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

Riemerella anatipestifer UvrC is required for iron utilization, biofilm formation and virulence

UvrC is a subunit of excinuclease ABC, which mediates nucleotide excision repair (NER) in bacteria. Our previous studies showed that transposon Tn4531 insertion in the UvrC encoding gene Riean_1413 results in reduced biofilm formation by Riemerella anatipestifer strain CH3 and attenuates virulence of strain YZb1. In this study, whether R. anatipestifer UvrC has some biological functions other than NER was investigated. Firstly, the uvrC of R. anatipestifer strain Yb2 was in-frame deleted by homologous recombination, generating deletion mutant ΔuvrC, and its complemented strain cΔuvrC was constructed based on Escherichia coli - R. anatipestifer shuttle plasmid pRES. Compared to the wild-type (WT) R. anatipestifer strain Yb2, uvrC deleted mutant ΔuvrC significantly reduced biofilm formation, tolerance to H2O2- and HOCl-induced oxidative stress, iron utilization, and adhesion to and invasion of duck embryonic hepatocytes, but not its growth curve and proteolytic activity. In addition, animal experiments showed that the LD50 value of ΔuvrC in ducklings was about 13-fold higher than that of the WT, and the bacterial loads in ΔuvrC infected ducklings were significantly lower than those in Yb2-infected ducklings, indicating uvrC deletion in R. anatipestifer attenuated virulence. Taken together, the results of this study indicate that R. anatipestifer UvrC is required for iron utilization, biofilm formation, oxidative stress tolerance and virulence of strain Yb2, demonstrating multiple functions of UvrC.RESEARCH HIGHLIGHTSDeletion of uvrC in R. anatipestfer Yb2 significantly reduced its biofilm formation.uvrC deletion led to reduced tolerance to H2O2- and HOCl-induced oxidative stress.The iron utilization of uvrC deleted mutant was significantly reduced.The uvrC deletion in R. anatipestifer Yb2 attenuated its virulence.

Isolation, identification, and proteomic analysis of outer membrane vesicles of Riemerella anatipestifer SX-1

Riemerella anatipestifer, belonging to Weeksellaceae family Riemerella, is a bacterium that can infect ducks, geese, and turkeys, causing diseases known as duck infectious serositis, new duck disease, and duck septicemia. We collected diseased materials from ducks on a duck farm in China and then isolated and purified a strain of serotype 1 R. anatipestifer named SX-1. Animal experiments showed that SX-1 is a highly virulent strain with an LD50 value of 101 CFU/mL. The complete genome sequence was obtained. The complete genome sequence of R. anatipestifer SX-1 was 2,112,539 bp; 847 genes were involved in catalytic activity, and 445 genes were related to the cell membrane. The total length of the repetitive sequences was 8746 bp. Four CRISPR loci were predicted in R. anatipestifer strain SX-1, and 4 genomic islands were predicted. Concentration and ultra-high-speed centrifugation were used to extract the outer membrane vesicles of R. anatipestifer SX-1. The OMVs were extracted successfully. Particle size analysis revealed the size and abundance of particles: 147.4 nm, 94.9%; 293.6 nm, 1.1%; 327.2 nm, 1.1%; 397.2 nm, 0.3%; and 371.8 nm, 1.1%. The average size was 173.5 nm. Label-free proteomic technology was used to identify proteins in the outer membrane vesicles. ATCC 11845 served as the reference genome sequence, and 148 proteins were identified using proteomic analysis, which were classified into 5 categories based on their sources. Among them, 24 originated from cytoplasmic proteins, 4 from extracellular secreted proteins, 27 from outer membrane proteins, 10 from periplasmic proteins, and 83 from unknown sources. This study conducted a proteomic analysis of OMVs to provide a theoretical basis for the development of R. anatipestifer OMVs vaccines and adjuvants and lays the foundation for further research on the relationship between the pathogenicity of R. anatipestifer and OMVs.

