A exhibited elevated fitness throughout the infection period. Collectively, these findings deepen our understanding of how RpoS can inhibit gene expression and demonstrate the temporal variation in the requirement for this sigma factor during infection.

Edwardsiella piscicida, a major fish pathogen, relies on T3/T6SSs for virulence and the EsrB transcription activator promotes the expression of these secretion systems and many other genes that enable growth in fish. Here, we found that the alternative sigma factor RpoS inhibits expression of esrB thereby diminishing expression of virulence-associated genes. Transcriptome profiling revealed that, as in many other organisms, RpoS enables expression of hundreds of genes, many of which are linked to stress responses, suggesting that RpoS may mediate a trade-off between stress adaptation and virulence. Consistent with this idea, we found that an rpoS mutant was attenuated early, but not late in infection of turbot, whereas an esrB mutant was attenuated late and not early in infection. Molecular analyses demonstrated that RpoS inhibition of esrB expression involves a direct interaction between RpoS and the esrB promoter; in particular, interactions between RpoS residue R99 and the -6G nucleotide in the esrB promoter discriminator appear to be critical for repression of esrB expression. These findings provide new insight into how a sigma factor can impede transcription and demonstrate the temporal dynamics of the requirement for a sigma factor during the course of infection.

Construction and evaluation of type III secretion system mutants of the catfish pathogen Edwardsiella piscicida

Catfish is the largest aquaculture industry in the United States. Edwardsiellosis is considered one of the most significant problems affecting this industry. Edwardsiella piscicida is a newly described species within the genus Edwardsiella, and it was previously classified as Edwardsiella tarda. It causes gastrointestinal septicaemia, primarily in summer months, in farmed channel catfish in the south-eastern United States. In the current study, we adapted gene deletion methods used for Edwardsiella to E. piscicida strain C07-087, which was isolated from a disease outbreak in a catfish production pond. Four genes encoding structural proteins in the type III secretion system (T3SS) apparatus of E. piscicida were deleted by homologous recombination and allelic exchange to produce in-frame deletion mutants (EpΔssaV, EpΔesaM, EpΔyscR and EpΔescT). The mutants were phenotypically characterized, and virulence and vaccine efficacy were evaluated. Three of the mutants, EpΔssaV, EpΔyscR and EpΔesaM, were significantly attenuated compared to the parent strain (p < .05), but EpΔescT strain was not. Vaccination of catfish with the four mutant strains (EpΔssaV, EpΔesaM, EpΔyscR and EpΔescT) provided significant protection when subsequently challenged with wild-type strain. In conclusion, we report methods for gene deletion in E. piscicida and development of vaccine candidates derived from a virulent catfish isolate.

Role of the sseK1 gene in the pathogenicity of Salmonella enterica serovar enteritidis in vitro and in vivo

Salmonella enteritidis is a common food-borne pathogen associated with consumption of contaminated poultry meat and eggs, which frequently causes gastroenteritis in humans. Salmonella secreted effector K1 (SseK1), as a translocated and secreted protein has been identified to be essential for the virulence of Salmonella typhimurium in host cells. However, the role of the sseK1 gene in the pathogenicity of S. enteritidis remain unclear. In this study, a sseK1 deletion mutant of S. enteritidis was constructed and its biological characteristics were examined. It was found that the sseK1 deletion mutant did not affect the growth, adherence and invasion of Salmonella enteritidis when compared to the wild-type S. enteritidis. However, the mutant showed decreased formation of biofilm and significantly reduced intracellular survival of bacteria in activated mouse peritoneal macrophages, as well as showed reduced pathogenicity to a murine model by increasing the lethal dose 50% (LD₅₀) value and decreasing the proliferation ratio of bacteria in vivo. Taken together, this study determined an important role for SseK1 in the pathogenicity of S. enteritidis in vitro and in vivo.

Time-Resolved Transposon Insertion Sequencing Reveals Genome-Wide Fitness Dynamics during Infection

Transposon insertion sequencing (TIS) is a powerful high-throughput genetic technique that is transforming functional genomics in prokaryotes, because it enables genome-wide mapping of the determinants of fitness. However, current approaches for analyzing TIS data assume that selective pressures are constant over time and thus do not yield information regarding changes in the genetic requirements for growth in dynamic environments (e.g., during infection). Here, we describe structured analysis of TIS data collected as a time series, termed pattern analysis of conditional essentiality (PACE). From a temporal series of TIS data, PACE derives a quantitative assessment of each mutant’s fitness over the course of an experiment and identifies mutants with related fitness profiles. In so doing, PACE circumvents major limitations of existing methodologies, specifically the need for artificial effect size thresholds and enumeration of bacterial population expansion. We used PACE to analyze TIS samples of Edwardsiella piscicida (a fish pathogen) collected over a 2-week infection period from a natural host (the flatfish turbot). PACE uncovered more genes that affect E. piscicida’s fitness in vivo than were detected using a cutoff at a terminal sampling point, and it identified subpopulations of mutants with distinct fitness profiles, one of which informed the design of new live vaccine candidates. Overall, PACE enables efficient mining of time series TIS data and enhances the power and sensitivity of TIS-based analyses.

