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

Mycoplasma glycine cleavage system key subunit GcvH is an apoptosis inhibitor targeting host endoplasmic reticulum

Mycoplasmas are minimal but notorious bacteria that infect humans and animals. These genome-reduced organisms have evolved strategies to overcome host apoptotic defense and establish persistent infection. Here, using Mycoplasma bovis as a model, we demonstrate that mycoplasma glycine cleavage system (GCS) H protein (GcvH) targets the endoplasmic reticulum (ER) to hijack host apoptosis facilitating bacterial infection. Mechanically, GcvH interacts with the ER-resident kinase Brsk2 and stabilizes it by blocking its autophagic degradation. Brsk2 subsequently disturbs unfolded protein response (UPR) signaling, thereby inhibiting the key apoptotic molecule CHOP expression and ER-mediated intrinsic apoptotic pathway. CHOP mediates a cross-talk between ER- and mitochondria-mediated intrinsic apoptosis. The GcvH N-terminal amino acid 31-35 region is necessary for GcvH interaction with Brsk2, as well as for GcvH to exert anti-apoptotic and potentially pro-infective functions. Notably, targeting Brsk2 to dampen apoptosis may be a conserved strategy for GCS-containing mycoplasmas. Our study reveals a novel role for the conserved metabolic route protein GcvH in Mycoplasma species. It also sheds light on how genome-reduced bacteria exploit a limited number of genomic proteins to resist host cell apoptosis thereby facilitating pathogenesis.

Characterization of Type VI secretion system in Edwardsiella ictaluri

Edwardsiella ictaluri is a Gram-negative facultative intracellular fish pathogen causing enteric septicemia of catfish (ESC). While various secretion systems contribute to E. ictaluri virulence, the Type VI secretion system (T6SS) remains poorly understood. In this study, we constructed 13 E. ictaluri T6SS mutants using splicing by overlap extension PCR and characterized them, assessing their uptake and survival in channel catfish (Ictalurus punctatus) peritoneal macrophages, attachment and invasion in channel catfish ovary (CCO) cells, in vitro stress resistance, and virulence and efficacy in channel catfish. Among the mutants, EiΔevpA, EiΔevpH, EiΔevpM, EiΔevpN, and EiΔevpO exhibited reduced replication inside peritoneal macrophages. EiΔevpM, EiΔevpN, and EiΔevpO showed significantly decreased attachment to CCO cells, while EiΔevpN and EiΔevpO also displayed reduced invasion of CCO cells (p < 0.05). Overall, T6SS mutants demonstrated enhanced resistance to oxidative and nitrosative stress in the nutrient-rich medium compared to the minimal medium. However, EiΔevpA, EiΔevpH, EiΔevpM, EiΔevpN, and EiΔevpO were susceptible to oxidative stress in both nutrient-rich and minimal medium. In fish challenges, EiΔevpD, EiΔevpE, EiΔevpG, EiΔevpJ, and EiΔevpK exhibited attenuation and provided effective protection against E. ictaluri wild-type (EiWT) infection in catfish fingerlings. However, their attenuation and protective efficacy were lower in catfish fry. These findings shed light on the role of the T6SS in E. ictaluri pathogenesis, highlighting its significance in intracellular survival, host cell attachment and invasion, stress resistance, and virulence. The attenuated T6SS mutants hold promise as potential candidates for protective immunization strategies in catfish fingerlings.

Mining the Flavoproteome of Brucella ovis, the Brucellosis Causing Agent in Ovis aries

