Infections associated with the genus Edwardsiella: the role of Edwardsiella tarda in human disease

DNA vaccines: roles against diseases

Vaccination is the most successful application of immunological principles to human health. Vaccine efficacy needs to be reviewed from time to time and its safety is an overriding consideration. DNA vaccines offer simple yet effective means of inducing broad-based immunity. These vaccines work by allowing the expression of the microbial antigen inside host cells that take up the plasmid. These vaccines function by generating the desired antigen inside the cells, with the advantage that this may facilitate presentation through the major histocompatibility complex. This review article is based on a literature survey and it describes the working and designing strategies of DNA vaccines. Advantages and disadvantages for this type of vaccines have also been explained, together with applications of DNA vaccines. DNA vaccines against cancer, tuberculosis, Edwardsiella tarda, HIV, anthrax, influenza, malaria, dengue, typhoid and other diseases were explored.

Edwardsiella tarda--a case report

Edwardsiella Tarda, a bacterium associated with fresh water ecosystems, can cause life-threatening illnesses in susceptible hosts. To date, very few cases of neonatal Edwardsiella Tarda sepsis have been reported in the literature, none from India. The author reports a 4-d-old preterm with E. tarda septicemia.

Upper extremity myonecrosis caused by Edwardsiella tarda resulting in transhumeral amputation: case report

Necrotizing soft tissue infections are rapidly progressive infections with a high rate of mortality. One type of necrotizing soft tissue infection is caused by marine gram-negative bacteria and commonly occurs in immunocompromised hosts. These types of infections are more common in patients with chronic liver disease, possibly because of impaired iron metabolism. We present the case of a rapidly progressive necrotizing soft tissue infection caused by Edwardsiella tarda, a marine gram-negative pathogen common in catfish. Few extraintestinal infections of E tarda have been described previously. Our patient had hepatitis C and was exposed to the bacteria by a puncture injury from a wild catfish. His infection required multiple debridements and ultimately required a transhumeral amputation for local control of the infection.

Structure of the O-polysaccharide of Edwardsiella tarda PCM 1150 containing an amide of D-glucuronic acid with L-alanine

Mild acid degradation of the lipopolysaccharide of Edwardsiella tarda PCM 1150 afforded an O-polysaccharide, which was isolated by GPC on Sephadex G-50 and studied by sugar and methylation analyses along with 1D and 2D 1H and 13C NMR spectroscopies, including experiments performed in a 9:1 H2O/D2O mixture to detect NH protons and their correlations with CH protons. The O-polysaccharide was found to contain an amide of d-glucuronic acid with l-alanine (d-GlcA6Ala) and the following structure of the branched hexasaccharide repeating unit was established: -->4)-β-D-GlepA6Ala-(1-->4)-α-L-Fucp-(1-->4)-α-D-Glcp-(1-->4)-α-D-Quip-(1-->3)-β-D-GlcpNAc-(1-->3<--1α-D-GalpNAc.

Edwardsiella tarda sialidase: pathogenicity involvement and vaccine potential

Bacterial sialidases are a group of glycohydrolases that are known to play an important role in invasion of host cells and tissues. In this study, we examined in a model of Japanese flounder (Paralichthys olivaceus) the potential function of NanA, a sialidase from the fish pathogen Edwardsiella tarda. NanA is composed of 670 residues and shares low sequence identities with known bacterial sialidases. In silico analysis indicated that NanA possesses a sialidase domain and an autotransporter domain, the former containing five Asp-boxes, a RIP motif, and the conserved catalytic site of bacterial sialidases. Purified recombinant NanA (rNanA) corresponding to the sialidase domain exhibited glycohydrolase activity against sialic acid substrate in a manner that is pH and temperature dependent. Immunofluorescence microscopy showed binding of anti-rNanA antibodies to E. tarda, suggesting that NanA was localized on cell surface. Mutation of nanA caused drastic attenuation in the ability of E. tarda to disseminate into and colonize fish tissues and to induce mortality in infected fish. Likewise, cellular study showed that the nanA mutant was significantly impaired in the infectivity against cultured flounder cells. Immunoprotective analysis showed that rNanA in the form of a subunit vaccine conferred effective protection upon flounder against lethal E. tarda challenge. rNanA vaccination induced the production of specific serum antibodies, which enhanced complement-mediated bactericidal activity and reduced infection of E. tarda into flounder cells. Together these results indicate that NanA plays an important role in the pathogenesis of E. tarda and may be exploited for the control of E. tarda infection in aquaculture.