Transposon insertion sequencing (TIS) enables genome-wide mapping of the genetic determinants of fitness, typically based on observations at a single sampling point. Here, we move beyond analysis of endpoint TIS data to create a framework for analysis of time series TIS data, termed pattern analysis of conditional essentiality (PACE). We applied PACE to identify genes that contribute to colonization of a natural host by the fish pathogen Edwardsiella piscicida. PACE uncovered more genes that affect E. piscicida’s fitness in vivo than were detected using a terminal sampling point, and its clustering of mutants with related fitness profiles informed design of new live vaccine candidates. PACE yields insights into patterns of fitness dynamics and circumvents major limitations of existing methodologies. Finally, the PACE method should be applicable to additional “omic” time series data, including screens based on clustered regularly interspaced short palindromic repeats with Cas9 (CRISPR/Cas9).

Transcriptomic dissection of the horizontally acquired response regulator EsrB reveals its global regulatory roles in the physiological adaptation and activation of T3SS and the cognate effector repertoire in Edwardsiella piscicida during infection toward turbot

Edwardsiella piscicida is the leading pathogen threatening worldwide aquaculture industries. The 2-component system (TCS) EsrA-EsrB is essential for the pathogenesis of this bacterium. However, little is known about the regulon and regulatory mechanism of EsrA-EsrB or about the factors that mediate the interaction of TCS with bacterial hosts. Here, our RNA-seq analysis indicated that EsrB strongly induces type III and type VI secretion systems (T3/T6SS) expression and that it modulates the expression of both physiology- and virulence-associated genes in E. piscicida grown in DMEM. EsrB binds directly to a highly conserved 18-bp DNA motif to regulate the expression of T3SS and other genes. EsrB/DMEM-activated genes include 3 known and 6 novel T3SS-dependent effectors. All these effector genes are highly induced by EsrB during the late stage of in vivo infection in fish. Furthermore, although in vivo colonization by the bacterium relies on EsrB and T3/T6SS expression, it does not require the expression of individual effectors other than EseJ. The mutant lacking these 9 effectors showed significant defects in in vivo colonization and virulence toward turbot, and, more importantly, a high level of protection against challenges by wild-type E. piscicida, suggesting that it may represent a promising live attenuated vaccine. Taken together, our data demonstrate that EsrB plays a global regulatory role in controlling physiologic responses and the expression of T3SS and its cognate effector genes. Our findings will facilitate further work on the mechanism of molecular pathogenesis of this bacterium during infection.

The influence of concentration of inactivated Edwardsiella tarda bacterin and immersion time on antigen uptake and expression of immune-related genes in Japanese flounder (Paralichthys olivaceus)

Our previous work has demonstrated that the immune response of Japanese flounder was associated with the concentration of formalin-inactivated Edwardsiella tarda and immersion time. In order to further investigate the influence of immersion vaccine dose and bath time on the antigen uptake, formalin-killed Edwardsiella tarda bacterin was prepared and adjusted to four concentrations (10⁹, 10⁸, 10⁷, 10⁶ cfu ml^-1) for 30, 60 and 90 min immersion in Japanese flounder model, respectively. Absolute quantitative real-time PCR was employed to examine the bacterin uptake in gill, skin, spleen and kidney at 3 and 6 h post vaccination. The results showed that the antigen uptaken in gills and skin were significant higher than spleen and kidney, and the antigen amounts in gill and skin both declined from 3 to 6 h, whereas the antigen amounts in spleen and kidney gradually increased. Significant higher antigen amounts were detected in 10⁹-30, 10⁹-60, 10⁸-60, 10⁸-90 and 10⁸-90 groups than other groups (P < 0.05), especially the 10⁸-60min group displayed the highest antigen uptaken. Meanwhile, the expression profiles of antigen recognization and presentation genes (MHCⅡα, TcRα, CD4-1), immunoglobulins (IgM, IgT), inflammatory cytokines (IL-1β, IL-6), heat shock protein 70 (HSP70) and c-type lysozyme were analyzed using real-time PCR. On the whole, the transcription levels of the eight genes exhibited to be higher in 10⁷-90, 10⁸ and 10⁹ cfu ml^-1 groups than other groups (P < 0.05), especially the 10⁸-60 group displayed the highest up-regulation. These results demonstrated that immersion with formalin-inactivated E. tarda, especially under 10⁸-60 min condition could efficiently enhance the antigen uptake and the expression of immune-related genes, which provided evidences for an enhanced vaccination effects under an optimized combination of vaccine dose and immersion time.

