The Edwardsiella ictaluri O polysaccharide biosynthesis gene cluster and the role of O polysaccharide in resistance to normal catfish serum and catfish neutrophils

Characterization of a novel Yersinia ruckeri serotype O1-specific bacteriophage with virulence-neutralizing activity

A lytic bacteriophage (φNC10) specific to serotype O1 Yersinia ruckeri has been identified and evaluated as a model to assess the potential use of bacteriophages and their products for disease control in aquaculture. Electron microscopy of purified φNC10 revealed a virion particle with a small (70 nm) polyhedral head and short tail. φNC10 infected only serotype O1 strains of Y. ruckeri and failed to bind a defined Y. ruckeri mutant strain lacking O1 lipopolysaccharides (O1-LPS), suggesting that φNC10 uses O1-LPS as its receptor. In addition, spontaneous φNC10-resistant mutants of Y. ruckeri exhibited defects in O1-LPS production and were sensitive to rainbow trout serum. Purified φNC10 displayed a polysaccharide depolymerase activity capable of degrading Y. ruckeri O1-LPS and thereby sensitizing Y. ruckeri to the bactericidal effects of rainbow trout serum. The φNC10-associated polysaccharide depolymerase activity also reduced the ability of Y. ruckeri cells to cause mortality following intraperitoneal injection into rainbow trout. These data demonstrate a potential utility of φNC10 and its associated polysaccharide depolymerase activity for Y. ruckeri disease prevention.

Development and potential use of an Edwardsiella ictaluri wzz mutant as a live attenuated vaccine against enteric septicemia in Pangasius hypophthalmus (Tra catfish)

Edwardsiella ictaluri is a causative agent of enteric septicemia of catfish (ESC), a seriously lethal disease in Vietnamese catfish (Pangasius hypophthalmus). A safe and effective vaccine against ESC is currently an urgent demand due to antibiotic overuse in pangasius farms has led to an alarming antimicrobial resistance. In this study, two E. ictaluri wzzE mutants (WzM-L3, deficient in a 1038bp-entire wzzE gene and WzM-S3, a 245bp-partial deletion of wzzE) were developed and their protection efficiacy was evaluated in hatched pangasius against ESC by immersion vaccination. As comparing to the high virulent wild-type strain who caused 73.33% of death on pangasius fingerlings immersed at 7.1 × 10⁶ CFU ml^-1, both mutants showed extremely low mortality rates at 3.33% (WzM-S3) and 0% (WzM-L3) on pangasius fingerlings immersed at high concentration of 1.5 × 10⁷ CFU mL^-1 and 9.7 × 10⁶ CFU ml^-1, respectively. Interestingly, both WzM-S3 and WzM-L3 had a remarkably high protection against ESC, as RPS % were found at 89.29% and 90%, respectively. The mutant WzM-L3 is a potential live attenuated vaccine against ESC in Vietnamese catfish farms with good protection and simple practice.

Influence of lipopolysaccharide outer-core in the intrinsic resistance to antimicrobial peptides and virulence in Edwardsiella ictaluri

The genus Edwardsiella consists of bacteria with an intrinsic resistance to cyclic cationic antimicrobial peptides (CAMPs). Edwardsiella ictaluri, a pathogen of the catfish (Ictalurus punctatus) and the causative agent of a systemic infection, is highly resistant to CAMPs. Previously, we determined that the oligo-polysaccharide (O-PS) of the lipopolysaccharide (LPS) does not play a role in the E. ictaluri CAMP resistance and an intact core-lipid A structure is necessary for CAMPs resistance. Here, we evaluated the influence of the outer-core in the CAMPs resistance and fish virulence. E. ictaluri wabG, a gene that encodes for the UDP-glucuronic acid transferase that links the lipid A-inner-core to the outer-core-oligopolysaccharides, was deleted. Deletion of ΔwabG caused a pleiotropic effect, influencing LPS synthesis, CAMPs resistance, growth, and biofilm formation. E. ictaluri ΔwabG was attenuated in zebrafish indicating the important role of LPS during fish pathogenesis. Also, we evaluated the inflammatory effects of wabG LPS in catfish ligated loop model, showing a decreased inflammatory effect at the gut level respects to the E. ictaluri wild type. We conclude that E. ictaluri CAMPs resistance is related to the molecules present in the LPS outer-core and that fish gut inflammation triggered by E. ictaluri is LPS dependent, reinforcing the hypothesis that fish gut recognizes LPS in an O-PS dependent fashion.