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.

Development of a multiplex polymerase chain reaction to detect five common Gram-negative bacteria of aquatic animals

A multiplex polymerase chain reaction (m-PCR) technique was developed as a rapid and accurate diagnostic tool for identifying five major Gram-negative bacilli -Vibrio vulnificus, V. parahaemolyticus, Aeromonas hydrophila, Chryseobacterium meningosepticum and Edwardsiella tarda- that cause major diseases in cultured aquatic animals in Taiwan. The expected amplicons for V. vulnificus, V. parahaemolyticus, A. hydrophila, C. meningosepticum and E. tarda were 410, 368, 685, 180 and 230bp, respectively. The assay was shown to be specific for the target pathogens. The sensitivities of detection were estimated to be 20.5fg∼200pg of genomic DNA or 10(2) ∼10(4) colony-forming units (cfu) of bacterial isolates when adopted as PCR templates. The m-PCR was capable of simultaneously amplifying target fragments from bacterial genome DNA mixed with the DNA extracted from viscera and tissues taken from fish without affecting the performance of the method.

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 ictaluri as the causative agent of mortality in cultured Nile tilapia

Edwardsiella ictaluri was consistently isolated from the spleens, livers, and head kidneys of diseased Nile tilapia Oreochromis niloticus from a farm experiencing mortality events in several culture ponds. We describe the first published outbreak of E. ictaluri-induced edwardsiellosis in Nile tilapia. Pure cultures of the isolated bacteria were characterized both biochemically and molecularly. Biochemical analysis was performed using the API-20E and RapID One systems, and antimicrobial susceptibility was determined by the broth microdilution method. Molecular analysis involved sequencing of the 16S rRNA gene, species-specific real-time polymerase chain reaction (PCR), and PCR-mediated genomic fingerprinting (rep-PCR). Pairwise sequence analysis of the 16S rRNA gene identified the case isolates to be a 100% match to E. ictaluri cultured from channel catfish in the southeastern United States. However, rep-PCR analysis identified the case isolates to be genetically different from representative strains isolated from disease outbreaks in cultured channel catfish in Mississippi. Infectivity challenges (intraperitoneal injection and immersion) demonstrated that a representative E. ictaluri strain isolated from tilapia was pathogenic to naive tilapia, reproducing clinical signs and mortality, thereby establishing Koch's postulates.

Edwardsiella tarda Eta1, an in vivo-induced antigen that is involved in host infection

Edwardsiella tarda, a Gram-negative bacterium, is a severe fish pathogen that can also infect humans. In this study, we identified, via in vivo-induced antigen technology, an E. tarda antigen, Eta1, and analyzed its function in a Japanese flounder (Paralichthys olivaceus) model. Eta1 is composed of 226 residues and shares homology with putative bacterial adhesins. Quantitative real-time reverse transcriptase (RT)-PCR analysis indicated that when cultured in vitro, eta1 expression was growth phase dependent and reached maximum at mid-logarithmic phase. During infection of flounder lymphocytes, eta1 expression was drastically increased at the early stage of infection. Compared to the wild type, the eta1-defective mutant, TXeta1, was unaffected in growth but exhibited attenuated overall virulence, reduced tissue dissemination and colonization capacity, and impaired ability to invade flounder lymphocytes and to block the immune response of host cells. The lost virulence of TXeta1 was restored when a functional eta1 gene was reintroduced into the strain. Western blot and immunodetection analyses showed that Eta1 is localized to the outer membrane and exposed on the surface of E. tarda and that recombinant Eta1 (rEta1) was able to interact with flounder lymphocytes. Consistent with these observations, antibody blocking of Eta1 inhibited E. tarda infection at the cellular level. Furthermore, when used as a subunit vaccine, rEta1 induced strong protective immunity in flounder against lethal E. tarda challenge. Taken together, these results indicate that Eta1 is an in vivo-induced antigen that mediates pathogen-host interaction and, as a result, is required for optimal bacterial infection.

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.