Comparative proteomic analysis unravels a role for EsrB in the regulation of reactive oxygen species stress responses in Edwardsiella piscicida

As a leading pathogen, Edwardsiella piscicida can cause hemorrhagic septicemia in fish and gastro-intestinal infections in humans. The two-component regulatory system EsrA-EsrB plays essential roles in pathogenesis through the type III and type VI secretion systems, and hemolysin production in E. piscicida It is unclear whether other virulence- or stress response-associated genes are regulated by EsrA-EsrB. In this study, the proteomes of wild-type E. piscicida EIB202 and esrB mutant strains were compared to reveal EsrB regulon components after growth in Luria-Bertani broth (LB). Overall, the expression levels of nine genes exhibited significant changes, and five of them required the presence of EsrB, while others exhibited higher levels in the esrB mutant. The diverse functions of these proteins were identified, including amino acid metabolism, oxidative stress defense and energy production. Interestingly, superoxidase dismutase and thiol peroxidase were the most significantly down-regulated by EsrB. Furthermore, other reported reactive oxygen species (ROS) resistance-related genes were also down-regulated by EsrB as revealed by quantitative real-time. Compared with the wild-type and the complement strain esrB⁺, ΔesrB displayed a significantly enhanced ROS resistance. These results demonstrated that EsrB plays important roles in the ROS resistance pathway in E. piscicida grown in LB conditions.

Comparative analysis of sigma factors RpoS, FliA, and RpoN in Edwardsiella tarda

Sigma factors are important regulators that bacteria employ to cope with environmental changes. Studies on the functions of sigma factors have uncovered their roles in many important cellular activities, such as growth, stress tolerance, motility, biofilm formation, and virulence. However, comparative analyses of sigma factors that examine their common and unique features or elucidate their cross-regulatory relationships have rarely been conducted for Edwardsiella tarda. Here, we characterized and compared motility and resistance to oxidative stress of E. tarda strains complemented with rpoS, fliA, and rpoN mutants. The results suggest that the sigma factors FliA and RpoN regulated motility, whereas RpoS exhibited no such function. RpoS and RpoN were essential for oxidative stress resistance, whereas FliA had no obvious impact under oxidative stress conditions. Furthermore, 2-dimensional gel electrophoresis based proteomics analysis combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry revealed 12 differentially expressed protein spots that represented 11 proteins between the mutant and wild-type strains. Quantification of the expression of target genes by quantitative reverse transcription PCR confirmed the results of our proteomics analysis. Collectively, these results suggest that these sigma factors are multifunctional mediators involved in controlling the expression of many metabolic pathway genes.

Deletion of Invasion Protein B in Salmonella enterica Serovar Typhimurium Influences Bacterial Invasion and Virulence

Salmonella enterica serovar Typhimurium (S. Typhimurium) has a wide host range and causes infections ranging from severe gastroenteritis to systemic infections in human, as well as causing typhoid-like disease in murine models of infection. S. Typhimurium translocates its effector proteins through the Salmonella pathogenicity island-I (SPI-I)-encoded T3SS-I needle complex. This study focuses on invasion protein B (SipB) of S. Typhimurium, which plays an active role in SPI-I invasion efficiency. To test our hypothesis, a sipB deletion mutant was constructed through double-crossover allelic using the suicide vector pRE112ΔsipB, and its biological characteristics were analyzed. The results showed that the SipB does not affect the growth of Salmonella, but the adherence, invasion, and virulence of the mutant were significantly decreased compared with wild-type S. Typhimurium (SL1344). This research indicates that SipB is an important virulence factor in the pathogenicity of S. Typhimurium.