Identification of Differentially Abundant Proteins of Edwardsiella ictaluri during Iron Restriction

Edwardsiella ictaluri is a Gram-negative facultative anaerobe intracellular bacterium that causes enteric septicemia in channel catfish. Iron is an essential inorganic nutrient of bacteria and is crucial for bacterial invasion. Reduced availability of iron by the host may cause significant stress for bacterial pathogens and is considered a signal that leads to significant alteration in virulence gene expression. However, the precise effect of iron-restriction on E. ictaluri protein abundance is unknown. The purpose of this study was to identify differentially abundant proteins of E. ictaluri during in vitro iron-restricted conditions. We applied two-dimensional difference in gel electrophoresis (2D-DIGE) for determining differentially abundant proteins and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF/TOF MS) for protein identification. Gene ontology and pathway-based functional modeling of differentially abundant proteins was also conducted. A total of 50 unique differentially abundant proteins at a minimum of 2-fold (p ≤ 0.05) difference in abundance due to iron-restriction were detected. The numbers of up- and down-regulated proteins were 37 and 13, respectively. We noted several proteins, including EsrB, LamB, MalM, MalE, FdaA, and TonB-dependent heme/hemoglobin receptor family proteins responded to iron restriction in E. ictaluri.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

Inflammatory effects of Edwardsiella ictaluri lipopolysaccharide modifications in catfish gut

Bacterial lipopolysaccharides (LPS) are structural components of the outer membranes of Gram-negative bacteria and also are potent inducers of inflammation in mammals. Higher vertebrates are extremely sensitive to LPS, but lower vertebrates, like fish, are resistant to their systemic toxic effects. However, the effects of LPS on the fish intestinal mucosa remain unknown. Edwardsiella ictaluri is a primitive member of the Enterobacteriaceae family that causes enteric septicemia in channel catfish (Ictalurus punctatus). E. ictaluri infects and colonizes deep lymphoid tissues upon oral or immersion infection. Both gut and olfactory organs are the primary sites of invasion. At the systemic level, E. ictaluri pathogenesis is relatively well characterized, but our knowledge about E. ictaluri intestinal interaction is limited. Recently, we observed that E. ictaluri oligo-polysaccharide (O-PS) LPS mutants have differential effects on the intestinal epithelia of orally inoculated catfish. Here we evaluate the effects of E. ictaluri O-PS LPS mutants by using a novel catfish intestinal loop model and compare it to the rabbit ileal loop model inoculated with Salmonella enterica serovar Typhimurium LPS. We found evident differences in rabbit ileal loop and catfish ileal loop responses to E. ictaluri and S. Typhimurium LPS. We determined that catfish respond to E. ictaluri LPS but not to S. Typhimurium LPS. We also determined that E. ictaluri inhibits cytokine production and induces disruption of the intestinal fish epithelia in an O-PS-dependent fashion. The E. ictaluri wild type and ΔwibT LPS mutant caused intestinal tissue damage and inhibited proinflammatory cytokine synthesis, in contrast to E. ictaluri Δgne and Δugd LPS mutants. We concluded that the E. ictaluri O-PS subunits play a major role during pathogenesis, since they influence the recognition of the LPS by the intestinal mucosal immune system of the catfish. The LPS structure of E. ictaluri mutants is needed to understand the mechanism of interaction.