Flavoproteins are a diverse class of proteins that are mostly enzymes and contain as cofactors flavin mononucleotide (FMN) and/or flavin adenine dinucleotide (FAD), which enable them to participate in a wide range of physiological reactions. We have compiled 78 potential proteins building the flavoproteome of Brucella ovis (B. ovis), the causative agent of ovine brucellosis. The curated list of flavoproteins here reported is based on (i) the analysis of sequence, structure and function of homologous proteins, and their classification according to their structural domains, clans, and expected enzymatic functions; (ii) the constructed phylogenetic trees of enzyme functional classes using 19 Brucella strains and 26 pathogenic and/or biotechnological relevant alphaproteobacteria together with B. ovis; and (iii) the evaluation of the genetic context for each entry. Candidates account for ∼2.7% of the B. ovis proteome, and 75% of them use FAD as cofactor. Only 55% of these flavoproteins belong to the core proteome of Brucella and contribute to B. ovis processes involved in maintenance activities, survival and response to stress, virulence, and/or infectivity. Several of the predicted flavoproteins are highly divergent in Brucella genus from revised proteins and for them it is difficult to envisage a clear function. This might indicate modified catalytic activities or even divergent processes and mechanisms still not identified. We have also detected the lack of some functional flavoenzymes in B. ovis, which might contribute to it being nonzoonotic. Finally, potentiality of B. ovis flavoproteome as the source of antimicrobial targets or biocatalyst is discussed. IMPORTANCE Some microorganisms depend heavily on flavin-dependent activities, but others maintain them at a minimum. Knowledge about flavoprotein content and functions in different microorganisms will help to identify their metabolic requirements, as well as to benefit either industry or health. Currently, most flavoproteins from the sheep pathogen Brucella ovis are only automatically annotated in databases, and only two have been experimentally studied. Indeed, certain homologues with unknown function are not characterized, and they might relate to still not identified mechanisms or processes. Our research has identified 78 members that comprise its flavoproteome, 76 of them flavoenzymes, which mainly relate to bacteria survival, virulence, and/or infectivity. The list of flavoproteins here presented allows us to better understand the peculiarities of Brucella ovis and can be applied as a tool to search for candidates as new biocatalyst or antimicrobial targets.

Identification of novel vaccine candidates in the whole-cell Aeromonas hydrophila biofilm vaccine through reverse vaccinology approach

Biofilm vaccine has been recognised as one of the successful strategy to reduce the Aeromonas hydrophila infection in fish. But, the vaccine contains the protective and non-protective proteins, which may lead to show altered heterologous adaptive immunity response. Moreover, cross protection and effectiveness of previously developed biofilm vaccine was not tested against different geographical A. hydrophila isolates. Therefore, in the present study, whole-cell A. hydrophila biofilm vaccine was evaluated in rohu, vaccinated group showed increased antibody titer and protection against the different geographical A. hydrophila isolates namely KAH₁ and AAH₂ with 78.9% and 84.2% relative percentage survival, respectively. In addition, by using the immune sera of biofilm vaccinated group, a total of six protective proteins were detected using western blot assay. Further, the same proteins were identified by nano LC-MS/MS method, a total of fourteen candidate proteins showing the immunogenic property including highly expressed OMP's tolC, bamA, lamb, AH4AK4_2542, AHGSH82_029580 were identified as potential vaccine candidates. The STRING analysis revealed that, top candidate proteins identified may potentially interact with other intracellular proteins; involved in ribosomal and (tricarboxylic acid) TCA pathway. Importantly, all the selected vaccine candidate proteins contain the B-cell epitope region. Finally, the present study concludes that, whole-cell A. hydrophila biofilm vaccine able to protect the fish against the different geographical A. hydrophila isolates. Further, through reverse vaccinology approach, a total of fourteen proteins were identified as potential vaccine candidates against A. hydrophila pathogen.

Hemolysin Co-regulated Family Proteins Hcp1 and Hcp2 Contribute to Edwardsiella ictaluri Pathogenesis

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of catfish (ESC), a devastating disease resulting in significant economic losses in the U.S. catfish industry. Bacterial secretion systems are involved in many bacteria's virulence, and Type VI Secretion System (T6SS) is a critical apparatus utilized by several pathogenic Gram-negative bacteria. E. ictaluri strain 93-146 genome has a complete T6SS operon with 16 genes, but the roles of these genes are still not explored. In this research, we aimed to understand the roles of two hemolysin co-regulated family proteins, Hcp1 (EvpC) and Hcp2. To achieve this goal, single and double E. ictaluri mutants (EiΔevpC, EiΔhcp2, and EiΔevpCΔhcp2) were generated and characterized. Catfish peritoneal macrophages were able to kill EiΔhcp2 better than EiΔevpC, EiΔevpCΔhcp2, and E. ictaluri wild-type (EiWT). The attachment of EiΔhcp2 and EiΔevpCΔhcp2 to ovary cells significantly decreased compared to EiWT whereas the cell invasion rates of these mutants were the same as that of EiWT. Mutants exposed to normal catfish serum in vitro showed serum resistance. The fish challenges demonstrated that EiΔevpC and EiΔevpCΔhcp2 were attenuated completely and provided excellent protection against EiWT infection in catfish fingerlings. Interestingly, EiΔhcp2 caused higher mortality than that of EiWT in catfish fingerlings, and severe clinical signs were observed. Although fry were more susceptible to vaccination with EiΔevpC and EiΔevpCΔhcp2, their attenuation and protection were significantly higher compared to EiWT and sham groups, respectively. Taken together, our data indicated that evpC (hcp1) is involved in E. ictaluri virulence in catfish while hcp2 is involved in adhesion to epithelial cells and survival inside catfish macrophages.