Occludin and collagen IV degradation mediated by the T9SS effector SspA contributes to blood-brain barrier damage in ducks during Riemerella anatipestifer infection

Riemerella anatipestifer infection is characterized by meningitis with neurological symptoms in ducklings and has adversely affected the poultry industry. R. anatipestifer strains can invade the duck brain to cause meningitis and neurological symptoms, but the underlying mechanism remains unknown. In this study, we showed that obvious clinical symptoms, an increase in blood‒brain barrier (BBB) permeability, and the accumulation of inflammatory cytokines occurred after intravenous infection with the Yb2 strain but not the mutant strain Yb2ΔsspA, indicating that Yb2 infection can lead to cerebrovascular dysfunction and that the type IX secretion system (T9SS) effector SspA plays a critical role in this pathological process. In addition, we showed that Yb2 infection led to rapid degradation of occludin (a tight junction protein) and collagen IV (a basement membrane protein), which contributed to endothelial barrier disruption. The interaction between SspA and occludin was confirmed by coimmunoprecipitation. Furthermore, we found that SspA was the main enzyme mediating occludin and collagen IV degradation. These data indicate that R. anatipestifer SspA mediates occludin and collagen IV degradation, which functions in BBB disruption in R. anatipestifer-infected ducks. These findings establish the molecular mechanisms by which R. anatipestifer targets duckling endothelial cell junctions and provide new perspectives for the treatment and prevention of R. anatipestifer infection.

Sifting through the core-genome to identify putative cross-protective antigens against Riemerella anatipestifer

Infectious serositis of ducks, caused by Riemerella anatipestifer, is one of the main infectious diseases that harm commercial ducks. Whole-strain-based vaccines with no or few cross-protection were observed between different serotypes of R. anatipestifer, and so far, control of infection is hampered by a lack of effective vaccines, especially subunit vaccines with cross-protection. Since the concept of reverse vaccinology was introduced, it has been widely used to screen for protective antigens in important pathogens. In this study, pan-genome binding reverse vaccinology, an emerging approach to vaccine candidate screening, was used to screen for cross-protective antigens against R. anatipestifer. Thirty proteins were identified from the core-genome as potential cross-protective antigens. Three of these proteins were recombinantly expressed, and their immunoreactivity with five antisera (anti-serotypes 1, 2, 6, 10, and 11) was demonstrated by Western blotting. Our study established a method for high-throughput screening of cross-protective antigens against R. anatipestifer in silico, which will lay the foundation for the development of a cross-protective subunit vaccine controlling R. anatipestifer infection. KEY POINTS: • Pan-genome binding reverse vaccine approach was first established in R. anatipestifer to screen for subunit vaccine candidates. • Thirty potential cross-protective antigens against R. anatipestifer were identified by this method. • The reliability of the method was verified preliminarily by the results of Western blotting of three of these potential antigens.

Riemerella anatipestifer Endonuclease I displays enzymatic activity and is associated with bacterial virulence

Riemerella anatipestifer is an important pathogen of waterfowl, causing septicemic and exudative diseases. We previously reported that the R. anatipestifer AS87_RS02625 is a secretory protein of the type IX secretion system (T9SS). In this study, R. anatipestifer T9SS protein AS87_RS02625 was determined to be a functional Endonuclease I (EndoI), which has DNase and RNase activities. Optimal temperature and pH of the recombinant R. anatipestifer EndoI (rEndoI) to cleave λDNA were determined as 55-60 °C and 7.5 respectively. The DNase activity of the rEndoI was dependent on the presence of divalent metal ions. Presence of Mg²⁺ at a concentration range of 7.5-15 mM in the rEndoI reaction buffer displayed the highest DNase activity. In addition, the rEndoI displayed RNase activity to cleave MS2-RNA (ssRNA), either in the absence or presence of divalent cations Mg²⁺, Mn²⁺, Ca²⁺, Zn²⁺ and Cu²⁺. The DNase activity of the rEndoI was significantly enhanced by Mg²⁺, Mn²⁺ and Ca²⁺ but not Zn²⁺ and Cu²⁺. Moreover, we indicated that R. anatipestifer EndoI functioned on the bacterial adherence, invasion, in vivo survival and inducing inflammatory cytokines. These results indicate that the R. anatipestifer T9SS protein AS87_RS02625 is a novel EndoI, displays endonuclease activity and plays an important role in bacterial virulence.

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 Reveals that PhoP Regulates Pathogenicity in Riemerella anatipestifer