Generation of killed but metabolically active (KBMA) Edwardsiella tarda and evaluation of its potential as a protective vaccine

A technology for inactivation of pathogens in human blood products by treatment with amotosalen hydrochloride (S-59) in combination with long wavelength ultraviolet light (UVA) to decrease transfusion-mediated sepsis has been applied to make safe vaccines against human pathogenic bacteria, and the resultants were called killed but metabolically active (KBMA) bacteria. In the present study, we first generated KBMA Edwardsiella tarda and evaluated its potential as a protective vaccine in olive flounder (Paralichthys olivaceus). To prevent the restoration of division ability by removal of psoralen adducts in the bacterial chromosome through the nucleotide excision repair (NER), the uvrA and uvrB genes knock-out E. tarda (ΔuvrAB E. tarda) was produced by the allelic exchange method. The optimal condition for generation of KBMA E. tarda was exposure of the ΔuvrAB E. tarda to 100 ng/ml of S-59 and 2.8 J/cm(2) of UVA irradiation. The KBMA E. tarda could not replicate but showed a high metabolic activity (measured by lactate dehydrogenase activity) that was comparable to the wild-type E. tarda. In comparison of survival rates between groups vaccinated with the same dose of bacteria, fish immunized with KBMA E. tarda showed significantly higher survival rates than fish immunized with formalin-killed cell (FKC) E. tarda. Furthermore, fish immunized with 1 × 10(7) CFU/fish of KBMA E. tarda showed no mortality, while PBS-injected fish showed 100% mortality. The serum agglutination titer was sharply increased by 10(7) CFU/fish of KBMA E. tarda compared to those of fish immunized with 10(6) CFU/fish of KBMA E. tarda or 10(7) CFU/fish of FKC E. tarda. The consistently lower serum agglutination titers against KBMA E. tarda than against FKC E. tarda in both KBMA and FKC E. tarda immunized groups suggest that some factors secreted from KBMA E. tarda might inhibit the serum agglutination activity. In conclusion, the present results showed the higher potential of KBMA E. tarda than FKC E. tarda as a prophylactic vaccine.

Generation and evaluation of virulence attenuated mutants of Edwardsiella tarda as vaccine candidates to combat edwardsiellosis in flounder (Paralichthys olivaceus)

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. The development of a live attenuated vaccine may be an effective approach for preventing this disease in fish. In this study, we introduced deletions of esrB, esaC, evpH, rpoS, and purA into the E. tarda LSE40ΔaroA strain, thereby generating five double-gene mutants (ΔaroAΔesrB, ΔaroAΔesaC, ΔaroAΔrpoS, ΔaroAΔevpH, and ΔaroAΔpurA) and two triple-gene mutants (ΔaroAΔesrBΔevpH and ΔaroAΔesaCΔevpH). When blue gourami (Trichogaster trichopterus) was used as a fish model for the primary screening and evaluation of the vaccine candidates, all mutants were attenuated significantly by more than 2 to 3 logs in terms of the 50% lethal dose (LD(50)). Five double-gene mutants yielded relative percentage survival (RPS) rates of 26.1-82.6% after challenge with wild-type E. tarda. The ΔaroAΔesrB mutant that conferred the highest RPS (82.6%) in blue gourami was also evaluated in flounder (Paralichthys olivaceus). After vaccination via intramuscular (i.m.) injection or immersion, this mutant could persist in the flounder for 14-35 days and it induced higher serum antibody titers than the control fish (P < 0.01). Flounder vaccinated via i.m. injection at doses of 10(3)-10(7) CFU/fish had RPS rates of 14.3-66.7% after i.m. challenge with 10(4) CFU/fish using wild-type E. tarda. Flounder vaccinated via immersion at a dose of 10(7) CFU/ml exhibited 100% RPS against immersion challenge with 10(7) CFU/ml using wild-type E. tarda. These results indicate that the ΔaroAΔesrB mutant could be used as an effective live vaccine to combat edwardsiellosis in flounder.

Mutations of flagellar genes fliC12, fliA and flhDC of Edwardsiella tarda attenuated bacterial motility, biofilm formation and virulence to fish

AIMS

The aim of this study was to investigate functions of flagellar genes fliC2, fliC12, fliA and flhDC in a bacterial fish pathogen Edwardsiella tarda.

METHODS AND RESULTS

In this study, functions of flagellar genes, fliC2, fliC12 (fliC1 + fliC2), fliA and flhDC (flhD + flhC) of Edw. tarda H1 were analysed by constructing in-frame deletion mutants respectively and complementary strains fliC2(+) and fliA(+) . Electron microscopy revealed that in-frame deletion of fliC12, fliA and flhDC significantly impaired the number and length of flagellar filaments, resulting in loss of both swimming and swarming motilities of the bacteria. In addition, compared to the wild-type strain and complementary strains, the flagellum-impaired mutants exhibited reduced biofilm formation ability, showed decreased ability in adherence and internalization to Epithelioma papulosum cyprini (EPC) cells and reduced pathogenicity to zebrafish.

CONCLUSIONS

These results indicated that fliC12, fliA and flhDC of Edw. tarda played essential roles in flagellar filaments structure, bacteria motility, biofilm formation, adherence, internalization and pathogenicity of this bacterium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that flagella function in facilitating virulence and it may provide a new target for vaccines against Edw. tarda infection.