Edwardsiella ictaluri evpP is required for colonisation of channel catfish ovary cells and necrosis in anterior kidney macrophages

Edwardsiella ictaluri is a Gram-negative facultative anaerobe that can survive inside channel catfish phagocytes. E. ictaluri can orchestrate Type VI Secretion System (T6SS) for survival in catfish macrophages. evpP encodes one of the T6SS translocated effector proteins. However, the role of evpP in E. ictaluri is still unexplored. In this work, we constructed an E. ictaluri evpP mutant (EiΔevpP) and assessed its survival under complement and oxidative stress. Persistence of EiΔevpP in catfish as well as attachment and invasion in catfish macrophage and ovary cells were determined. Further, virulence of EiΔevpP in catfish and apoptosis it caused in macrophages were explored. EiΔevpP behaved same as wild type (EiWT) under complement and oxidative stress in complex media, whereas oxidative stress affected mutant's survival significantly in minimal media (p < .05). Persistence of EiΔevpP in live catfish and uptake and survival inside peritoneal macrophages were similar. The attachment and invasion capabilities of EiΔevpP in catfish ovary cells were significantly less than that of EiWT (p < .05). Although EiΔevpP showed reduced attenuation in catfish, causing decreased catfish mortality compared with EiWT (44.73% vs. 67.53%), this difference was not significant. The apoptosis assay using anterior kidney macrophages indicated that the number of live macrophages exposed to EiΔevpP was significantly higher compared with EiWT exposed macrophages at 24-hr post-treatment (p < .05). However, there were no significant differences in the early and late apoptosis. Remarkably, necrosis in EiΔevpP exposed macrophages was significantly less than that of EiWT exposed macrophages at 24 hr (p < .05). Our results demonstrated that evpP is required for colonisation of catfish ovary cells and increased apoptosis and necrosis in anterior kidney macrophages.

Transposon mutagenesis and identification of mutated genes in growth-delayed Edwardsiella ictaluri

Background

Edwardsiella ictaluri is a Gram-negative facultative intracellular anaerobe and the etiologic agent of enteric septicemia of channel catfish (ESC). To the catfish industry, ESC is a devastating disease due to production losses and treatment costs. Identification of virulence mechanisms of E. ictaluri is critical to developing novel therapeutic approaches for the disease. Here, we report construction of a transposon insertion library and identification of mutated genes in growth-delayed E. ictaluri colonies. We also provide safety and efficacy of transposon insertion mutants in catfish.

Results

An E. ictaluri transposon insertion library with 45,000 transposants and saturating 30.92% of the TA locations present in the E. ictaluri genome was constructed. Transposon end mapping of 250 growth-delayed E. ictaluri colonies and bioinformatic analysis of sequences revealed 56 unique E. ictaluri genes interrupted by the MAR2xT7 transposon, which are involved in metabolic and cellular processes and mostly localized in the cytoplasm or cytoplasmic membrane. Of the 56 genes, 30 were associated with bacterial virulence. Safety and vaccine efficacy testing of 19 mutants showed that mutants containing transposon insertions in hypothetical protein (Eis::004), and Fe-S cluster assembly protein (IscX, Eis::039), sulfurtransferase (TusA, Eis::158), and universal stress protein A (UspA, Eis::194) were safe and provided significant protection (p < 0.05) against wild-type E. ictaluri.

Conclusions

The results indicate that random transposon mutagenesis causing growth-delayed phenotype results in identification bacterial virulence genes, and attenuated strains with transposon interrupted virulence genes could be used as vaccine to activate fish immune system.