Structures of a unique O-polysaccharide of Edwardsiella tarda PCM 1153 containing an amide of galacturonic acid with 2-aminopropane-1,3-diol and an abequose-containing O-polysaccharide shared by E. tarda PCM 1145, PCM 1151 and PCM 1158

Lipopolysaccharides of four strains of Edwardsiella tarda were degraded by mild acid hydrolysis, and the released O-polysaccharides were isolated by GPC and studied by sugar and methylation analyses along with (1)H and (13)C NMR spectroscopy, including 2D (1)H, (1)H COSY, TOCSY, ROESY, (1)H, (13)C HMBC, HSQC and HSQC-TOCSY experiments. The O-polysaccharide from E. tarda PCM 1153 was found to contain D-GalA, D-GlcNAc, D-Gal and 2-amino-1,3-propanediol (GroN). In the tetrasaccharide repeating unit, GroN is amide-linked to one of the GalA residues, and Gal is non-stoichiometrically 2- or 3-O-acetylated (~45% at each position): [structure: see text]. Three other E. tarda strains examined (PCM 1145, PCM 1151 and PCM 1158) share the following O-polysaccharide structure: [structure: see text] where Abe indicates 3,6-dideoxy-D-xylo-hexose (abequose). This structure resembles those of Citrobacter freundii O22 (PCM 1555) and Salmonella enterica O4. In accordance with the structural data, SDS-PAGE and immunoblotting of the lipopolysaccharides with anti-C. freundii O22 serum demonstrated that the O-antigens of the three E. tarda strains are serologically identical to each other and to the O-antigens of C. freundii O22 and S. enterica O4.

Classical labeling of bacterial pathogens according to their lifestyle in the host: inconsistencies and alternatives

An ample understanding of the complex interactions between host and pathogen will improve our ability to develop new prophylactic and therapeutic measures against infection. Precise classification of infectious agents in regards to their infective lifestyles in the host and corresponding pathogenic implications are required because clear concepts are essential to plan fruitful research. Classically, pathogenic bacteria are classified as extracellular, facultative intracellular, and obligate intracellular. In my opinion, this classification is inadequate because, as concluded from data here discussed, it is based on inconsistencies and hyper-valorizes the capacity of the infectious agent replicate in vitro in cell-free media. For a microbial pathogen, what matters is whether intra- or extracellularity is in the context of the in vivo life and in association with pathogenicity. When living as a pathogen in association with its host, what is relevant in microbiological terms is not the ability to grow in artificial cell-free bacteriological media or in environmental niches but whether the intracellular infectious agent, besides the phase of intracellular growth which is behind its label, also is able to live extracellularly in the natural settings of the extracellular territories of their hosts. To eliminate the inconsistencies associated with the classical labeling of bacterial pathogens, I propose that bacterial pathogens be labeled exclusive extracellular, dual intracellular/extracellular and exclusive intracellular based on their infective lifestyle in the host, not in the ability to grow in artificial bacteriological media.

Crohn's Disease Exacerbation Induced by Edwardsiella tarda Gastroenteritis

Exacerbations of Crohn's disease are not infrequently associated with bacterial gastroenteritis. The recognition of synchronous infections in such patients is vital for the initiation of appropriate antimicrobial therapy. Furthermore, the detection of active bacterial infections may lead the clinician to delay starting biological therapy. We report here a man presenting with an exacerbation of his Crohn's disease during a trip to Thailand. Stool cultures were positive for the unusual gut pathogen Edwardsiella tarda. The patient's symptoms resolved with concurrent antibiotic and steroid therapy. This finding demonstrates the value of performing stool culture in all patients presenting with exacerbations of inflammatory bowel diseases.

[A case of fasciitis localized in the calf muscles associated with Edwardsiella tarda sepsis]

A 49-year-old man presented with fever and pain, redness, swelling, and difficulty in walking. The serum C-reactive protein (CRP), creatin kinase (CK), and endotoxin levels were elevated. A blood culture revealed Edwardsiella tarda(E. tarda). Computed tomography (CT) showed subfascial and subcutaneous low-density areas in the lower legs, suggesting focal abscesses and edema. The patient was likely to have necrotizing fasciitis or cellulitis. He was successfully treated with several antibiotics and discharged after 43 days. Because E. tarda causes sepsis and fulminating necrotizing fasciitis with a high mortality rate in patients with an underlying illness, it should be considered a potentially important pathogen. The lack of an underlying illness may be a factor for a good outcome in this case.