Comparative genomics reveals that a fish pathogenic bacterium Edwardsiella tarda has acquired the locus of enterocyte effacement (LEE) through horizontal gene transfer

Background

Edwardsiella tarda is an enterobacterium which causes edwardsiellosis, a fatal disease of cultured fishes such as red sea bream, eel, and flounder. Preventing the occurrence of E. tarda infection has thus been an important issue in aquaculture. E. tarda has been isolated from other animals and from many environments; however, the relationship between the genotype and evolutionary process of this pathogen is not fully understood. To clarify this relationship, we sequenced and compared the genomes of pathogenic and non-pathogenic E. tarda strains isolated from fish, human, and eel pond using next-generation sequencing technology.

Results

Eight strains of E. tarda were sequenced with high accuracy (>99.9%) with coverages from 50- to 400-fold. The obtained reads were mapped to a public reference genome. By comparing single nucleotide and insertion/deletion polymorphisms, we found that an attenuated strain of E. tarda had a loss-of-function mutation in a gene related to the type III secretion system (T3SS), suggesting that this gene is involved in the virulence of E. tarda. A comprehensive gene comparison indicated that fish pathogenic strains possessed a type VI secretion system (T6SS) and pilus assembly genes in addition to the T3SS. Moreover, we found that an E. tarda strain isolated from red sea bream harbored two pathogenicity islands of T3SS and T6SS, which were absent in other strains. In particular, this T3SS was homologous to the locus of enterocyte effacement (LEE) in enteropathogenic and enterohemorrhagic Escherichia coli. Evolutionary analysis suggested that this locus, here named Et-LEE (E. tarda LEE), was introgressed into the E. tarda genome through horizontal transfer.

Conclusions

We found significant differences in the presence/absence of virulence-related genes among E. tarda strains, reflecting their evolutionary relationship. In particular, a single genotype previously proposed for fish-pathogenic strains may be further divided into two subgroups. Furthermore, the current study demonstrated, for the first time, that a fish pathogenic bacterium carried a LEE-like pathogenicity island which was previously reported only in zoonotic pathogenic enterobacteria. These findings will contribute to the exploration of strain-specific drug targets against E. tarda in aquafarms, while also shedding light on the evolution of pathogenesis in enterobacteria.

Edwardsiella tarda mutant disrupted in type III secretion system and chorismic acid synthesis and cured of a plasmid as a live attenuated vaccine in turbot

Edwardsiella tarda is an intractable Gram-negative pathogen in many fish species to cause edwardsiellosis. Its infection leads to extensive losses in a diverse array of commercially important fish. The type III secretion system (T3SS) has been considered as one of the major virulence factors and plays important roles in its intracellular lifestyle. In this study, an E. tarda EIB202 mutant WED with deletions in the T3SS genes for EseB, EseC, EseD and EscA, along with the aroC gene for the biosynthesis of chorismic acid, as well as the curing of endogenous plasmid pEIB202 was constructed by allelic exchange strategy. Compared to the wild-type EIB202 which was highly virulent towards turbot (Scophthamus maximus) via intraperitoneal (i.p.), intramuscular (i.m.) injection or immersion and caused systemic infection in turbot as well as the unexpected red mouth symptom when immersion challenged, WED was highly attenuated when inoculated into turbot via i.m., i.p. and immersion routes, and exhibited significantly impaired capacity to survive in fish tissues. WED showed 5700-fold higher 50% lethal dose (LD50) than that of the wild type when i.m. or i.p. challenged. Inoculation with WED by i.p. or immersion injection routes elicited significant protection against the challenge of the wild-type E. tarda after 5 weeks of vaccination. The vaccinated fish produced low while significant level of specific antibody and showed increased expression of immune-related factors including IL-1β, IFN-γ, MHC II, MHC-I and CD8, indicating that WED possesses significant immunoprotective potential. Furthermore, our data indicated that a single dose of i.p. and immersion vaccination with WED could produce significant protection as long as 12 and 6 months, respectively. These results demonstrated the feasibility of WED as a live attenuated vaccine in turbot against edwardsiellosis by immersion or i.p. injection routes.

