Ferric hydroxamate uptake system contributes to Edwardsiella ictaluri virulence

Flavobacterium columnare ferric iron uptake systems are required for virulence

Flavobacterium columnare, which causes columnaris disease, is one of the costliest pathogens in the freshwater fish-farming industry. The virulence mechanisms of F. columnare are not well understood and current methods to control columnaris outbreaks are inadequate. Iron is an essential nutrient needed for metabolic processes and is often required for bacterial virulence. F. columnare produces siderophores that bind ferric iron for transport into the cell. The genes needed for siderophore production have been identified, but other components involved in F. columnare iron uptake have not been studied in detail. We identified the genes encoding the predicted secreted heme-binding protein HmuY, the outer membrane iron receptors FhuA, FhuE, and FecA, and components of an ATP binding cassette (ABC) transporter predicted to transport ferric iron across the cytoplasmic membrane. Deletion mutants were constructed and examined for growth defects under iron-limited conditions and for virulence against zebrafish and rainbow trout. Mutants with deletions in genes encoding outer membrane receptors, and ABC transporter components exhibited growth defects under iron-limited conditions. Mutants lacking multiple outer membrane receptors, the ABC transporter, or HmuY retained virulence against zebrafish and rainbow trout mirroring that exhibited by the wild type. Some mutants predicted to be deficient in multiple steps of iron uptake exhibited decreased virulence. Survivors of exposure to such mutants were partially protected against later infection by wild-type F. columnare.

Siderophores Produced by the Fish Pathogen Flavobacterium columnare Strain MS-FC-4 Are Not Essential for Its Virulence

Flavobacterium columnare causes columnaris disease in wild and aquaculture-reared freshwater fish. F. columnare virulence mechanisms are not well understood, and current methods to control columnaris disease are inadequate. Iron acquisition from the host is important for the pathogenicity and virulence of many bacterial pathogens. F. columnare iron acquisition has not been studied in detail. We identified genes predicted to function in siderophore production for ferric iron uptake. Genes predicted to encode the proteins needed for siderophore synthesis, export, uptake, and regulation were deleted from F. columnare strain MS-FC-4. The mutants were examined for defects in siderophore production, for growth defects in iron-limited conditions, and for virulence against zebrafish and rainbow trout. Mutants lacking all siderophore activity were obtained. These mutants displayed growth defects when cultured under iron-limited conditions, but they retained virulence against zebrafish and rainbow trout similar to that exhibited by the wild type, indicating that the F. columnare MS-FC-4 siderophores are not required for virulence under the conditions tested. IMPORTANCE Columnaris disease, which is caused by Flavobacterium columnare, is a major problem for freshwater aquaculture. Little is known regarding F. columnare virulence factors, and control measures are limited. Iron acquisition mechanisms such as siderophores are important for virulence of other pathogens. We identified F. columnare siderophore biosynthesis, export, and uptake genes. Deletion of these genes eliminated siderophore production and resulted in growth defects under iron-limited conditions but did not alter virulence in rainbow trout or zebrafish. The results indicate that the F. columnare strain MS-FC-4 siderophores are not critical virulence factors under the conditions tested but may be important for survival under iron-limited conditions in natural aquatic environments or aquaculture systems.

Fluorescent sensors of siderophores produced by bacterial pathogens

Siderophores are iron-chelating molecules that solubilize Fe3+ for microbial utilization and facilitate colonization or infection of eukaryotes by liberating host iron for bacterial uptake. By fluorescently labeling membrane receptors and binding proteins, we created 20 sensors that detect, discriminate, and quantify apo- and ferric siderophores. The sensor proteins originated from TonB-dependent ligand-gated porins (LGPs) of Escherichia coli (Fiu, FepA, Cir, FhuA, IutA, BtuB), Klebsiella pneumoniae (IroN, FepA, FyuA), Acinetobacter baumannii (PiuA, FepA, PirA, BauA), Pseudomonas aeruginosa (FepA, FpvA), and Caulobacter crescentus (HutA) from a periplasmic E. coli binding protein (FepB) and from a human serum binding protein (siderocalin). They detected ferric catecholates (enterobactin, degraded enterobactin, glucosylated enterobactin, dihydroxybenzoate, dihydroxybenzoyl serine, cefidericol, MB-1), ferric hydroxamates (ferrichromes, aerobactin), mixed iron complexes (yersiniabactin, acinetobactin, pyoverdine), and porphyrins (hemin, vitamin B12). The sensors defined the specificities and corresponding affinities of the LGPs and binding proteins and monitored ferric siderophore and porphyrin transport by microbial pathogens. We also quantified, for the first time, broad recognition of diverse ferric complexes by some LGPs, as well as monospecificity for a single metal chelate by others. In addition to their primary ferric siderophore ligands, most LGPs bound the corresponding aposiderophore with ∼100-fold lower affinity. These sensors provide insights into ferric siderophore biosynthesis and uptake pathways in free-living, commensal, and pathogenic Gram-negative bacteria.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

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

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

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.