Mechanisms of intrinsic resistance to antimicrobial peptides of Edwardsiella ictaluri and its influence on fish gut inflammation and virulence

The genus Edwardsiella comprises a genetically distinct taxon related to other members of the family Enterobacteriaceae. It consists of bacteria differing strongly in their biochemical and physiological features, natural habitats, and pathogenic properties. Intrinsic resistance to cationic antimicrobial peptides (CAMPs) is a specific property of the genus Edwardsiella. In particular, Edwardsiella ictaluri, an important pathogen of the catfish (Ictalurus punctatus) aquaculture and the causative agent of a fatal systemic infection, is highly resistant to CAMPs. E. ictaluri mechanisms of resistance to CAMPs are unknown. We hypothesized that E. ictaluri lipopolysaccharide (LPS) plays a role in both virulence and resistance to CAMPs. The putative genes related to LPS oligo-polysaccharide (O-PS) synthesis were in-frame deleted. Individual deletions of wibT, gne and ugd eliminated synthesis of the O-PS, causing auto-agglutination, rough colonies, biofilm-like formation and motility defects. Deletion of ugd, the gene that encodes the UDP-glucose dehydrogenase enzyme responsible for synthesis of UDP-glucuronic acid, causes sensitivity to CAMPs, indicating that UDP-glucuronic acid and its derivatives are related to CAMP intrinsic resistance. E. ictaluri OP-S mutants showed different levels of attenuation, colonization of lymphoid tissues and immune protection in zebrafish (Danio rerio) and catfish. Orally inoculated catfish with O-PS mutant strains presented different degrees of gut inflammation and colonization of lymphoid tissues. Here we conclude that intrinsic resistance to CAMPs is mediated by Ugd enzyme, which has a pleiotropic effect in E. ictaluri influencing LPS synthesis, motility, agglutination, fish gut inflammation and virulence.

Characterization of the sialic acid synthase from Aliivibrio salmonicida suggests a novel pathway for bacterial synthesis of 7-O-acetylated sialic acids

Resolving the enzymatic pathways leading to sialic acids (Sias) in bacteria are vitally important for understanding their roles in pathogenesis and for subsequent development of tools to combat infections. A detailed characterization of the involved enzymes is also essential due to the highly applicable properties of Sias, i.e., as used in a wide range of medical applications and human nutrition. Bacterial strains that produce Sias display them mainly on their cell surface to mimic animal cells thereby evading the host's immune system. Despite several studies, little is known about the virulence mechanisms of the fish pathogen Aliivibrio salmonicida. The genome of A. salmonicida LFI1238 contains a gene cluster homologous to the Escherichia coli neuraminic acid (Neu) gene cluster involved in biosynthesis of Sias found in the E. coli capsule. This cluster is probably responsible for the biosynthesis of Neu found in A. salmonicida. In this work, we have produced and characterized the sialic acid (Sia) synthase NeuB1, the key enzyme in the pathway. The Sia synthase is an enzyme producing N-acetylneuraminic acid by the condensation of N-acetylmannosamine and phosphoenolpyruvate. Genome content, kinetic data obtained, together with structural considerations, have led us to the prediction that the substrate for NeuB1 from A. salmonicida, E. coli and Streptococcus agalactiae among others, is 4-O-acetyl-N-acetylmannosamine. This means that the product of its enzymatic reaction is 7-O-acetyl-N-acetylneuraminic acid. We propose a pathway for production of this Sia in A. salmonicida, and present evidence for the presence of diacetylated Neu in the bacterium.

High-throughput bioluminescence-based mutant screening strategy for identification of bacterial virulence genes

A high-throughput bioluminescence screening procedure for identification of virulence genes in bacteria was developed and applied to the fish pathogen Edwardsiella ictaluri. A random transposon mutant library expressing bioluminescence was constructed and robotically arrayed on 384-well plates. Mutants were cultivated and mixed with catfish serum and neutrophils in 96-well plates, and bioluminescence was used to detect mutants that are more susceptible to killing by these host factors. The virulence and vaccine efficacy of selected mutants were determined in channel catfish. Transposon insertion sites in 13 mutants attenuated in the natural host were mapped to the E. ictaluri genome. Ten unique genes were mutated, including genes encoding a negative regulator of sigmaE activity, a glycine cleavage system protein, tricarboxylic acid cycle enzymes, an O polysaccharide biosynthesis enzyme, proteins encoded on the native plasmid pEI1, and a fimbrial chaperon protein. Three of these mutants were found to have potential as live attenuated vaccines. This study demonstrates a novel application of bioluminescence to identify bacterial genes required for host resistance; as a result, efficacious and genetically defined live attenuated vaccine candidates were developed.