Immune effects of a bivalent expressed outer membrane protein to American eels (Anguilla rostrota)

The specific and non-specific immune parameters and protection of American eels (Anguilla rostrata) were evaluated after immunized eels with a bivalent expressed out membrane protein (OMP) of porin Ⅱ of Aeromonas hydrophila and ompS2 of Edwardsiella tarda. One hundred eighty eels were distributed into 3 equal groups and intraperitoneal (i.p) injection with phosphate-buffered saline (PBS group), formalin-killed-whole-cell (FKC) of A. hydrophila and E. tarda (FKC group) or the bivalent OMP (OMP group). The lymphocytes and red blood cells collected on 14, 21 and 42 days post-vaccination were used to evaluate the stimulation index (SI) and the sera collected on 14, 21, 28 and 42 days were used to assize the titers of specific antibody as well as lysozyme activity. Lysozyme activities in skin mucus, suspension of liver and kidney were also recorded on 14, 21 and 28 days. On 28 d post-vaccination, eels from all three groups were challenged by i.p injection of live A. hydrophila or E. tarda. The results show that, compared with the PBS group, proliferation of lymphocytes in OMP group was significantly (P < 0.05) enhanced on 21 days, and the serum titers of anti-A. hydrophila and anti- E. tarda antibody in eels of FKC and OMP group were significant increased (P < 0.05 or P < 0.01) on 14, 21 and 28 days. Activity of the lysozyme in serum, skin mucus, liver and kidney were significant changed (P < 0.05 or P < 0.01) between the three groups. Relative Percent Survival (RPS) after challenged with A. hydrophila on 28 days post immunization in two vaccinated groups vs. PBS group were 50%, and the RPS challenge E. tarda in FKC and OMP vs. PBS group were 50% and 37.5% respectively. These results suggest that American eels immunized with the bivalent OMP would positively affect specific as well as non-specific immune parameters and protect against infection by the two pathogens in freshwater farming.

Tricarboxylic acid cycle and one-carbon metabolism pathways are important in Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of channel catfish (ESC). The disease causes considerable economic losses in the commercial catfish industry in the United States. Although antibiotics are used as feed additive, vaccination is a better alternative for prevention of the disease. Here we report the development and characterization of novel live attenuated E. ictaluri mutants. To accomplish this, several tricarboxylic acid cycle (sdhC, mdh, and frdA) and one-carbon metabolism genes (gcvP and glyA) were deleted in wild type E. ictaluri strain 93-146 by allelic exchange. Following bioluminescence tagging of the E. ictaluri ΔsdhC, Δmdh, ΔfrdA, ΔgcvP, and ΔglyA mutants, their dissemination, attenuation, and vaccine efficacy were determined in catfish fingerlings by in vivo imaging technology. Immunogenicity of each mutant was also determined in catfish fingerlings. Results indicated that all of the E. ictaluri mutants were attenuated significantly in catfish compared to the parent strain as evidenced by 2,265-fold average reduction in bioluminescence signal from all the mutants at 144 h post-infection. Catfish immunized with the E. ictaluri ΔsdhC, Δmdh, ΔfrdA, and ΔglyA mutants had 100% relative percent survival (RPS), while E. ictaluri ΔgcvP vaccinated catfish had 31.23% RPS after re-challenge with the wild type E. ictaluri.

Gene expression profiling in live attenuated Edwardsiella tarda vaccine immunized and challenged zebrafish: insights into the basic mechanisms of protection seen in immunized fish

Despite the importance and success of vaccine immunization against bacterial diseases in fish, little is known about the molecular mechanisms of vaccine-induced immune protection in teleost fish. In this study, the live attenuated Edwardsiella tarda vaccine strain WED, which has been shown to evoke efficacious protection against edwardsiellosis and ascites diseases in fish, was extensively evaluated for multiple parameters in a 5-week immunization and challenge experiment in zebrafish. The parameters evaluated included the immunologic potency (relative percent survival, RPS), the specific IgM antibody titers and the expression profiles of multiple immune-related gene markers at multiple time points following immunization and challenge. During the 4-week immunization phase, the toll-like receptor (TLR) 5 signaling pathway, the MHC-I antigen processing pathway and cytotoxic T lymphocyte (CTL) responses were activated in succession. In contrast, the MHC-II antigen processing pathway and the markers of CD4(+) T lymphocyte activation were down-regulated, and IgM transcription and specific IgM antibody titers were not significantly induced following immunization. During the 1-week challenge phase, the induction of MHC-I and CTL responses and the inhibition of MHC-II and CD4(+) T cell responses were similarly observed in immunized zebrafish following challenge with wild E. tarda. With the 5-week immunization and challenge model, our data suggest the basic mechanism that underlying the long-lasting protective immunity elicited by WED in zebrafish. This mechanism involved the induction of the TLR-5 signaling pathway, the MHC-I antigen processing pathway and CTL effector function, and CTL function seems play a major role in the protection against E. tarda infection in zebrafish.