Two-component PhoB-PhoR regulatory system and ferric uptake regulator sense phosphate and iron to control virulence genes in type III and VI secretion systems of Edwardsiella tarda

Inorganic phosphate (P(i)) and iron are essential nutrients that are depleted by vertebrates as a protective mechanism against bacterial infection. This depletion, however, is sensed by some pathogens as a signal to turn on the expression of virulence genes. Here, we show that the PhoB-PhoR two-component system senses changes in P(i) concentration, whereas the ferric uptake regulator (Fur) senses changes in iron concentration in Edwardsiella tarda PPD130/91 to regulate the expression of type III and VI secretion systems (T3SS and T6SS) through an E. tarda secretion regulator, EsrC. In sensing low P(i) concentration, PhoB-PhoR autoregulates and activates the phosphate-specific transport operon, pstSCAB-phoU, by binding directly to the Pho box in the promoters of phoB and pstS. PhoB also binds with EsrC simultaneously on the promoter of an E. tarda virulence protein, evpA, to regulate directly the transcription of genes from T6SS. In addition, PhoB requires and interacts with PhoU to activate esrC and suppress fur indirectly through unidentified regulators. Fur, on the other hand, senses high iron concentration and binds directly to the Fur box in the promoter of evpP to inhibit EsrC binding to the same region. In addition, Fur suppresses transcription of phoB, pstSCAB-phoU, and esrC indirectly via unidentified regulators, suggesting negative cross-talk with the Pho regulon. Physical interactions exist between Fur and PhoU and between Fur and EsrC. Our findings suggest that T3SS and T6SS may carry out distinct roles in the pathogenicity of E. tarda by responding to different environmental factors.

Vibrio vulnificus Septicemia After Handling Tilapia Species Fish: A Canadian Case Report and Review

BACKGROUND

Vibrio vulnificus can cause a necrotizing soft tissue infection or primary septicemia; these infections are collectively known as vibriosis. This bacterium is commonly found within molluscan shellfish. Primary septicemia is often fatal, principally affecting persons with chronic liver disease.

CASE PRESENTATION

A fatal case of V vulnificus sepsis that developed in a patient with chronic hepatitis B and chronic renal failure is reported. Diagnosis was made by isolation of the pathogen by blood culture. Upon further questioning, the patient's family recounted that the patient had handled and ingested Tilapia species fish in the hours preceding the patient's presentation. Despite treatment with doxycycline and cefotaxime, in conjunction with supportive care in the intensive care unit, the patient died on day 7 from multiple organ dysfunction.

CONCLUSION

The present case highlights the need to consider V vulnificus in the microbiological differential diagnosis when a person presents with sepsis and bullous cutaneous lesions. The importance of educating patients with liver disease (and certain other chronic diseases) about the need to be cautious when handling or consuming seafood is underscored.

A case of liver abscess caused by Edwardsiella tarda

Edwardsiella tarda, a member of the Enterobacteriaceae family, is found in freshwater and marine environments. Extraintestinal infections of Edwardsiella tarda have been rarely reported. We describe a 70-year-old Japanese woman suffering from autoimmune hemolytic anemia, with liver abscess caused by Edwardsiella tarda. She had a history of cholecystectomy for gallbladder stone 10 years prior to this admission. She was successfully treated with percutaneous transhepatic abscess aspiration and meropenem. This is the first report of liver abscess caused by Edwardsiella tarda in Japan.

Evaluation of the selective and differential ET medium for detection of Edwardsiella tarda in aquaculture systems

AIMS

Edwardsiella tarda is an important pathogen in aquaculture where it can cause serious losses. A rapid detection of it is vital to minimize the mortalities caused by this disease, and in this work, the effectiveness of the selective differential Edw. tarda medium (ET) was evaluated for the diagnosis of edwardsiellosis as well as for its possible use in epidemiological studies.

METHODS AND RESULTS

ET medium was evaluated in parallel with the commercial Salmonella-Shigella agar (SS), which is usually employed for the selective isolation of enteric bacilli. Moreover, two general media (TSA-1 and MA) were employed as a control. The results obtained showed that ET is distinctly selective for the isolation of Edw. tarda, allowing its recovery from mixed cultures and natural samples as a unique species. In contrast, although colonies of Edw. tarda could be clearly distinguishable in SS because of the appearance of a characteristic black centre, other enteric and nonenteric bacterial species were also able to grow on this medium.

CONCLUSIONS

We recommend ET agar as an useful medium for the primary isolation of Edw. tarda from aquaculture samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained support ET medium as the most appropriate to develop epidemiological studies of edwardsiellosis in aquaculture and permits an earlier diagnosis of this important disease.