Universal Stress Proteins Contribute Edwardsiella ictaluri Virulence in Catfish

Edwardsiella ictaluri is an intracellular Gram-negative facultative pathogen causing enteric septicemia of catfish (ESC), a common disease resulting in substantial economic losses in the U.S. catfish industry. Previously, we demonstrated that several universal stress proteins (USPs) are highly expressed under in vitro and in vivo stress conditions, indicating their importance for E. ictaluri survival. However, the roles of these USPs in E. ictaluri virulence is not known yet. In this work, 10 usp genes of E. ictaluri were in-frame deleted and characterized in vitro and in vivo. Results show that all USP mutants were sensitive to acidic condition (pH 5.5), and EiΔusp05 and EiΔusp08 were very sensitive to oxidative stress (0.1% H₂O₂). Virulence studies indicated that EiΔusp05, EiΔusp07, EiΔusp08, EiΔusp09, EiΔusp10, and EiΔusp13 were attenuated significantly compared to E. ictaluri wild-type (EiWT; 20, 45, 20, 20, 55, and 10% vs. 74.1% mortality, respectively). Efficacy experiments showed that vaccination of catfish fingerlings with EiΔusp05, EiΔusp07, EiΔusp08, EiΔusp09, EiΔusp10, and EiΔusp13 provided complete protection against EiWT compared to sham-vaccinated fish (0% vs. 58.33% mortality). Our results support that USPs contribute E. ictaluri virulence in catfish.

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.

The virulence and immune protection of Edwardsiella ictaluri HemR mutants in catfish

Edwardsiella ictaluri is a Gram-negative facultative intracellular rod, causing enteric septicemia of catfish (ESC). Several heme uptake systems have been described in bacterial pathogens, most of which involve outer membrane proteins (OMPs). We have shown recently that heme/hemoglobin receptor family protein (HemR) is significantly up-regulated in E. ictaluri under iron-restricted conditions. In this work, our goal was to construct E. ictaluri HemR mutants and assess their virulence and immune protection potentials in catfish. To accomplish this, an in-frame deletion mutant (EiΔhemR) was constructed, and its virulence and immune protection were determined in catfish fingerlings and fry. The results indicated that the EiΔhemR was attenuated completely in catfish fingerlings, but it was virulent in 14 day-old catfish fry. To increase the attenuation of EiΔhemR in fry, we introduced frdA and sdhC gene deletions to the mutant, yielding two double (EiΔhemRΔfrdA and EiΔhemRΔsdhC) and one triple (EiΔhemRΔfrdAΔsdhC) mutants. Results indicated that two double HemR mutants did not exhibit increased attenuation, but the triple HemR mutant showed significantly less virulence and high protection in fry (p < 0.05). Histological examination of fry tissues vaccinated with the triple mutant displayed similar inflammation to that of wild-type infected fry, but much less necrosis and far fewer bacteria were observed. Immunohistochemistry (IHC) result indicated fewer numbers of bacteria around blood vessel and in the hematopoietic tissue in fry infected with triple mutant compared to control group infected with E. ictaluri wild-type. Our data indicated that EiΔhemR was safe and protective in catfish fingerlings, while EiΔhemRΔfrdAΔsdhC was much safer in catfish fry.

The Role of TonB Gene in Edwardsiella ictaluri Virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen that causes enteric septicemia in catfish (ESC). Stress factors including poor water quality, poor diet, rough handling, overcrowding, and water temperature fluctuations increase fish susceptibility to ESC. The TonB energy transducing system (TonB-ExbB-ExbD) and TonB-dependent transporters of Gram-negative bacteria support active transport of scarce resources including iron, an essential micronutrient for bacterial virulence. Deletion of the tonB gene attenuates virulence in several pathogenic bacteria. In the current study, the role of TonB (NT01EI_RS07425) in iron acquisition and E. ictaluri virulence were investigated. To accomplish this, the E. ictaluri tonB gene was in-frame deleted. Growth kinetics, iron utilization, and virulence of the EiΔtonB mutant were determined. Loss of TonB caused a significant reduction in bacterial growth in iron-depleted medium (p > 0.05). The EiΔtonB mutant grew similarly to wild-type E. ictaluri when ferric iron was added to the iron-depleted medium. The EiΔtonB mutant was significantly attenuated in catfish compared with the parent strain (21.69 vs. 46.91% mortality). Catfish surviving infection with EiΔtonB had significant protection against ESC compared with naïve fish (100 vs. 40.47% survival). These findings indicate that TonB participates in pathogenesis of ESC and is an important E. ictaluri virulence factor.

