Disruption of the M949_RS01915 gene changed the bacterial lipopolysaccharide pattern, pathogenicity and gene expression of Riemerella anatipestifer

Capsular polysaccharide determines the serotyping of Riemerella anatipestifer

IMPORTANCE

Riemerella anatipestifer (R. anatipestifer) is one of the most important veterinary pathogens with at least 21 serotypes. However, the exact polysaccharide(s) that determine R. anatipestifer serotype is still unknown. This study has provided a preliminary exploration of the relationship between capsular polysaccharides and serotyping in R. anatipestifer and suggests possible directions for further investigation of the genetic basis of serotypes in this bacterium.

Riemerella anatipestifer GldG is necessary for secretion of effectors by type IX secretion system

Riemerella anatipestifer secretes proteins through the type IX secretion system (T9SS). Recent studies have shown that the R. anatipestifer T9SS component proteins GldM and GldK also act as crucial virulence factors. In our previous study, the disruption of AS87_RS00460 gene, which encodes the predicted protein GldG, significantly reduced the bacterial virulence of R. anatipestifer wild-type strain Yb2, but the mechanism was unclear. In this study, we investigated the function of the GldG in bacterial virulence and protein secretion using the mutant strain Yb2ΔgldG and complementation strain cYb2ΔgldG. Our results demonstrate that the gldG gene encodes a gliding-motility-associated ABC transporter substrate-binding protein GldG, which was localized to the bacterial membrane in an immunoblotting analysis, and functions in the bacterium's adherence to and invasion of host cells and its survival in host blood. The resistance of mutant strain Yb2ΔgldG to complement-dependent killing was significantly reduced. Yb2ΔgldG displayed reduced gliding motility and deficient protein secretion. Label-free quantification (LFQ) with liquid chromatography-mass spectrometry (LC-MS) showed that 10 proteins with a conserved T9SS C-terminal domain were differentially secreted by Yb2ΔgldG and Yb2. The secretion levels of those 10 proteins were determined with immunoblotting, and the results were consistent with the LFQ LC-MS data. All of these effects were rescued by complementation with a plasmid encoding Yb2 gldG. Our results demonstrate that the R. anatipestifer gldG gene encodes the protein GldG, which is involved in bacterial virulence and protein secretion.

Genome-Wide Analysis and Characterization of the Riemerella anatipestifer Putative T9SS Secretory Proteins with a Conserved C-Terminal Domain

Riemerella anatipestifer is a major pathogenic agent of duck septicemic and exudative diseases. Recent studies have shown that the R. anatipestifer type IX secretion system (T9SS) acts as a crucial virulence factor. We previously identified two T9SS component proteins, GldK and GldM, and one T9SS effector metallophosphoesterase, which play important roles in bacterial virulence. In this study, 19 T9SS-secreted proteins that contained a conserved T9SS C-terminal domain (CTD) were predicted in R. anatipestifer strain Yb2 by searching for CTD-encoding sequences in the whole genome. The proteins were confirmed with a liquid chromatography-tandem mass spectrometry analysis of the bacterial culture supernatant. Nine of them were reported in our previous study. We generated recombinant proteins and mouse antisera for the 19 predicted proteins to confirm their expression in the bacterial culture supernatant and in bacterial cells. Western blotting indicated that the levels of 14 proteins were significantly reduced in the T9SS mutant Yb2ΔgldM culture medium but were increased in the bacterial cells. RT-qPCR indicated that the expression of these genes did not differ between the wild-type strain Yb2 and the T9SS mutant Yb2ΔgldM. Nineteen mutant strains were successfully constructed to determine their virulence and proteolytic activity, which indicated that seven proteins are associated with bacterial virulence, and two proteins, AS87_RS04190 and AS87_RS07295, are protease-activity-associated virulence factors. In summary, we have identified at least 19 genes encoding T9SS-secreted proteins in the R. anatipestifer strain Yb2 genome, which encode multiple functions associated with the bacterium's virulence and proteolytic activity. IMPORTANCE Riemerella anatipestifer T9SS plays an important role in bacterial virulence. We have previously reported nine R. anatipestifer T9SS-secreted proteins and clarified the function of the metallophosphoesterase. In this study, we identified 10 more secreted proteins associated with the R. anatipestifer T9SS, in addition to the nine previously reported. Of these, 14 proteins showed significantly reduced secretion into the bacterial culture medium but increased expression in the bacterial cells of the T9SS mutant Yb2ΔgldM; seven proteins were shown to be associated with bacterial virulence; and two proteins, AS87_RS04190 and AS87_RS07295, were shown to be protease-activity-associated virulence factors. Thus, we have demonstrated that multiple R. anatipestifer T9SS-secreted proteins function in virulence and proteolytic activity.