PoCRIP1, Paralichthys olivaceus cysteine-rich intestinal protein 1: molecular characterization, expression analysis upon Edwardsiella tarda challenge and a possible role in the immune regulation

Cysteine-rich intestinal protein (CRIP) is a LIM domain protein containing a zinc-finger motif and plays a role in the regulation of the inflammatory immune response. In the present study, we isolated a CRIP1 cDNA, designated PoCRIP1, from an olive flounder Paralichthys olivaceus intestine cDNA library by EST analysis. The PoCRIP cDNA consists of 421 bp with a polyadenylation signal sequence, AATAAA, and a poly(A) tail; it encodes a polypeptide of 76 amino acids containing a double zinc-finger motif (Cys(2)HisCys and Cys(4) sequences). The deduced amino acid sequence of PoCRIP1 showed 75.3-94.7% homology with CRIP1s of other species, including mammals. The PoCRIP1 transcript was highly expressed in the intestine and pyloric ceca and moderately expressed in the gill, heart, kidney, liver, muscle, spleen, skin, and stomach of normal conditioned flounder. Inducible expression of the PoCRIP1 transcript was observed in flounder challenged with Edwardsiella tarda, an economically important pathogen for aquaculture of flounder. Over-expression of PoCRIP1 augmented p65-driven flounder IL-6 promoter activity in HINAE cells. These results suggest that PoCRIP1 may function in the immune response of the flounder through the regulation of cytokine expression.

Foodborne agents associated with the consumption of aquaculture catfish

In the Food, Conservation, and Energy Act (Farm Bill) of 2008, Congress amended the Federal Meat Inspection Act to provide that catfish be inspected by the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS). As part of the development of its inspection program, the FSIS conducted an assessment of the food safety risk associated with consuming farm-raised catfish. To thoroughly identify hazards for consideration in the risk assessment, the scientific literature was surveyed for all potential agents that have been linked to illness associated with farm-raised catfish consumption. A review of microbial hazards suggested that Salmonella is the foodborne pathogen most likely to be associated with catfish, but the impact of other pathogens remains unclear. This review also summarizes the current data available on chemical residues in catfish, including pesticides and heavy metals, and any regulatory levels that have been established for these compounds. The current usage of veterinary drugs in aquaculture also is outlined, including information on unapproved usage of drugs in catfish.

Detection of type III secretion gene as an indicator for pathogenic Edwardsiella tarda

AIMS

To differentiate pathogenic and nonpathogenic Edwardsiella tarda strains based on the detection of type III secretion system (T3SS) gene using polymerase chain reaction (PCR).

METHODS AND RESULTS

Primers were designed to amplify Edw. tarda T3SS component gene esaV, catalase gene katB, haemolysin gene hlyA and 16S rRNA gene as an internal positive control. Genomic DNAs were extracted using a commercial isolation kit from 36 Edw. tarda strains consisting of 18 pathogenic and 18 nonpathogenic strains, and 50 ng of each DNA was used as the template for PCR amplification. PCR was performed with a thermocycler (TaKaRa TP600) in a 25-μl volume. Products of esaV were detected in all pathogenic strains, but not in nonpathogenic strains; katB was detected in all pathogenic strains and one of nonpathogenic strains; hlyA was not detected in any strains.

CONCLUSIONS

The detection of esaV gene can be used for the assessment of pathogenic Edw. tarda strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

The strategy using T3SS gene as the virulence indicator provides a useful tool for the clinical assessment of pathogenic Edw. tarda strains and prediction of edwardsiellosis risk in fish culture environments.

Neonatal sepsis following maternal amnionitis by Edwardsiella tarda: a case report and a review of the literature

Edwardsiella tarda, a gram-negative bacterium, is a rare pathogen in the neonatal period. We present a term newborn that developed E. tarda septicemia following maternal amnionitis. The severe neurological outcome in this case, as well as in all other reported cases, highlights the need for meticulous neurological evaluation in neonates presenting with E. tarda septicemia even in the absence of bona fide meningitis.