Comparative Analysis of the Flavobacterium columnare Genomovar I and II Genomes

Columnaris disease caused by Gram-negative rod Flavobacterium columnare is one of the most common diseases of catfish. F. columnare is also a common problem in other cultured fish species worldwide. F. columnare has three major genomovars; we have sequenced a representative strain from genomovar I (ATCC 49512, which is avirulent in catfish) and genomovar II (94-081, which is highly pathogenic in catfish). Here, we present a comparative analysis of the two genomes. Interestingly, F. columnare ATCC 49512 and 94-081 meet criteria to be considered different species based on the Average Nucleotide Identity (90.71% similar) and DNA–DNA Hybridization (42.6% similar). Genome alignment indicated the two genomes have a large number of rearrangements. However, function-based comparative genomics analysis indicated that the two strains have similar functional capabilities with 2,263 conserved orthologous clusters; strain ATCC 49512 has 290 unique orthologous clusters while strain 94-081 has 391. Both strains carry type I secretion system, type VI secretion system, and type IX secretion system. The two genomes also have similar CRISPR capacities. The F. columnare strain ATCC 49512 genome contains a higher number of insertion sequence families and phage regions, while the F. columnare strain 94-081 genome has more genomic islands and more regulatory gene capacity. Transposon mutagenesis using Tn4351 in pathogenic strain 94-081 yielded six mutants, and experimental infections of fish showed hemolysin and glycine cleavage protein mutants had 15 and 10% mortalities, respectively, while the wild-type strain caused 100% mortalities. Our comparative and mutational analysis yielded important information on classification of genomovars I and II F. columnare as well as potential virulence genes in F. columnare strain 94-081.

Examination of Host Phenotypes in Gambusia affinis Following Antibiotic Treatment

The commonality of antibiotic usage in medicine means that understanding the resulting consequences to the host is vital. Antibiotics often decrease host microbiome community diversity and alter the microbial community composition. Many diseases such as antibiotic-associated enterocolitis, inflammatory bowel disease, and metabolic disorders have been linked to a disrupted microbiota. The complex interplay between host, microbiome, and antibiotics needs a tractable model for studying host-microbiome interactions. Our freshwater vertebrate fish serves as a useful model for investigating the universal aspects of mucosal microbiome structure and function as well as analyzing consequential host effects from altering the microbial community. Methods include host challenges such as infection by a known fish pathogen, exposure to fecal or soil microbes, osmotic stress, nitrate toxicity, growth analysis, and measurement of gut motility. These techniques demonstrate a flexible and useful model system for rapid determination of host phenotypes.

Ferric hydroxamate uptake system contributes to Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia in fish, particularly in channel catfish. Ferric iron is an essential micronutrient for bacterial survival, and some bacterial pathogens use secreted hydroxamate-type siderophores to chelate iron in host tissues. Siderophore-iron complexes are taken up by these bacteria via the ferric hydroxamate uptake (Fhu) system. In E. ictaluri, the Fhu system consists of fhuC, fhuD, fhuB, and fhuA genes. However, the importance of the Fhu system in E. ictaluri virulence has not been investigated completely. Here, we present construction of E. ictaluri fhuD and fhuB mutants (EiΔfhuD and EiΔfhuB) by in-frame gene deletion and evaluation of the mutants' virulence and immunogenicity in channel catfish fingerlings and fry. Immersion challenges showed that EiΔfhuD was not significantly attenuated (p < 0.05) in catfish fingerlings, whereas EiΔfhuB was significantly attenuated (p < 0.01). Catfish fingerlings immunized with EiΔfhuD and EiΔfhuB showed 100% and 97.62% survival, respectively. Fry immersion challenges indicated EiΔfhuB was also significantly attenuated (p < 0.05) in two-week old fry compared to the wild-type (48.96% vs. 82.14% mortalities). The survival rate in the fry vaccinated with EiΔfhuB was significantly higher (p < 0.05) than that of non-vaccinated fry (96.77% vs. 21.42% survival). Our data indicates that the fhuB gene, but not the fhuD gene, contributes to E. ictaluri virulence.

Identification of Salmonella enterica serovar Kentucky genes involved in attachment to chicken skin

BACKGROUND

Regardless of sanitation practices implemented to reduce Salmonella prevalence in poultry processing plants, the problem continues to be an issue. To gain an understanding of the attachment mechanism of Salmonella to broiler skin, a bioluminescent-based mutant screening assay was used. A random mutant library of a field-isolated bioluminescent strain of Salmonella enterica serovar Kentucky was constructed. Mutants' attachment to chicken skin was assessed in 96-well plates containing uniform 6 mm diameter pieces of circular chicken skin. After washing steps, mutants with reduced attachment were selected based on reduced bioluminescence, and transposon insertion sites were identified.