Duck infectious serositis, also known as Riemerella anatipestifer disease, infects domestic ducks, geese, and turkeys and wild birds. However, the regulatory mechanism of its pathogenicity remains unclear. The PhoPR two-component system (TCS) was first reported in Gram-negative bacteria in our previous research and was demonstrated to be involved in virulence and gene expression. Here, DNA affinity purification sequencing (DAP-seq) was applied to further explore the regulation of PhoPR in relation to pathogenicity in R. anatipestifer. A conserved motif was identified upstream of 583 candidate target genes which were directly regulated by PhoP. To further confirm the genes which are regulated by PhoR and PhoP, single-gene-deletion strains were constructed. The results of transcriptome analysis using next-generation RNA sequencing showed 136 differentially expressed genes (DEGs) between the ΔphoP strain and the wild type (WT) and 183 DEGs between the ΔphoR strain and the WT. The candidate target genes of PhoP were further identified by combining transcriptome analysis and DAP-seq, which revealed that the main direct regulons of PhoP are located on the membrane and PhoP is involved in regulating aerotolerance. Using the in vivo duck model, the pathogenicity of ΔphoP and ΔphoR mutants was found to be significantly lower than that of the WT. Together, our findings provide insight into the direct regulation of PhoP and suggest that phoPR is essential for the pathogenicity of R. anatipestifer. The gene deletion strains are expected to be candidate live vaccine strains of R. anatipestifer which can be used as ideal genetic engineering vector strains for the expression of foreign antigens. IMPORTANCE Riemerella anatipestifer is a significant pathogen with high mortality in the poultry industry that causes acute septicemia and infectious polyserositis in ducks, chickens, geese, and other avian species. Previously, we characterized the two-component system encoded by phoPR and found that R. anatipestifer almost completely lost its pathogenicity for ducklings when phoPR was deleted. However, the mechanism of PhoPR regulation of virulence in R. anatipestifer had not been deeply explored. In this study, we utilized DAP-seq to explore the DNA-binding sites of PhoP as a response regulator in the global genome. Furthermore, phoP and phoR were deleted separately, and transcriptomics analysis of the corresponding gene deletion strains was performed. We identified a series of directly regulated genes of the PhoPR two-component system. The duckling model showed that both PhoP and PhoR are essential virulence-related factors in R. anatipestifer.

An improved joint non-negative matrix factorization for identifying surgical treatment timing of neonatal necrotizing enterocolitis

Neonatal necrotizing enterocolitis is a severe neonatal intestinal disease. Timely identification of surgical indications is essential for newborns in order to seek the best time for treatment and improve prognosis. This paper attempts to establish an algorithm model based on multimodal clinical data to determine the features of surgical indications and construct an auxiliary diagnosis model. The proposed algorithm adds hypergraph constraints on the two modal data based on Joint Nonnegative Matrix Factorization (JNMF), aiming to mine the higher-order correlations of the two data features. In addition, the adjacency matrix of the two kinds of data is used as a network regularization constraint to prevent overfitting. Orthogonal and L1-norm regulations were introduced to avoid feature redundancy and perform feature selection, respectively, and confirmed 14 clinical features. Finally, we used three classifiers, random forest, support vector machine, and logistic regression, to perform binary classification of patients requiring surgery. The results show that when the features selected by the proposed algorithm model are classified by random forest, the area under the ROC curve is 0.8, which has high prediction accuracy.

Riemerella anatipestifer AS87_RS02955 Acts as a Virulence Factor and Displays Endonuclease Activity

Riemerella anatipestifer is an important bacterial pathogen in the global duck industry and causes heavy economic losses. In our previous study, we demonstrated that R. anatipestifer type IX secretion system components GldK and GldM, and the secretion protein metallophosphoesterase, acted as virulence factors. In this study, R. anatipestifer AS87_RS02955 was investigated for virulence and enzymatic activity properties. We constructed AS87_RS02955 mutation and complementation strains to assess bacterial virulence. In vivo bacterial loads showed a significantly reduced bacterial loads in the blood of ducks infected with mutant strain Yb2Δ02955, which was recovered in the blood of ducks infected with the complementation strain cYb2Δ02955, demonstrating that AS87_RS02955 was associated with virulence. Further studies showed AS87_RS02955 was a novel nonspecific endonuclease with no functionally conserved domain, but enzymatic activity toward DNA and RNA was indicated. DNase activity was activated by Zn²⁺, Cu²⁺, Mg²⁺, Ca²⁺, and Mn²⁺ ions but inhibited by ethylenediaminetetraacetic acid. RNase activity was independent of metal cations, but stimulated by Mg²⁺, Ca²⁺, and Mn²⁺. RAS87_RS02955 enzymatic activity was active across a broad pH and temperature range. Moreover, we identified four sites in rAS87_RS02955, F39, F92, I134, and F145, which were critical for enzymatic activity. In summary, we showed that R. anatipestifer AS87_RS02955 encoded a novel endonuclease with important roles in bacterial virulence. IMPORTANCE R. anatipestifer AS87_RS02955 was identified as a novel T9SS effector and displayed a nonspecific endonuclease activity in this study. The protein did not contain a conserved His-Asn-His motif structure, which is similar to the endonuclease from Prevotella sp. Its mutant strain Yb2Δ02955 demonstrated significantly attenuated virulence, suggesting AS87_RS02955 is an important virulence factor. Moreover, AS87_RS02955 displayed nonspecific endonuclease activity to cleave λ DNA and MS2 RNA, while four protein sites were critical for endonuclease activity. In conclusion, R. anatipestifer AS87_RS02955 plays important roles in bacterial virulence.