Riemerella anatipestifer T9SS Effector SspA Functions in Bacterial Virulence and Defending Natural Host Immunity

Riemerella anatipestifer is a major pathogenic agent of duck septicemic and exudative diseases. Recent studies have shown that the R. anatipestifer type IX secretion system (T9SS) is a crucial factor in bacterial virulence. The AS87_RS04190 protein was obviously missing from the secreted proteins of the T9SS mutant strain Yb2ΔgldM. A bioinformatic analysis indicated that the AS87_RS04190 protein contains a T9SS C-terminal domain sequence and encodes a putative subtilisin-like serine protease (SspA). To determine the role of the putative SspA protein in R. anatipestifer pathogenesis and proteolysis, we constructed two strains with an sspA mutation and complementation, respectively, and determined their median lethal doses, their bacterial loads in infected duck blood, and their adherence to and invasion of cells. Our results demonstrate that the SspA protein functions in bacterial virulence. It is also associated with the bacterial protease activity and has a conserved catalytic triad structure (Asp126, His158, and Ser410), which is necessary for protein function. The optimal reactive pH and temperature were determined to be 7.0 and 50°C, respectively, and K_m and V_max were determined to be 10.15 mM and 246.96 U/mg, respectively. The enzymatic activity of SspA is activated by Ca²⁺, Mg²⁺, and Mn²⁺ and inhibited by Cu²⁺ and EDTA. SspA degrades gelatin, fibrinogen, and bacitracin LL-37. These results demonstrate that SspA is an effector protein of T9SS and functions in R. anatipestifer virulence and its proteolysis of gelatin, fibrinogen, and bacitracin LL-37. IMPORTANCE In recent years, Riemerella anatipestifer T9SS has been reported to act as a virulence factor. However, the functions of the proteins secreted by R. anatipestifer T9SS are not entirely clear. In this study, a secreted subtilisin-like serine protease SspA was shown to be associated with R. anatipestifer virulence, host complement evasion, and degradation of gelatin, fibrinogen, and LL-37. The enzymatic activity of recombinant SspA was determined, and its K_m and V_max were 10.15 mM and 246.96 U/mg, respectively. Three conserved sites (Asp126, His158, and Ser410) are necessary for the protein's function. The median lethal dose of the sspA-deleted mutant strain was reduced >10,000-fold, indicating that SspA is an important virulence factor. In summary, we demonstrate that the R. anatipestifer AS87_RS04190 gene encodes an important T9SS effector, SspA, which plays an important role in bacterial virulence.

VasH Contributes to Virulence of Aeromonas hydrophila and Is Necessary to the T6SS-mediated Bactericidal Effect

Aeromonas hydrophila is a Gram-negative bacterium that is commonly distributed in aquatic surroundings and has been considered as a pathogen of fish, amphibians, reptiles, and mammals. In this study, a virulent strain A. hydrophila GD18, isolated from grass carp (Ctenopharyngodon idella), was characterized to belong to a new sequence type ST656. Whole-genome sequencing and phylogenetic analysis showed that GD18 was closer to environmental isolates, however distantly away from the epidemic ST251 clonal group. The type VI secretion system (T6SS) was known to target both eukaryotic and prokaryotic cells by delivering various effector proteins in diverse niches by Gram-negative bacteria. Genome-wide searching and hemolysin co-regulated protein (Hcp) expression test showed that GD18 possessed a functional T6SS and is conditionally regulated. Further analysis revealed that VasH, a σ54-transcriptional activator, was strictly required for the functionality of T6SS in A. hydrophila GD18. Mutation of vasH gene by homologous recombination significantly abolished the bactericidal property. Then the virulence contribution of VasH was characterized in both in vitro and in vivo models. The results supported that VasH not only contributed to the bacterial cytotoxicity and resistance against host immune cleaning, but also was required for virulence and systemic dissemination of A. hydrophila GD18. Taken together, these findings provide a perspective for understanding the VasH-mediated regulation mechanism and T6SS-mediated virulence and bactericidal effect of A. hydrophila.