RESULTS

Attachment attenuation was detected in transposon mutants with insertion in genes encoding flagella biosynthesis, lipopolysaccharide core biosynthesis protein, tryptophan biosynthesis, amino acid catabolism pathway, shikimate pathway, tricarboxylic acid (TCA) cycle, conjugative transfer system, multidrug resistant protein, and ATP-binding cassette (ABC) transporter system. In particular, mutations in S. Kentucky flagellar biosynthesis genes (flgA, flgC, flgK, flhB, and flgJ) led to the poorest attachment of the bacterium to skin.

CONCLUSIONS

The current study indicates that attachment of Salmonella to broiler skin is a multifactorial process, in which flagella play an important role.

Intramacrophage Infection Reinforces the Virulence of Edwardsiella tarda

Edwardsiella tarda is an important pathogenic bacterium that can replicate in macrophages. However, how the intramacrophage infection process affects the virulence of this bacterium is essentially unknown. Here, we show that E. tarda replicates and induces a caspase-1-dependent cell pyroptosis in a murine macrophage model. Via pyroptosis, intracellular E. tarda escapes to the extracellular milieu, forming a unique bacterial population. Being different from the bacteria cultured alone, this unique population possesses a reprogrammed transcriptional profile, particularly with upregulated type III secretion system (T3SS)/T6SS cluster genes. Subsequent studies revealed that the macrophage-released population gains enhanced infectivity for host epithelial cells and increases resistance to multiple host defenses and hence displays significantly promoted virulence in vivo Further studies indicated that T3SS is essentially required for the macrophage infection process, while T6SS contributes to infection-induced bacterial virulence. Altogether, this work demonstrates that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, suggesting a positive role for intramacrophage infection in bacterial pathogenesis.

IMPORTANCE

Many pathogens can replicate in macrophages, which is crucial for their pathogenesis. To survive in the macrophage cell, pathogens are likely to require fitness genes to counteract multiple host-killing mechanisms. Here, Edwardsiella tarda is proved to exit from macrophages during infection. This macrophage-released population displays a reprogrammed transcriptional profile with significantly upregulated type III secretion system (T3SS)/T6SS-related genes. Furthermore, both enhanced infectivity in epithelial cells and activated resistance to complex host defenses were conferred on this macrophage-primed population, which consequently promoted the full virulence of E. tarda in vivo Our work provides evidence that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, highlighting the importance of the intramacrophage infection cycle for the pathogenesis of E. tarda.

The host effects of Gambusia affinis with an antibiotic-disrupted microbiome

While serving as critical tools against bacterial infections, antimicrobial therapies can also result in serious side effects, such as antibiotic-associated entercolitis. Recent studies utilizing next generation sequencing to generate community 16S gene profiles have shown that antibiotics can strongly alter community composition and deplete diversity. However, how these community changes in the microbiota are related to the host side effects is still unclear. We have used the freshwater Western mosquitofish (Gambusia affinis) as a tractable vertebrate model system to study host effects following exposure to a broad spectrum antibiotic, rifampicin. After 3days of exposure, the bacterial communities of the mucosal skin and gut microbiomes lost diversity and shifted composition. Compared to unexposed controls, treated fish were more susceptible to a specific pathogen, Edwardsiella ictaluri, yet displayed no survival differences when subjected to a polymicrobial water challenge of soil or feces. Treated fish were more susceptible to osmotic stress from NaCl, but not to the toxin nitrate. Treated fish failed to gain weight as well as controls over one month when fed a matched diet. Because of small sample sizes, pathogen susceptibility and weight gain differences were not statistically significant. This study provides supporting evidence in an experimental laboratory system that an antibiotic can have significant and persistent negative host effects, and provides for future study into the mechanisms of these effects.

Succinate dehydrogenase mutant of Listonella anguillarum protects rainbow trout against vibriosis

Listonella anguillarum is a Gram-negative facultative anaerobic rod causing hemorrhagic septicemia in marine and rarely in freshwater fish. Succinate dehydrogenase (SDH) plays an important role in the tricarboxylic acid (TCA) cycle by oxidizing succinate to fumarate while reducing ubiquinone to ubiquinol. Recent studies indicate that central metabolic pathways, including the TCA cycle, contribute to bacterial virulence. However, the role of SDH in L. anguillarum virulence has not been studied. Here, we report in-frame deletion of the succinate dehydrogenase iron-sulfur protein (SDHB) and its role in L. anguillarum virulence in rainbow trout. To accomplish this goal, upstream and downstream regions of the L. anguillarum sdhB gene were amplified in-frame and cloned into a suicide plasmid. The chromosomal sdhB gene of L. anguillarum was deleted by homologous recombination. Virulence and immunogenicity of the L. anguillarum ΔsdhB mutant (LaΔsdhB) were determined in rainbow trout. Results show that LaΔsdhB was highly attenuated in rainbow trout, and fish immunized with LaΔsdhB displayed high relative survival rate after exposure to wild type L. anguillarum. These findings indicate SDH is important in L. anguillarum virulence in rainbow trout, and LaΔsdhB could be used as an immersion, oral, or injection vaccine to protect rainbow trout against vibriosis.