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.

Identification and Pathogenicity Analysis of the Pathogen Causing Spotted Spleen in Muscovy Duck

Since September 2020, the clinical symptoms of Muscovy duck spleen spots have appeared in Guangdong, Guangxi, Jiangxi, Hunan, Hubei, and other provinces, resulting in a large number of Muscovy duck deaths and great economic losses. The absence of the typical clinical symptoms caused by pathogenic microorganisms makes the cause of the spotted spleen a mystery. High-throughput sequencing results suggested that Riemerella anatipestifer (R. anatipestifer) may be the pathogen. Then, R. anatipestifer was regarded as the research target for isolation, identification, and pathogenicity assessment. After biochemical test, PCR amplification, and serotype determination, it was confirmed that the isolated strain CZG-1 was serotype 15 R. anatipestifer. Typical spotted spleen symptoms were observed after CZG-1 infection. Furthermore, drug sensitivity assays showed the similar drug-resistant spectrum of R. anatipestifer serotype 15 to other serotypes; for example, all test strains were resistant to polymyxin, gentamicin, and neomycin. The CZG-1 strain has high pathogenicity, and its lethal dose of 50% (LD50) is 35.122 CFU/ml. Virulence gene determination showed that the CZG-1 strain had at least five virulence genes, bioF, TSS9-1, TSS9-2, PncA, and 0373Right. Above all, this study identified and proved that the pathogen of spotted spleen in ducks was R. anatipestifer serotype 15, which caused death of ducks without the typical symptoms of bacterial infection. The results of this study enriched the knowledge of symptom after R. anatipestifer infection, provided a reference to the identification of the pathogen of spotted spleen, and provided theoretical basis for prevention and control of spotted spleen.

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.

Type IX secretion system effectors and virulence of the model Flavobacterium columnare strain MS-FC-4

Flavobacterium columnare causes columnaris disease in wild and cultured freshwater fish and is a major problem for sustainable aquaculture worldwide. The F. columnare type IX secretion system (T9SS) secretes many proteins and is required for virulence. The T9SS component GldN is required for secretion and for gliding motility over surfaces. Genetic manipulation of F. columnare is inefficient, which has impeded identification of secreted proteins that are critical for virulence. Here we identified a virulent wild-type F. columnare strain (MS-FC-4) that is highly amenable to genetic manipulation. This facilitated isolation and characterization of two deletion mutants lacking core components of the T9SS. Deletion of gldN disrupted protein secretion and gliding motility and eliminated virulence in zebrafish and rainbow trout. Deletion of porV disrupted secretion and virulence but not motility. Both mutants exhibited decreased extracellular proteolytic, hemolytic, and chondroitin sulfate lyase activities. They also exhibited decreased biofilm formation and decreased attachment to fish fins and to other surfaces. Using genomic and proteomic approaches, we identified proteins secreted by the T9SS. We deleted ten genes encoding secreted proteins and characterized the virulence of mutants lacking individual or multiple secreted proteins. A mutant lacking two genes encoding predicted peptidases exhibited reduced virulence in rainbow trout, and mutants lacking a predicted cytolysin showed reduced virulence in zebrafish and rainbow trout. The results establish F. columnare strain MS-FC-4 as a genetically amenable model to identify virulence factors. This may aid development of measures to control columnaris disease and impact fish health and sustainable aquaculture. IMPORTANCE: Flavobacterium columnare causes columnaris disease in wild and aquaculture-reared freshwater fish and is a major problem for aquaculture. Little is known regarding the virulence factors involved in this disease and control measures are inadequate. The type IX secretion system (T9SS) secretes many proteins and is required for virulence, but the secreted virulence factors are not known. We identified a strain of F. columnare (MS-FC-4) that is well suited for genetic manipulation. The components of the T9SS and the proteins secreted by this system were identified. Deletion of core T9SS genes eliminated virulence. Genes encoding ten secreted proteins were deleted. Deletion of two peptidase-encoding genes resulted in decreased virulence in rainbow trout, and deletion of a cytolysin-encoding gene resulted in decreased virulence in rainbow trout and zebrafish. Secreted peptidases and cytolysins are likely virulence factors and are targets for the development of control measures.