XRE-Type Regulator BioX Acts as a Negative Transcriptional Factor of Biotin Metabolism in Riemerella anatipestifer

Biotin is essential for the growth and pathogenicity of microorganisms. Damage to biotin biosynthesis results in impaired bacterial growth and decreased virulence in vivo. However, the mechanisms of biotin biosynthesis in Riemerella anatipestifer remain unclear. In this study, two R. anatipestifer genes associated with biotin biosynthesis were identified. AS87_RS05840 encoded a BirA protein lacking the N-terminal winged helix-turn-helix DNA binding domain, identifying it as a group I biotin protein ligase, and AS87_RS09325 encoded a BioX protein, which was in the helix-turn-helix xenobiotic response element family of transcription factors. Electrophoretic mobility shift assays demonstrated that BioX bound to the promoter region of bioF. In addition, the R. anatipestifer genes bioF (encoding 7-keto-8-aminopelargonic acid synthase), bioD (encoding dethiobiotin synthase), and bioA (encoding 7,8-diaminopelargonic acid synthase) were in an operon and were regulated by BioX. Quantitative reverse transcription-PCR showed that transcription of the bioFDA operon increased in the mutant Yb2ΔbioX in the presence of excessive biotin, compared with that in the wild-type strain Yb2, suggesting that BioX acted as a repressor of biotin biosynthesis. Streptavidin blot analysis showed that BirA caused biotinylation of BioX, indicating that biotinylated BioX was involved in metabolic pathways. Moreover, as determined by the median lethal dose, the virulence of Yb2ΔbioX was attenuated 500-fold compared with that of Yb2. To summarize, the genes birA and bioX were identified in R. anatipestifer, and BioX was found to act as a repressor of the bioFDA operon involved in the biotin biosynthesis pathway and identified as a bacterial virulence factor. IMPORTANCE Riemerella anatipestifer is a causative agent of diseases in ducks, geese, turkeys, and various other domestic and wild birds. Our study reveals that biotin synthesis of R. anatipestifer is regulated by the BioX through binding to the promoter region of the bioF gene to inhibit transcription of the bioFDA operon. Moreover, bioX is required for R. anatipestifer pathogenicity, suggesting that BioX is a potential target for treatment of the pathogen. R. anatipestifer BioX has thus been identified as a novel negative regulator involved in biotin metabolism and associated with bacterial virulence in this study.

Molecular and serological characterization of Riemerella isolates associated with poultry in Australia

A total of 62 isolates of Riemerella-like organisms, originally isolated from Australian poultry (10 from chickens, 46 from ducks, five from unknown hosts and one vaccine strain), were included in this study. On the basis of two published polymerase chain reaction (PCR) assays that are reported to be specific for Riemerella anatipestifer, 51 of the isolates were identified as R. anatipestifer. Forty-six of these isolates had a detailed history and were sourced from ducks, while five were of unknown origin. The 11 remaining isolates failed to yield a positive reaction in either PCR with 10 originating from chickens and one from a duck. Amplification and sequencing of the 16S rRNA gene of these isolates identified the duck isolate as Moraxella lacunta. Phylogenetic analysis of the 10 chicken isolates identified one as R. columbina and the remaining nine isolates as Riemerella-like taxon 2. The 51 Australian R. anatipestifer isolates were assigned by gel diffusion test to serovars 1 (26 isolates), 6 (seven isolates), 8 (five isolates), 9 (two isolates), 13 (one isolate) and 14 (one isolate) while nine isolates gave no reaction to any antiserum. A commercial system was used to perform DNA fingerprinting using rep-PCR analysis, which revealed different clusters with a lack of a clear relationship between the clusters and the serovars.