Key evolutionary events in the emergence of a globally disseminated, carbapenem resistant clone in the Escherichia coli ST410 lineage

There is an urgent need to understand the global epidemiological landscape of carbapenem-resistant Escherichia coli (CREC). Here we provide combined genomic and phenotypic characterization of the emergence of a CREC clone from the ST410 lineage. We show that the clone expands with a single plasmid, within which there is frequent switching of the carbapenemase gene type between blaNDM and blaOXA-181 with no impact on plasmid stability or fitness. A search for clone-specific traits identified unique alleles of genes involved in adhesion and iron acquisition, which have been imported via recombination. These recombination-derived allelic switches had no impact on virulence in a simple infection model, but decreased efficiency in binding to abiotic surfaces and greatly enhanced fitness in iron limited conditions. Together our data show a footprint for evolution of a CREC clone, whereby recombination drives new alleles into the clone which provide a competitive advantage in colonizing mammalian hosts.

Evolution and Sequence Diversity of FhuA in Salmonella and Escherichia

The fhuACDB operon, present in a number of Enterobacteriaceae, encodes components essential for the uptake of ferric hydroxamate type siderophores. FhuA acts not only as a transporter for physiologically important chelated ferric iron but also as a receptor for various bacteriophages, toxins, and antibiotics, which are pathogenic to bacterial cells. In this research, fhuA gene distribution and sequence diversity were investigated in Enterobacteriaceae, especially Salmonella and Escherichia Comparative sequence analysis resulted in a fhuA phylogenetic tree that did not match the expected phylogeny of species or trees of the fhuCDB genes. The fhuA sequences showed a unique mosaic clustering pattern. On the other hand, the gene sequences showed high conservation for strains from the same serovar or serotype. In total, six clusters were identified from FhuA proteins in Salmonella and Escherichia, among which typical peptide fragment variations could be defined. Six fragmental insertions/deletions and two substitution fragments were discovered, for which the combination of polymorphism patterns could well classify the different clusters. Structural modeling demonstrated that all the six featured insertions/deletions and one substitution fragment are located at the apexes of the long loops present as part of the FhuA external pocket. These frequently mutated regions are likely under high selection pressure, with bacterial strains balancing escape from phage infection or toxin/antibiotics attack via fhuA gene mutations while maintaining the siderophore uptake activity essential for bacterial survival. The unusual fhuA clustering suggests that high-frequency exchange of fhuA genes has occurred between enterobacterial strains after distinctive species were established.

Stress-related genes promote Edwardsiella ictaluri pathogenesis

Edwardsiella ictaluri is a Gram-negative facultative anaerobic rod and the causative agent of enteric septicemia of channel catfish (ESC), which is one of the most prevalent diseases of catfish, causing significant economic losses in the catfish industry. E. ictaluri is resistant to complement system and macrophage killing, which results in rapid systemic septicemia. However, mechanisms of E. ictaluri stress responses under conditions of host environment are not studied well. Therefore, in this work, we report E. ictaluri stress responses during hydrogen peroxide, low pH, and catfish serum stresses as well as during catfish invasion. E. ictaluri stress responses were characterized by identifying expression of 13 universal stress protein (USP) genes (usp01-usp13) and seven USP-interacting protein genes (groEL, groES, dnaK, grpE, and clpB, grpE, relA). Data indicated that three usp genes (usp05, usp07, and usp13) were highly expressed in all stress conditions. Similarly, E. ictaluri heat shock proteins groEL, groES, dnaK, grpE, and clpB were highly expressed in oxidative stress. Also, E. ictaluri grpE and relA were highly expressed in catfish spleen and head kidney. These findings contribute to our understanding of stress response mechanisms in E. ictaluri stress response, and stress-related proteins that are essential for E. ictaluri could be potential targets for live attenuated vaccine development against ESC.