Comparative analysis of the phenotypic characteristics of high- and low-virulent strains of Edwardsiella tarda

Edwardsiella tarda is a causative agent of edwardsiellosis in freshwater and marine fish. Extracellular enzymic, haemolytic, hydrophobic and serum resistance activities, haemagglutination, autoagglutination and siderophores of high- and low- virulent E. tarda strains were examined. The results revealed different haemagglutination, autoagglutination, haemolytic, hydrophobic and serum resistance activities in different strains. Analysis of extracellular proteins (ECPs) and outer membrane proteins (OMPs) demonstrated several major, low molecular weight, virulent-strain-specific proteins, which could be virulence-related. Based on the database search with MALDI-TOF MS data, the closest homologies of the three protein bands Ed1, Ed2 and Ed3 were phosphotransferase enzyme family protein, nitrite reductase [NAD(P)H], large subunit and ATP-dependent Lon protease, respectively. A comparison of pathogenicity of purified lipopolysaccharide (LPS) and lipid A from virulent and avirulent strains demonstrated that LPS was one of the virulence factors of the E. tarda isolates, and lipid A was a biologically active determinant of LPS.

Temperature and Mg2+ sensing by a novel PhoP-PhoQ two-component system for regulation of virulence in Edwardsiella tarda

The PhoP-PhoQ two-component system is commonly used by bacteria to sense environmental factors. Here we show that the PhoP-PhoQ system of Edwardsiella tarda detects changes in environmental temperature and Mg(2+) concentration as well as regulates the type III and VI secretion systems through direct activation of esrB. Protein secretion is activated from 23 to 35 °C or at low Mg(2+) concentrations, but it is suppressed at or below 20 °C, at or above 37 °C, or at high Mg(2+) concentrations. The effects of temperature and Mg(2+) concentration are additive. The PhoQ sensor domain has a low T(m) of 37.9 °C, and it detects temperatures through a conformational change of its secondary structure. Mutation of specific Pro or Thr residues increased the stability of the PhoQ sensor drastically, altering its temperature-sensing ability. The PhoQ sensor detects Mg(2+) concentration through the direct binding of Mg(2+) to a cluster of acidic residues (DDDSAD) and through changes that likely affect its tertiary structure. Here, we describe for the first time the use of PhoP-PhoQ as a temperature sensor for bacterial virulence control.

Surgical wound infection, tuboovarian abscess, and sepsis caused by Edwardsiella tarda: case reports and literature review

Edwardsiella tarda is a freshwater pathogen that may cause mild to invasive infections with high mortality in humans. We describe two patients with serious E. tarda infections. The first patient was a woman with a tuboovarian abscess (TOA) and bilateral salpingitis requiring surgical resection and drainage. Her hospital course was complicated by a postoperative wound infection. TOA fluid as well as surgical wound culture revealed pure growth of E. tarda resistant to several antibiotics. The second patient was a man with a bloodstream E. tarda infection and cholangitis who recently traveled to Ecuador. He presented with hypoxia and further workup revealed choledocholithiasis and common bile duct benign polyps. Both patients made a full recovery.

Structural basis for the secretion of EvpC: a key type VI secretion system protein from Edwardsiella tarda

The recently identified type VI secretion system (T6SS) is implicated in the virulence of many Gram-negative bacteria. Edwardsiella tarda is an important cause of hemorrhagic septicemia in fish and also gastro- and extra-intestinal infections in humans. The E. tardavirulent protein (EVP) gene cluster encodes a conserved T6SS which contains 16 open reading frames. EvpC is one of the three major EVP secreted proteins and shares high sequence similarity with Hcp1, a key T6SS virulence factor from Pseudomonas aeruginosa. EvpC contributes to the virulence of E. tarda by playing an essential role in functional T6SS. Here, we report the crystal structure of EvpC from E. tarda PPD130/91 at a 2.8 Å resolution, along with functional studies of the protein. EvpC has a β-barrel domain with extended loops. The β-barrel consists of 11 anti-parallel β-strands with an α-helix located on one side. In solution, EvpC exists as a dimer at low concentration and as a hexamer at higher concentration. In the crystal, the symmetry related EvpC molecules form hexameric rings which stack together to form a tube similar to Hcp1. Structure based mutagenesis revealed that N-terminal negatively charged residues, Asp4, Glu15 and Glu26, and C-terminal positively charged residues, Lys161, Lys162 and Lys163, played crucial roles in the secretion of EvpC. Moreover, the localization study indicates the presence of wild type EvpC in cytoplasm, periplasm and secreted fractions, whereas the N-terminal and C-terminal mutants were found mostly in the periplasmic region and was completely absent in the secreted fraction. Results reported here provide insight into the structure, assembly and function of EvpC. Further, these findings can be extended to other EvpC homologs for understanding the mechanism of T6SS and targeting T6SS mediated virulence in Gram-negative pathogens.