Tissue persistence and vaccine efficacy of tricarboxylic acid cycle and one-carbon metabolism mutant strains of Edwardsiella ictaluri

Edwardsiella ictaluri causes enteric septicemia in fish. Recently, we reported construction of E. ictaluri mutants with single and double gene deletions in tricarboxylic acid cycle (TCA) and one-carbon (C-1) metabolism. Here, we report the tissue persistence, virulence, and vaccine efficacy of TCA cycle (EiΔsdhC, EiΔfrdA, and EiΔmdh), C-1 metabolism (EiΔgcvP and EiΔglyA), and combination mutants (EiΔfrdAΔsdhC, EiΔgcvPΔsdhC, EiΔmdhΔsdhC, and EiΔgcvPΔglyA) in channel catfish. The tissue persistence study showed that EiΔsdhC, EiΔfrdA, EiΔfrdAΔsdhC, and EiΔgcvPΔsdhC were able to invade catfish and persist until 11 days post-infection. Vaccination of catfish fingerlings with all nine mutants provided significant (P<0.05) protection against subsequent challenge with the virulent parental strain. Vaccinated catfish fingerlings had 100% survival when subsequently challenged by immersion with wild-type E. ictaluri except for EiΔgcvPΔglyA and EiΔgcvP. Mutant EiΔgcvPΔsdhC was found to be very good at protecting catfish fry, as evidenced by 10-fold higher survival compared to non-vaccinated fish.

The contribution of the glycine cleavage system to the pathogenesis of Francisella tularensis

Biosynthesis and acquisition of nutrients during infection are integral to pathogenesis. Members of a metabolic pathway, the glycine cleavage system, have been identified in virulence screens of the intracellular bacterium Francisella tularensis but their role in pathogenesis remains unknown. This system generates 5,10-methylenetetrahydrofolate, a precursor of amino acid and DNA synthesis, from glycine degradation. To characterize this pathway, deletion of the gcvT homolog, an essential member of this system, was performed in attenuated and virulent F. tularensis strains. Deletion mutants were auxotrophic for serine but behaved similar to wild-type strains with respect to host cell invasion, intracellular replication, and stimulation of TNF-α. Unexpectedly, the glycine cleavage system was required for the pathogenesis of virulent F. tularensis in a murine model. Deletion of the gcvT homolog delayed mortality and lowered bacterial burden, particularly in the liver and bloodstream. To reconcile differences between the cell culture model and animal model, minimal tissue culture media was employed to mimic the nutritionally limiting environment of the host. This reevaluation demonstrated that the glycine cleavage system contributes to the intracellular replication of virulent F. tularensis in serine limiting environments. Thus, the glycine cleavage system is the serine biosynthetic pathway of F. tularensis and contributes to pathogenesis in vivo.

Effect of multiple mutations in tricarboxylic acid cycle and one-carbon metabolism pathways on Edwardsiella ictaluri pathogenesis

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia of catfish (ESC). We have shown recently that tricarboxylic acid cycle (TCA) and one-carbon (C1) metabolism are involved in E. ictaluri pathogenesis. However, the effect of multiple mutations in these pathways is unknown. Here, we report four novel E. ictaluri mutants carrying double gene mutations in TCA cycle (EiΔmdhΔsdhC, EiΔfrdAΔsdhC), C1 metabolism (EiΔglyAΔgcvP), and both TCA and C1 metabolism pathways (EiΔgcvPΔsdhC). In-frame gene deletions were constructed by allelic exchange and mutants' virulence and vaccine efficacy were evaluated using in vivo bioluminescence imaging (BLI) as well as end point mortality counts in catfish fingerlings. Results indicated that all the double gene mutants were attenuated compared to wild-type (wt) E. ictaluri. There was a 1.39-fold average reduction in bioluminescence, and hence bacterial numbers, from all the mutants except for EiΔfrdAΔsdhC at 144 h post-infection. Vaccination with mutants was very effective in protecting channel catfish against subsequent infection with virulent E. ictaluri 93-146 strain. In particular, immersion vaccination resulted in complete protection. Our results provide further evidence on the importance of TCA and C1 metabolism pathways in bacterial pathogenesis.