Characterization of the Riemerella anatipestifer M949_RS00050 gene

Based on its causing ever-increasing heavy economic losses, Riemerella anatipestifer has been viewed as an important bacterial pathogen in the duck industry worldwide. However, the molecular mechanisms regarding its pathogenicity are poorly understood. In our previous study, we have built a random mutagenesis library of Riemerella anatipestifer CH3 using transposon Tn4351. In this study, we screened the library by determining bacterial median lethal dose in ducklings. A mutant strain showed about 376-fold attenuated virulence in comparison with the wild-type strain CH3 was obtained. Subsequently, the Tn4351 inserted gene was identified as M949_RS00050, which encodes a putative protein containing an outer membrane protein beta-barrel domain by genome walking and sequence analyses. Southern blot analysis indicated a single Tn4351 insertion in the CH3 chromosomal DNA. Inactivation of M949_RS00050 gene did not affect bacterial metabolic activity and the silver stained lipopolysaccharide pattern. However, the bacterial sensitivity to normal duck sera killing and bacterial hydrophobicity were dramatically enhanced in the M949_RS00050 gene inactivated mutant strain, compared to its wild-type strain CH3. Moreover, bacterial adherence and invasion abilities, bacterial capsular polysaccharide quantity, biofilm formation capacity and the bacterial virulence of the mutant strain were obviously decreased, compared to the wild-type strain CH3. Thus, our finding demonstrates that the M949_RS00050 gene functions on multiple bacterial biological properties and virulence in Riemerella anatipestifer.

New Perspectives on Galleria mellonella Larvae as a Host Model Using Riemerella anatipestifer as a Proof of Concept

Galleria mellonella larvae have been used as a host model to study interactions between pathogens and hosts for several years. However, whether the model is useful to interrogate Riemerella anatipestifer infection biology remained unknown. This study aimed to exploit the potential of G. mellonella larvae and reveal their limitations as a host model for R. anatipestifer infection. G. mellonella larvae were shown to be effective for virulence evaluations of different R. anatipestifer strains. Furthermore, the virulent strain R. anatipestifer CH-1 had a stronger ability to proliferate than the attenuated strain R. anatipestifer ATCC 11845 in both G. mellonella larvae and ducklings. Unconventionally it was shown that G. mellonella larvae cannot be used to evaluate the efficacy of antimicrobials and their combinations. Additionally, it was shown that certain virulence factors, such as OmpA (B739_0861), B739_1208, B739_1343, and Wza (B739_1124), were specific only for ducklings, suggesting that G. mellonella larvae must be cautiously used to identify virulence factors of R. anatipestifer Evaluation of heme uptake-related virulence genes, such as tonB1 and tonB2, required preincubating the strains with hemoglobin before infection of G. mellonella larvae since R. anatipestifer cannot obtain a heme source from G. mellonella larvae. In conclusion, this study revealed the applicability and limitations of G. mellonella as a model with which to study the pathogen-host interaction, particularly in the context of R. anatipestifer infection.

Development of a markerless gene deletion strategy using rpsL as a counterselectable marker and characterization of the function of RA0C_1534 in Riemerella anatipestifer ATCC11845 using this strategy

Riemerella anatipestifer is a gram-negative bacterium that mainly infects ducks, turkeys and other birds. In a previous study, we established a markerless mutation system based on the pheS mutant as a counterselectable marker. However, the toxic effect of p-Cl-Phe on the R. anatipestifer strain expressing the pheS mutant was weak on blood agar plates. In this study, we successfully obtained streptomycin-resistant derivative of R. anatipestifer ATCC11845 using 100 μg/mL streptomycin as a selection pressure. Then, we demonstrate that rpsL can be used as a counterselectable marker in the R. anatipestifer ATCC11845 rpsL mutant strain, namely, R. anatipestifer ATCCs. A suicide vector carrying wild-type rpsL, namely, pORS, was constructed and used for markerless deletion of the gene RA0C_1534, which encodes a putative sigma-70 family RNA polymerase sigma factor. Using rpsL as a counterselectable marker, markerless mutagenesis of RA0C_1534 was also performed based on natural transformation. R. anatipestifer ATCCsΔRA0C_1534 was more sensitive to H₂O₂-generated oxidative stress than R. anatipestifer ATCCs. Moreover, transcription of RA0C_1534 was upregulated under 10 mM H₂O₂ treatment and upon mutation of fur. These results suggest that RA0C_1534 is involved in oxidative stress response in R. anatipestifer. The markerless gene mutation method developed in this study provides new tools for investigation of the physiology and pathogenic mechanisms of this bacterium.