Interaction between two putative glycosyltransferases is required for glycosylation of a serine-rich streptococcal adhesin

Fap1, a serine-rich glycoprotein, is essential for fimbrial biogenesis and biofilm formation of Streptococcus parasanguinis (formerly S. parasanguis). Fap1-like proteins are conserved in many streptococci and staphylococci and have been implicated in bacterial virulence. Fap1 contains two serine-rich repeat regions that are modified by O-linked glycosylation. A seven-gene cluster has been identified, and this cluster is implicated in Fap1 biogenesis. In this study, we investigated the initial step of Fap1 glycosylation by using a recombinant Fap1 as a model. This recombinant molecule has the same monosaccharide composition profile as the native Fap1 protein. Glycosyl linkage analyses indicated that N-acetylglucosamine (GlcNAc) is among the first group of sugar residues transferred to the Fap1 peptide. Two putative glycosyltransferases, Gtf1 and Gtf2, were essential for the glycosylation of Fap1 with GlcNAc-containing oligosaccharide(s) in both S. parasanguinis as well as in the Fap1 glycosylation system in Escherichia coli. Yeast two-hybrid analysis as well as in vitro and in vivo glutathione S-transferase pull-down assays demonstrated the two putative glycosyltransferases interacted with each other. The interaction domain was mapped to an N-terminal region of Gtf1 that was required for the Fap1 glycosylation. The data in this study suggested that the formation of the Gtf1 and Gtf2 complex was required for the initiation of the Fap1 glycosylation and that the N-terminal region of Gtf1 was necessary.

Signature-tagged mutagenesis of Edwardsiella ictaluri identifies virulence-related genes, including a salmonella pathogenicity island 2 class of type III secretion systems

Edwardsiella ictaluri is the leading cause of mortality in channel catfish culture, but little is known about its pathogenesis. The use of signature-tagged mutagenesis in a waterborne infection model resulted in the identification of 50 mutants that were unable to infect/survive in catfish. Nineteen had minitransposon insertions in miscellaneous genes in the chromosome, 10 were in genes that matched to hypothetical proteins, and 13 were in genes that had no significant matches in the NCBI databases. Eight insertions were in genes encoding proteins associated with virulence in other pathogens, including three in genes involved in lipopolysaccharide biosynthesis, three in genes involved in type III secretion systems (TTSS), and two in genes involved in urease activity. With the use of a sequence from a lambda clone carrying several TTSS genes, Blastn analysis of the partially completed E. ictaluri genome identified a 26,135-bp pathogenicity island containing 33 genes of a TTSS with similarity to the Salmonella pathogenicity island 2 class of TTSS. The characterization of a TTSS apparatus mutant indicated that it retained its ability to invade catfish cell lines and macrophages but was defective in intracellular replication. The mutant also invaded catfish tissues in numbers equal to those of invading wild-type E. ictaluri bacteria but replicated poorly and was slowly cleared from the tissues, while the wild type increased in number.

Broad host range fluorescence and bioluminescence expression vectors for Gram-negative bacteria

Tagging of bacteria with living colors and living light allows increasingly valuable new imaging and detection technologies to be accessible to researchers. In this study, we aimed to create stable broad host range expression vectors for tagging Gram-negative bacteria with fluorescence and bioluminescence. To accomplish this, a mutated form of promoterless green fluorescent protein (gfp) gene, gfpmut3a, from Aequorea victoria and promoterless bacterial luciferase genes, luxCDABE, from Photorhabdus luminescens were inserted into broad host range plasmid pBBR1MCS4. Expression of gfp and luxCDABE genes was driven by lacZ promoter. In addition, dual versions with both gfpmut3a and luxCDABE genes and inducible versions carrying lacI(q) gene were also constructed. These new broad host range vectors containing a stable broad host range origin of replication and mobility genes can be transferred to Gram-negative bacteria by either electroporation or conjugal mating and maintained stably. Availability of these expression vectors should be useful in developing new approaches to study a broad variety of Gram-negative bacteria, particularly for applications investigating host-pathogen interactions in vivo and in vitro.