EseG, an effector of the type III secretion system of Edwardsiella tarda, triggers microtubule destabilization

Edwardsiella tarda is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish and both gastrointestinal and extraintestinal infections in humans. A type III secretion system (T3SS) was recently shown to contribute to pathogenesis, since deletions of various T3SS genes increased the 50% lethal dose (LD(50)) by about 1 log unit in the blue gourami infection model. In this study, we report EseG as the first identified effector protein of T3SS. EseG shares partial homology with two Salmonella T3SS effectors (SseG and SseF) over a conserved domain (amino acid residues 142 to 192). The secretion of EseG is dependent on a functional T3SS and, in particular, requires the chaperone EscB. Experiments using TEM-1 β-lactamase as a fluorescence-based reporter showed that EseG was translocated into HeLa cells at 35°C. Fractionation of infected HeLa cells demonstrated that EseG was localized to the host membrane fraction after translocation. EseG is able to disassemble microtubule structures when overexpressed in mammalian cells. This phenotype may require a conserved motif of EseG (EseG(142-192)), since truncated versions of EseG devoid of this motif lose their ability to cause microtubule destabilization. By demonstrating the function of EseG, our study contributes to the understanding of E. tarda pathogenesis. Moreover, the approach established in this study to identify type III effectors can be used to identify and characterize more type III and possible type VI effectors in Edwardsiella.

Analysis of Edwardsiella tarda DegP, a serine protease and a protective immunogen

Edwardsiella tarda is a severe aquaculture pathogen with a broad host range that includes humans, animal, and fish. A gene (degP(Et)) encoding a DegP homologue was cloned from TX01, a pathogenic E. tarda strain isolated from diseased fish. DegP(Et) shares high sequence identities with the DegP proteins of several bacterial species. Functional analyses showed that degP(Et) could complement the temperature-sensitive phenotype of an Escherichia coli degP null mutant. Expression of degP(Et) in TX01 was modulated by growth phase and temperature, the latter possibly through the action of the sigma(E)-like factor. Overexpression of degP(Et) (i) enhanced the ability of TX01 to disseminate in fish blood at the advanced stage of infection, (ii) heightened the activity of type 2 autoinducer, and (iii) increased the expression of luxS and the genes encoding components of the virulence-associated type III secretion system. Recombinant DegP(Et) purified from E. coli was a serine protease that exhibited maximum activity at 40 degrees C and pH8.0. The proteolytic activity of recombinant DegP(Et) depended on the catalytic triad and the PDZ domains. Immunoprotective analyses showed that purified recombinant DegP(Et) was a protective immunogen that could induce the production of specific serum antibodies and elicit strong protective immunity in fish vaccinated with DegP(Et).

Motility and the expression of the flagellin protein FliC are negatively regulated by quorum sensing in Edwardsiella tarda

Edwardsiella tarda is a gram-negative bacterial pathogen of fish and animals. A number of gram-negative bacteria have quorum-sensing systems and produce N-acylhomoserine lactone (AHL) that they use as a quorum-sensing signaling molecule. We have already reported that E. tarda NUF251 produces AHLs and has the AHL-synthase gene, edwI. Inactivation of NUF251 edwI induces expression of an approximately 45 kDa extracellular protein, identified as a flagellin encoded by FliC. Mutation of edwI also changes the motility pattern of NUF251 from a radial expansion pattern to concentric rings. The addition of exogenous AHL was capable of restoring normal motility to NUF251 edwI mutants. These results demonstrate that quorum sensing negatively regulates motility and expression of the FliC protein.

Edwardsiella tarda T6SS component evpP is regulated by esrB and iron, and plays essential roles in the invasion of fish

Edwardsiella tarda is a gram-negative pathogen for hemorrhagic septicemia in a broad range of hosts. The type VI secretion system (T6SS) has recently been dissected in E. tarda to secrete EvpC, EvpI and a novel effector protein EvpP. In this study, sequencing and genetic alignments showed that evpP genes from different E. tarda isolates were highly similar and an evpP homolog was also found in Aeromonas hydrophila 0865 isolated from a diseased eel, suggesting the possible lateral gene transfer of evpP or the whole T6SS gene island. With reporter strains carrying gfp gene fused to the evpP promoter region, flow cytometric analysis revealed that transcription of evpP was positively regulated by either the two-component system EsrA-EsrB in E. tarda or the iron concentration in media. Compared with the parental strain, in-frame deletion of evpP in E. tarda EIB202 led to the significantly increased 50% lethal doses in zebrafish (Danio rerio) and Japanese flounder (Paralichthys olivaceus), decreased hemolytic activities, failure to adhere to mucus and reduced serum resistance, and complementation of an intact evpP gene restored these phenotypes in the evpP mutant. Investigation of infection kinetics indicated that the evpP deletion mutant was unable to proliferate in vivo, particularly in immune organs of fish. Moreover, the evpP deletion mutant exhibited incapacity to internalize in EPC cell model in vitro, demonstrating that EvpP in T6SS plays critical roles for invasion mechanism of E. tarda and merits as potential target for attenuated live vaccine construction.