Transferable green fluorescence-tagged pEI2 in Edwardsiella ictaluri and preliminary investigation of its effects on virulence

Edwardsiella ictaluri is the etiologic agent of enteric septicemia of catfish, which causes substantial losses in catfish aquaculture. To determine pathogen-host interactions, previous studies have used the green fluorescence protein (GFP) gene. Here, the pEI2 plasmid of E. ictaluri isolate I49 was tagged using a Tn10-GFP-kan cassette to create the green fluorescence-expressing derivative I49-gfp. The Tn10-GFP-kan insertion site was mapped by plasmid sequencing to 663 bp upstream of open reading frame 2 and appeared to be at a neutral site in the plasmid. Purification of the pEI2::GFPKan plasmid and mobilization into E. coli resulted in GFP expression. The isolated pEI2::GFPkan plasmid was used to retransform the wild type I49 isolate (ensuring a single Tn10-GFP-kan insertion) and an independent E. ictaluri isolate, S97-73-3. The wild type and the green fluorescent-tagged strains were compared for modulation of pathogenicity in channel catfish Ictalurus punctatus by immersion challenge. A significant reduction in mortalities occurred for the I49GFPkan strain as compared to its isogenic parent, but no difference was observed between the S97-73-3GFPkan strain and the S97-73-3 wild type. This GFP-tagged plasmid will be useful for determining the effects that the pEI2::GFPkan plasmid has on virulence and host-pathogen interactions between E. ictaluri isolates.

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.

Global gene expression in channel catfish after vaccination with an attenuated Edwardsiella ictaluri

To understand the global gene expression in channel catfish after immersion vaccination with an attenuated Edwardsiella ictaluri (AquaVac-ESC™), microarray analysis of 65,182 UniGene transcripts was performed. With a filter of false-discovery rate less than 0.05 and fold change greater than 2, a total of 52 unique transcripts were found to be upregulated in vaccinated fish at 48 h post vaccination, whereas a total of 129 were downregulated. The 52 upregulated transcripts represent genes with putative functions in the following seven major categories: (1) hypothetical (25%); (2) novel (23%); (3) immune response (17%); (4) signal transduction (15%); (5) cell structure (8%); (6) metabolism (4%); and (7) others (8%). The 129 downregulated transcripts represent genes with putative functions in the following ten major categories: (1) novel (25%); (2) immune response (23%); (3) hypothetical (12%); (4) metabolism (10%); (5) signal transduction (7%); (6) protein synthesis (6.2%); (7) cell structure (5%); (8) apoptosis (3%); (9) transcription/translation (2%); and (10) others (6%). Microarray analysis revealed that apolipoprotein A-I was upregulated the most (8.5 fold, P = 0.011) at 48 h post vaccination whereas a novel protein (accession no. CV995854) was downregulated the most (342 fold, P = 0.001). Differential regulation of several randomly selected transcripts in vaccinated fish was also validated by quantitative PCR. Our results suggest that these differentially regulated genes elicited by the vaccination might play important roles in the protection of channel catfish against E. ictaluri.

Identification and expression profile of multiple genes in the anterior kidney of channel catfish induced by modified live Edwardsiella ictaluri vaccination

Using PCR-select subtractive cDNA hybridization technique, 57 expressed sequence tags (ESTs) were isolated from 240 clones of a modified live Edwardsiella ictaluri vaccinated vs. sham-vaccinated channel catfish anterior kidney subtractive library. The transcription levels of the 57 ESTs in response to E. ictaluri vaccination were then evaluated by quantitative PCR (QPCR). Of the 57 ESTs, 43 were induced at least 2-fold higher in all three vaccinated fish compared to unvaccinated control fish. Of the 43 upregulated genes, five were consistently upregulated greater than 10-fold, including two highly upregulated (>20-fold) glycosyltransferase and Toll-like receptor 5. The transcriptional levels of GTPase 1, coatomer protein complex zeta 1, and type II arginine deiminase were consistently induced greater than 10-fold. MHC class I alpha chain and transposase were upregulated greater than 10-fold in two of the three vaccinated fish. The 43 upregulated genes also included 19 moderately upregulated (3-10-fold) and 17 slightly upregulated (2-3-fold). Our results suggest that subtractive cDNA hybridization and QPCR are powerful cost-effective techniques to identify differentially expressed genes in response to modified live E. ictaluri vaccination.