Asd-based balanced-lethal system in attenuated Edwardsiella tarda to express a heterologous antigen for a multivalent bacterial vaccine

Edwardsiella tarda is an enteric Gram-negative invasive intracellular pathogen, which causes enteric septicemia in fish. It could be potentially used to develop a recombinant attenuated E. tarda vaccine for the aquaculture industry. Because live vaccine strains can potentially be released into the environment upon vaccination, medical and environmental safety issues must be considered. Deletion of the asdB gene in E. tarda resulted in a diaminopimelic acid (DAP)-dependent mutant. The wild type asdB gene was inserted in place of the antibiotic-resistance gene in the plasmid, and the resultant non-antibiotic resistant vector was transformed into the attenuated and DAP-dependent E. tarda vaccine strain (WEDΔasdB) to obtain a balanced-lethal system for heterologous antigen expression. The balanced-lethal expression system was further optimized by comparing plasmid replicons with different Shine-Dalgarno sequences and start codons for the asdB gene. Utilizing the optimized balanced-lethal expression system, the protective antigen gene gapA34 from the fish pathogen Aeromonas hydrophila LSA34 was expressed in the attenuated E. tarda to generate the multivalent vaccine candidate WEDΔasdB/pUTta4DGap. This vaccine was shown to evoke an effective immune response against both E. tarda and A. hydrophila LSA34 by vaccinating turbot via a simple immersion route. This multivalent E. tarda vector vaccine has great potential for broad applications in aquaculture.

Identification of antigenic Edwardsiella tarda surface proteins and their role in pathogenesis

Edwardsiella tarda causes an infectious fish disease called edwardsiellosis. Several outer membrane proteins (OMPs) are associated with virulence factors and are attractive as vaccine candidates. In this study, 4 immuno-reactive OMPs of E. tarda were detected using anti-sera from flounder infected with E. tarda. Using matrix-assisted laser desorption/ionization mass spectrometry analyses, 2 of the 4 OMPs were identified as OmpA and murein lipoprotein (Lpp), which are highly conserved surface proteins in gram-negative bacteria. For further characterization of these surface proteins, we generated ompA- and lpp-inactivated mutants by insertion of a kanamycin cassette in the corresponding genes, and named these mutants E. tarda CK99 and CK164, respectively. As expected, immuno-reactive OmpA and Lpp proteins were absent in E. tarda CK99 and CK164, respectively, confirming that OmpA and Lpp are antigenic surface proteins. Interestingly, the LD(50) value of E. tarda CK164 in fish (2.0 × 10(8) colony-forming unit [CFU]/fish) was greater than that of the parental strain (3.0 × 10(7) CFU/fish). The LD(50) of E. tarda CK99 did not differ from that of its parental strain. After administering attenuated E. tarda CK164 to fish, we monitored the E. tarda-specific immune response profile. We observed that the E. tarda-specific serum IgM titer increased in a time-dependent manner, and was much higher than the value observed after the administration of a heat-killed E. tarda control. Moreover, fish vaccinated with E. tarda CK164 were 100% protected when challenged by CK41, a pathogenic strain. Our results suggest that E. tarda CK164 can potentially be used for developing an effective live attenuated vaccine for edwardsiellosis that can be applied in the aquaculture industry.

Pathogenesis of and strategies for preventing Edwardsiella tarda infection in fish

Edwardsiella tarda is one of the serious fish pathogens, infecting both cultured and wild fish species. Research on edwardsiellosis has revealed that E. tarda has a broad host range and geographic distribution, and contains important virulence factors that enhance bacterial survival and pathogenesis in hosts. Although recent progress in edwardsiellosis research has enabled the development of numerous, highly effective vaccine candidates, these efforts have not been translated into a commercialized vaccine. The present review aims to provide an overview of the identification, pathology, diagnosis and virulence factors of E. tarda in fish, and describe recent strategies for developing vaccines against edwardsiellosis. The hope is that this presentation will be useful not only from the standpoint of understanding the pathogenesis of E. tarda, but also from the perspective of facilitating the development of effective vaccines.

An improved method for detection of Edwardsiella tarda by loop-mediated isothermal amplification by targeting the EsrB gene

Edwardsiella tarda is a major pathogen in aquatic environments that can cause heavy economic losses. An improved method for quick and accurate detection of E. tarda by loop-mediated isothermal amplification (LAMP) with two additional loop primers was developed by targeting the EsrB gene (EsrB - LAMP). In this method, the Mg2+ concentration, reaction temperature, and reaction time were optimized to 8 mmol/L, 61°C, and 40 min, respectively. The detection limit with the EsrB gene was as low as 10 copies, which is 100 times more sensitive than that of conventional polymerase chain reaction (PCR). The EsrB-LAMP assay was shown more sensitive and rapid than previously reported LAMP assays targeting the hemolysin gene ( hemolysin -LAMP) for detection of E. tarda. The EsrB -LAMP was also highly specific to E. tarda and had no cross-reaction with 13 other strains of bacteria. The assay can be carried out in a simple heating device and the EsrB-LAMP products can be visually detected by adding fluorescent dye to the reaction mixture. Taken together, the improved EsrB-LAMP diagnostic protocol has the potential for detection of E. tarda from indoor and outdoor samples.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