Comparative analysis of the production of nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) from macrophages exposed to high virulent and low virulent strains of Edwardsiella tarda

We previously reported that high virulent strain (NUF251) of Edwardsiella tarda has an ability to prevent the production of reactive oxygen species by macrophages, and is even capable of surviving and multiplying within Japanese flounder (Paralichthys olivaceus) peritoneal macrophages, whereas the low virulent strain (NUF194) has no such ability. In this study, we found that NUF251 and NUF194 induced NO and TNF-alpha production from Japanese flounder peritoneal macrophages, and NUF251 caused faster induction of NO release and much higher level of TNF-alpha production than NUF194. In addition, similar differences between two strains in terms of the induction of NO and TNF-alpha production were also observed in mouse macrophage cell line RAW264.7 cells. Our results suggest that the potent ability to induce the production of NO and TNF-alpha from macrophages may be one of the factors responsible for the virulence of E. tarda.

Investigation of EscA as a chaperone for the Edwardsiella tarda type III secretion system putative translocon component EseC

Edwardsiella tarda is an important Gram-negative enteric pathogen affecting both animals and humans. It possesses a type III secretion system (T3SS) essential for pathogenesis. EseB, EseC and EseD have been shown to form a translocon complex after secretion, while EscC functions as a T3SS chaperone for EseB and EseD. In this paper we identify EscA, a protein required for accumulation and proper secretion of another translocon component, EseC. The escA gene is located upstream of eseC and the EscA protein has the characteristics of T3SS chaperones. Cell fractionation experiments indicated that EscA is located in the cytoplasm and on the cytoplasmic membrane. Mutation with in-frame deletion of escA greatly decreased the secretion of EseC, while complementation of escA restored the wild-type secretion phenotype. The stabilization and accumulation of EseC in the cytoplasm were also affected in the absence of EscA. Mutation of escA did not affect the transcription of eseC but reduced the accumulation level of EseC as measured by using an EseC-LacZ fusion protein in Ed. tarda. Co-purification and co-immunoprecipitation studies demonstrated a specific interaction between EscA and EseC. Further analysis showed that residues 31-137 of EseC are required for EseC-EscA interaction. Mutation of EseC residues 31-137 reduced the secretion and accumulation of EseC in Ed. tarda. Finally, infection experiments showed that mutations of EscA and residues 31-137 of EseC increased the LD(50) by approximately 10-fold in blue gourami fish. These results indicated that EscA functions as a specific chaperone for EseC and contributes to the virulence of Ed. tarda.

Emerging infections of the gastrointestinal tract

Infections account for significant GI morbidity and mortality worldwide. New organisms are being identified, associated with diarrhoeal illness and some with other gastrointestinal illness as well. Among GI viruses, Sapovirus is now recognised to cause diarrhoea, especially in children. A hypervirulent strain of Clostridium difficile has caused epidemics in many countries. Newly identified bacterial species that may cause diarrhoea include Campylobacter concisus, Arcobacteria, Edwardsiella tarda, Aeromonas, Plesiomonas and Laribacter. Helicobacteria are reviewed, as well as the role of gastric acid suppression in predisposing to enteric infections.

A 12-year-old boy with acute gastroenteritis caused by Edwardsiella tarda O4:H4

A 12-year-old boy was brought to the hospital with a 3-week history of watery diarrhea mixed with mucus and colicky abdominal pain. Stool culture identified Edwardsiella tarda O4: H4, and no other pathogenic bacteria were detected. Acute gastroenteritis caused by Edwardsiella tarda O4: H4 was diagnosed. This bacterium was shown to be sensitive to ampicillin hydrate. When this antibiotic was administered, the condition of the patient improved within a week. The patient had a history of eating raw shrimp and fish while traveling with his parents.