Development of bioluminescent Edwardsiella ictaluri for noninvasive disease monitoring

Edwardsiella ictaluri is a facultative intracellular bacterium that causes enteric septicemia of catfish (ESC). In this study, we aimed to develop bioluminescent E. ictaluri that can be monitored by noninvasive bioluminescence imaging (BLI). To accomplish this, the luxCDABE operon of Photorhabdus luminescens was cloned downstream of the lacZ promoter in the broad host range plasmid pBBR1MCS4. Edwardsiella ictaluri strain 93-146 transformed with the new plasmid, pAKlux1, was highly bioluminescent. pAKlux1 was stably maintained in E. ictaluri without any apparent effect on growth or native plasmid stability. To assess the usefulness of the bioluminescent strain in disease studies, catfish were infected with 93-146 pAKlux1 by intraperitoneal injection and by bath immersion, and in vivo bacterial dissemination was observed using BLI. This study demonstrated that bioluminescent E. ictaluri can be used for real-time monitoring of ESC in live fish, which should enable observation of pathogen attachment sites and tissue predilections.

The structure of the antigenic O-polysaccharide of the lipopolysaccharide of Edwardsiella ictaluri strain MT104

The structural characterization of the antigenic O-polysaccharide component of the lipopolysaccharide produced by the fish pathogenic bacterium Edwardsiella ictaluri MT104 was undertaken by the application of NMR spectroscopy and chemical analysis. The O-chain was found to be a linear polymer of a repeating tetrasaccharide unit composed of D-glucose, 2-acetamido-2-deoxy-D-galactose, and D-galactose in a 1:2:1 ratio having the structure: [carbohydrate structure]; see text.

Identification and characterization of a two-component hemolysin from Edwardsiella ictaluri

The channel catfish pathogen Edwardsiella ictaluri possesses hemolysin activity, and strains that are adapted for growth in fish tend to have greater hemolysin activity than strains that are adapted for in vitro growth conditions. To investigate its potential role in virulence, an isogenic hemolysin mutant strain of E. ictaluri R4383 was constructed by transposon mutagenesis. Sequencing of the chromosomal insertion site identified two genes, designated eihA and eihB, that encode proteins with homology to the Serratia family of two-component hemolysins. EihB is similar to the secretion/activation proteins from this family, and EihA is similar to the cytolysin proteins from this family. Bacterial challenge in channel catfish fingerlings did not show a significant difference in virulence between the wild type E. ictaluri strain and the hemolysin deficient E. ictaluri mutant strain.

Evaluation of a loop-mediated isothermal amplification method for rapid detection of channel catfish Ictalurus punctatus important bacterial pathogen Edwardsiella ictaluri

Channel catfish Ictalurus punctatus infected with Edwardsiella ictaluri results in 40--50 million dollars annual losses in profits to catfish producers. Early detection of this pathogen is necessary for disease control and reduction of economic loss. In this communication, the loop-mediated isothermal amplification method (LAMP) that amplifies DNA with high specificity and rapidity at an isothermal condition was evaluated for rapid detection of E. ictaluri. A set of four primers, two outer and two inner, was designed specifically to recognize the eip 18 gene of this pathogen. The LAMP reaction mix was optimized. Reaction temperature and time of the LAMP assay for the eip 18 gene were also optimized at 65 degrees C for 60 min, respectively. Our results show that the ladder-like pattern of bands sizes from 234 bp specifically to the E. ictaluri gene was amplified. The detection limit of this LAMP assay was about 20 colony forming units. In addition, this optimized LAMP assay was used to detect the E. ictaluri eip 18 gene in brains of experimentally challenged channel catfish. Thus, we concluded that the LAMP assay can potentially be used for rapid diagnosis in hatcheries and ponds.