The Type 9 Secretion System Is Required for Flavobacterium johnsoniae Biofilm Formation

Flavobacterium johnsoniae forms biofilms in low nutrient conditions. Protein secretion and cell motility may have roles in biofilm formation. The F. johnsoniae type IX secretion system (T9SS) is important for both secretion and motility. To determine the roles of each process in biofilm formation, mutants defective in secretion, in motility, or in both processes were tested for their effects on biofilm production using a crystal violet microplate assay. All mutants that lacked both motility and T9SS-mediated secretion failed to produce biofilms. A porV deletion mutant, which was severely defective for secretion, but was competent for motility, also produced negligible biofilm. In contrast, mutants that retained secretion but had defects in gliding formed biofilms. An sprB mutant that is severely but incompletely defective in gliding motility but retains a fully functional T9SS was similar to the wild type in biofilm formation. Mutants with truncations of the gldJ gene that compromise motility but not secretion showed partial reduction in biofilm formation compared to wild type. Unlike the sprB mutant, these gldJ truncation mutants were essentially nonmotile. The results show that a functional T9SS is required for biofilm formation. Gliding motility, while not required for biofilm formation, also appears to contribute to formation of a robust biofilm.

A Riemerella anatipestifer Metallophosphoesterase That Displays Phosphatase Activity and Is Associated with Virulence

Riemerella anatipestifer is an important pathogen of waterfowl, causing septicemic and exudative diseases. In our previous study, we demonstrated that bacterial virulence and secretion proteins of the type IX secretion system (T9SS) mutant strains Yb2ΔgldK and Yb2ΔgldM were significantly reduced, in comparison to those of wild-type strain Yb2. In this study, the T9SS secretion protein AS87_RS00980, which is absent from the secretion proteins of Yb2ΔgldK and Yb2ΔgldM, was investigated by construction of gene mutation and complementation strains. The virulence assessment showed >1,000-fold attenuated virulence and significantly reduced bacterial loads in the blood of ducks infected with Yb2Δ00980, the AS87_RS00980 gene deletion mutant strain. Bacterial virulence was recovered in complementation strain cYb2Δ00980 Further study indicated that the T9SS secretion protein AS87_RS00980 is a metallophosphoesterase (MPPE), which displayed phosphatase activity and was cytomembrane localized. Moreover, the optimal reactive pH and temperature were determined to be 7.0 and 60°C, respectively, and the K_m and V _max were determined to be 3.53 mM and 198.1 U/mg. The rMPPE activity was activated by Zn²⁺ and Cu²⁺ but inhibited by Fe³⁺, Fe²⁺, and EDTA. There are five conserved sites, namely, N267, H268 H351, H389, and H391, in the metallophosphatase domain. Mutant proteins Y267-rMPPE and Y268-rMPPE retained 29.30% and 19.81% relative activity, respectively, and mutant proteins Y351-rMPPE, Y389-rMPPE, and Y391-rMPPE lost almost all MPPE activity. Taken together, these results indicate that the R. anatipestifer AS87_RS00980 gene encodes an MPPE that is a secretion protein of T9SS that plays an important role in bacterial virulence.IMPORTANCE Riemerella anatipestifer T9SS was recently discovered to be associated with bacterial gliding motility and secretion of virulence factors. Several T9SS genes have been identified, but no effector has been reported in R. anatipestifer to date. In this study, we identified the T9SS secretion protein AS87_RS00980 as an MPPE that displays phosphatase activity and is associated with bacterial virulence. The enzymatic activity of the rMPPE was determined, and the K_m and V _max were 3.53 mM and 198.1 U/mg, respectively. Five conserved sites were also identified. The AS87_RS00980 gene deletion mutant strain was attenuated >1,000-fold, indicating that MPPE is an important virulence factor. In summary, we identified that the R. anatipestifer AS87_RS00980 gene encodes an important T9SS effector, MPPE, which plays an important role in bacterial virulence.