Edwardsiella comparative phylogenomics reveal the new intra/inter-species taxonomic relationships, virulence evolution and niche adaptation mechanisms

Edwardsiella bacteria are leading fish pathogens causing huge losses to aquaculture industries worldwide. E. tarda is a broad-host range pathogen that infects more than 20 species of fish and other animals including humans while E. ictaluri is host-adapted to channel catfish causing enteric septicemia of catfish (ESC). Thus, these two species consist of a useful comparative system for studying the intricacies of pathogen evolution. Here we present for the first time the phylogenomic comparisons of 8 genomes of E. tarda and E. ictaluri isolates. Genome-based phylogenetic analysis revealed that E. tarda could be separate into two kinds of genotypes (genotype I, EdwGI and genotype II, EdwGII) based on the sequence similarity. E. tarda strains of EdwGI were clustered together with the E. ictaluri lineage and showed low sequence conservation to E. tarda strains of EdwGII. Multilocus sequence analysis (MLSA) of 48 distinct Edwardsiella strains also supports the new taxonomic relationship of the lineages. We identified the type III and VI secretion systems (T3SS and T6SS) as well as iron scavenging related genes that fulfilled the criteria of a key evolutionary factor likely facilitating the virulence evolution and adaptation to a broad range of hosts in EdwGI E. tarda. The surface structure-related genes may underlie the adaptive evolution of E. ictaluri in the host specification processes. Virulence and competition assays of the null mutants of the representative genes experimentally confirmed their contributive roles in the evolution/niche adaptive processes. We also reconstructed the hypothetical evolutionary pathway to highlight the virulence evolution and niche adaptation mechanisms of Edwardsiella. This study may facilitate the development of diagnostics, vaccines, and therapeutics for this under-studied pathogen.

Edwardsiella tarda flagellar protein FlgD: a protective immunogen against edwardsiellosis

Edwardsiella tarda is a gram-negative bacterium and a causative agent of edwardsiellosis, resulting to severe loss of the aquaculture industry. In this study, based on the reverse vaccinology, sixteen flagellar proteins were selected from highly pathogenic E. tarda EIB202 genome information and in silico analyzed as potential vaccine candidates. Among them, ten recombinant proteins were highly expressed in Escherichia coli and successfully purified. The immunoprotective potentials of these purified recombinant proteins were evaluated in zebrafish model. And recombinant FlgD and FliD were found to lead to a high relative percent survival (RPS, about 70%) against E. tarda EIB202. Furthermore, FlgD required in flagellum hook assembly brought about the similar immune protection in turbot. The immune responses of zebrafish and turbot to recombinant FlgD were also investigated, and the results indicated that its high protection was mainly involved in cellular mediated immune response, corresponding to the intracellular pathogenicity of E. tarda.

Overexpression of mltA in Edwardsiella tarda reduces resistance to antibiotics and enhances lethality in zebra fish

AIMS

The aim of this study was to investigate the role of membrane-bound lytic murein transglycosylase A (MltA) in a bacterial fish pathogen Edwardsiella tarda.

METHODS AND RESULTS

An mltA in-frame deletion mutant (ΔmltA) and an mltA overexpression strain (mltA(+)) of Edw. tarda were constructed through double-crossover allelic exchange and by transformation of a low-copy plasmid carrying the intact mltA into the ΔmltA mutant, respectively. Either inactivation or overexpression of MltA in Edw. tarda resulted in elevated sensitivity to β-lactam antibiotics and lower viability in oligotrophic or high osmotic environment than wild-type strain. Autolysis induced by EDTA was reduced in ΔmltA strain, while mltA(+) strain was virtually flimsy, indicating that MltA is responsible for the lysis effect. Moreover, mltA(+) strain exhibited significant increases in lipopolysaccharide (LPS) biosynthesis and virulence to zebra fish compared with wild-type strain.

CONCLUSIONS

The results indicated that MltA plays essential roles in β-lactam antibiotics and environmental stresses resistance, autolysis, LPS biosynthesis and pathogenicity of Edw. tarda. This is the first report that MltA has a virulence-related function in Edw. tarda.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided useful information for further studies on pathogenesis of Edw. tarda.