Riemerella anatipestifer GldM is required for bacterial gliding motility, protein secretion, and virulence

Riemerella anatipestifer is a major pathogenic agent of duck septicemic and exudative diseases. Genetic analyses suggest that this pathogen has a novel protein secretion system, known as the “type IX secretion system” (T9SS). We previously reported that deletion of the AS87_RS08465 gene significantly reduced the bacterial virulence of the R. anatipestifer strain Yb2, but the mechanism remained unclear. The AS87_RS08465 gene is predicted to encode the gliding motility protein GldM (GldM) protein, a key component of the T9SS complex. In this study, Western blotting analysis demonstrated that R. anatipestifer GldM was localized to the cytomembrane. Further study revealed that the adhesion and invasion capacities of the mutant strain RA2281 (designated Yb2ΔgldM) in Vero cells and the bacterial loads in the blood of infected ducks were significantly reduced. RNA-Seq and PCR analyses showed that six genes were upregulated and five genes were downregulated in the mutant strain Yb2ΔgldM and that these genes were mainly involved in the secretion of proteins. Yb2ΔgldM was also found to be defective in gliding motility and protein secretion. Liquid chromatography–tandem mass spectrometry analysis revealed that nine of the proteins had a conserved T9SS C-terminal domain and were differentially secreted by Yb2ΔgldM compared to Yb2. The complementation strain cYb2ΔgldM recovered the adhesion and invasion capacities in Vero cells and the bacterial loads in the blood of infected ducks as well as the bacterial gliding motility and most protein secretion in the mutant strain Yb2ΔgldM to the levels of the wild-type strain Yb2. Taken together, these results indicate that R. anatipestifer GldM is associated with T9SS and is important in bacterial virulence.

Riemerella anatipestifer gene AS87_08785 encodes a functional component, GldK, of the type IX secretion system

Riemerella anatipestifer is an important pathogen of waterfowl, causing septicemic and exudative diseases. In our previous study, we demonstrated that the deletion of the AS87_08785 gene significantly reduced the virulence of R. anatipestifer strain Yb2, but the mechanism remained unclear. In this study, R. anatipestifer strains with mutated or complemented AS87_08785 genes were constructed and characterized. A sequence analysis indicated that the AS87_08785 gene encoded a putative GldK protein, which localized to the membrane fraction in a western blotting analysis. The mutant strain Yb2ΔgldK displayed defective gliding motility on agar plates, reduced protease activity, and a reduced capacity for protein secretion. RNA sequencing and quantitative PCR analyses indicated that the transcription of 13 genes was downregulated in mutant Yb2ΔgldK. Animal experiments showed that the bacterial loads in the blood of Yb2ΔgldK-infected ducks were significantly reduced relative to those in wild-type strain Yb2 infected ducks. Most of the defective biological properties of the mutant were restored in complementation strain cYb2ΔgldK. Our results demonstrated that R. anatipestifer gene AS87_08785 encoded a component of the type IX secretion system, GldK, which functioned in bacterial gliding motility, protein secretion, and bacterial virulence.

The Riemerella anatipestifer M949_RS01035 gene is involved in bacterial lipopolysaccharide biosynthesis

In this study, the Riemerella anatipestifer mutant strain RA1062 was obtained by screening a random Tn4351 transposon mutant library. The mutant strain was unreactive with the anti-CH3 lipopolysaccharide monoclonal antibody, as demonstrated with an enzyme-linked immunosorbent assay, and its M949_RS01035 gene was inactivated. When cultured in trypticase soy broth, the late stage growth of the mutant RA1062 was significantly decreased. The mutant RA1062 was stained with crystal violet and presented a rough lipopolysaccharide phenotype, which differed from that of the wild-type strain CH3, suggesting that deletion of the M949_RS01035 gene resulted in defective lipopolysaccharide. Silver staining and Western blot analyses further confirmed that the RA1062 lipopolysaccharide had a deficiency in ladder-like binding pattern, as compared to lipopolysaccharide of the wild-type CH3 strain. In addition, the mutant RA1062 showed a higher susceptibility to complement-dependent killing, increased bacterial adhesion and invasion capacities to Vero cells, decreased blood bacterial loads, and attenuated virulence in infected ducks, when compared to the wild-type strain CH3. Moreover, RNA-Seq and real-time polymerase chain reaction analyses indicated that two genes were up-regulated and two were down-regulated in the mutant RA1062 genome. Furthermore, an animal protection experiment showed that immunization of ducks with inactivated RA1062 bacterin conferred effective cross-protection against challenge with the virulent R. anatipestifer serotypes 1, 2, and 10. This study presents evidence that the M949_RS01035 gene is involved in bacterial phenotype, virulence, and gene regulation in R. anatipestifer. The mutant strain RA1062 could be used as a cross-protective vaccine candidate.