Development of signature-tagged mutagenesis in Riemerella anatipestifer to identify genes essential for survival and pathogenesis

Riemerella anatipestifer causes epizootic infectious disease in ducks, geese, turkeys and other birds, and serious economic losses especially to the duck industry. However, little is known about the molecular basis of its pathogenesis. In this study, signature-tagged transposon mutagenesis based on Tn4351 was developed in R. anatipestifer to identify genes essential for survival and pathogenesis. Seventeen tagged Tn4351 random mutation libraries of the R. anatipestifer strain WJ4 containing 5100 mutants were screened for survive using a duckling infection model. Twenty mutants that could not be recovered from the infected ducklings, were identified, and 17 mutated genes were identified by inverse PCR or genome-walking PCR. Of these genes, FIP52_03215, FIP52_04350 and FIP52_09345, were inserted into two mutant strains, and FIP52_03215 and FIP52_03175 were found exclusively on the chromosome of serotype 1 R. anatipestifer strains. Twelve out of 17 genes encoding for proteins were predicted to be involved in amino acid, nucleotide, coenzyme, or lipid transport and metabolism, one gene was predicted to be involved in signal transduction, one gene was predicted to be involved in DNA replication, recombination and repair, the other three genes had an unknown function. Animal experiments showed that the virulence of mutants 16-284, 7-295, 24-231, 9-232 and 19-214 were significantly attenuated compared to that of the wild-type WJ4. Moreover, the median lethal dose of mutant 16-284 was greater than 10¹⁰ CFU, and its virulence to ducklings was partially restored when it was complemented with the shuttle expression plasmid pRES-FIP52_09345. The results in this study will be helpful to further study the molecular mechanisms of the pathogenesis of R. anatipestifer infection.

The Type IX Secretion System Is Required for Virulence of the Fish Pathogen Flavobacterium psychrophilum

Flavobacterium psychrophilum causes bacterial cold-water disease in wild and aquaculture-reared fish and is a major problem for salmonid aquaculture. The mechanisms responsible for cold-water disease are not known. It was recently demonstrated that the related fish pathogen, Flavobacterium columnare, requires a functional type IX protein secretion system (T9SS) to cause disease. T9SSs secrete cell surface adhesins, gliding motility proteins, peptidases, and other enzymes, any of which may be virulence factors. The F. psychrophilum genome has genes predicted to encode components of a T9SS. Here, we used a SacB-mediated gene deletion technique recently adapted for use in the Bacteroidetes to delete a core F. psychrophilum T9SS gene, gldN The ΔgldN mutant cells were deficient for secretion of many proteins in comparison to wild-type cells. Complementation of the mutant with wild-type gldN on a plasmid restored secretion. Compared to wild-type and complemented strains, the ΔgldN mutant was deficient in adhesion, gliding motility, and extracellular proteolytic and hemolytic activities. The ΔgldN mutant exhibited reduced virulence in rainbow trout and complementation restored virulence, suggesting that the T9SS plays an important role in the disease.IMPORTANCE Bacterial cold-water disease, caused by F. psychrophilum, is a major problem for salmonid aquaculture. Little is known regarding the virulence factors involved in this disease, and control measures are inadequate. A targeted gene deletion method was adapted to F. psychrophilum and used to demonstrate the importance of the T9SS in virulence. Proteins secreted by this system are likely virulence factors and targets for the development of control measures.

Development of a colloidal gold immunochromatographic strip for rapid detection of Riemerella anatipestifer in ducks

Riemerella anatipestifer is one of the major bacterial pathogens of ducks and causes significant economic losses in poultry agriculture. Usually, methods for detecting R. anatipestifer infection need specialized equipment and highly skilled personnel. In this study, a novel colloidal gold immunochromatographic strip was developed for rapid detection of R. anatipestifer in ducks. The monoclonal antibodies 2D5 and 2A6 against R. anatipestifer were used as colloidal gold-labeled protein and capture protein, respectively, to recognize the bacteria in tryptic soy broth medium culture and in hearts of infected ducks. The goat anti-mouse IgG antibody was labeled on nitrocellulose membrane as a control for C line. The labeling pH was optimized as 10.0, and the concentration of 2D5 labeled to colloidal gold particles was optimized as 18 μg/mL. The strip specifically detected serotypes 1, 2, and 10 R. anatipestifer strains and showed no cross-reaction with Escherichia coli, Salmonella enterica, and Pasteurella multocida strains. The sensitivity of the strip for detecting R. anatipestifer was 1.0 × 10⁶ colony forming unit. The strips remained stable for up to 8 mo at 4°C, and the detection can be completed within 15 min. The strip can detect R. anatipestifer in hearts of the ducks experimentally infected with R. anatipestifer but not infected with E. coli, which were also confirmed with bacterial isolation followed by multiplex polymerase chain reaction. These results suggested that the strips are reliable methods for identification of R. anatipestifer in laboratories and in duck farms.