GtxA from Gallibacterium anatis, a cytolytic RTX-toxin with a novel domain organisation

GWAS and comparative genomics reveal candidate antibiotic resistance genes in the avian pathogen Gallibacterium anatis for six widespread antibiotics

Gallibacterium anatis is a Gram-negative bacterium found in the respiratory and genital tracts of various animals, primarily poultry. Its association with septicemia and high mortality in poultry, along with the rise in multidrug-resistant strains, has amplified concerns. Recent research uncovered significant variability in antibiotic resistance profiles among G. anatis isolates from different Austrian flocks, and even between different organs within the same bird. In response, in the present study 60 of these isolates were sequenced and a combination of comparative genomics and genome-wide association study (GWAS) analysis was applied to understand the genetic variability of G. anatis across flocks and organs and to identify genes related to antibiotic resistance. The results showed that each flock harbored one or two strains of G. anatis with only a few strains shared between flocks, demonstrating a great variability among flocks. We identified genes associated with resistance to nalidixic acid, trimethoprim, cefoxitin, tetracycline, ampicillin and sulfamethoxazole. Our findings revealed that G. anatis may develop antibiotic resistance through two mechanisms: single-nucleotide mutations and the presence of specific genes that confer resistance. Unexpectedly, some tetracycline-resistant isolates lacked all known tetracycline-associated genes, suggesting the involvement of novel mechanisms of tetracycline resistance that require additional exploration. Furthermore, our functional annotation of resistance genes highlighted the citric acid cycle pathway as a potential key modulator of antibiotic resistance in G. anatis. In summary, this study describes the first application of GWAS analysis to G. anatis and provides new insights into the acquisition of multidrug resistance in this important avian pathogen.

Antibiotic resistance of Gallibacterium anatis biovar haemolytica isolates from chickens

Introduction

Gallibacterium anatis is an opportunistic bacteria inducing a range of clinical signs in poultry. Gallibacterium anatis strains show multidrug resistance to antibacterial substances. The purpose of this study was to examine the susceptibility of G. anatis biovar haemolytica isolates collected from the respiratory, reproduction and gastrointestinal tracts of chickens to different antibiotics from various classes.

Material and Methods

Gallibacterium anatis biovar haemolytica was identified in tracheal swab and gastrointestinal and reproductive tract tissue samples from Polish layer and broiler chicken flocks. Twenty six isolates with β-haemolysis capability, each from a different flock, obtained from the respiratory (n = 8), reproductive (n = 10) and gastrointestinal (n = 8) tracts were selected and identified by matrix-assisted laser desorption/ionisation-time-of-flight mass spectrometry after culturing. A PCR method targeting the 16S genes was used for verification of isolates. The isolates' susceptibility to 20 antimicrobials was evaluated using the disc diffusion method for 8 drugs and the dilution method for the other 12. In addition, they were tested for the presence of the GtxA, gyrB and flfA virulence genes and blaROB, aphA, tetB and tetH antibiotic resistance genes by PCR.

Results

The most prevalent antibiotic resistance was to tilmicosin, tylosin and quinupristin/dalfopristin (all 100%), erythromycin (96.2%), tetracycline (96.2%), linezolid (92.3%) and teicoplanin (92.3%). Universal susceptibility was to only one antibiotic, chloramphenicol. Statistically significant differences were found between the resistance of gastrointestinal tract strains and that of strains from other tracts to daptomycin, gentamicin, ciprofloxacin and colistin. The GtxA and gyrB genes were detected in 100% of isolates and flfA in 19.2%. The isolates most frequently contained tetB and less frequently tetH and aphA, and did not contain blaROB.

Conclusion

Most G. anatis biovar haemolytica isolates were resistant to many classes of antibiotics. Therefore, it is necessary and important to be vigilant for the occurrence of these bacteria and thorough in their diagnosis.

Gallibacterium anatis infection in poultry: a comprehensive review

Gallibacterium anatis (G. anatis), a member of the Pasteurellaceae family, normally inhabits the upper respiratory and lower genital tracts of poultry. However, under certain circumstances of immunosuppression, co-infection (especially with Escherichia coli or Mycoplasma), or various stressors, G. anatis caused respiratory, reproductive, and systemic diseases. Infection with G. anatis has emerged in different countries worldwide. The bacterium affects mainly chickens; however, other species of domestic and wild birds may get infected. Horizontal, vertical, and venereal routes of G. anatis infection have been reported. The pathogenicity of G. anatis is principally related to the presence of some essential virulence factors such as Gallibacterium toxin A, fimbriae, haemagglutinin, outer membrane vesicles, capsule, biofilms, and protease. The clinical picture of G. anatis infection is mainly represented as tracheitis, oophoritis, salpingitis, and peritonitis, while other lesions may be noted in cases of concomitant infection. Control of such infection depends mainly on applying biosecurity measures and vaccination. The antimicrobial sensitivity test is necessary for the correct treatment of G. anatis. However, the development of multiple drug resistance is common. This review article sheds light on G. anatis regarding history, susceptibility, dissemination, virulence factors, pathogenesis, clinical picture, diagnosis, and control measures.

Isolation and characterization of multidrug resistant Gallibacterium anatis biovar haemolytica strains from Polish geese and hens

Gallibacterium anatis biovar haemolytica is a bacterium that is frequently associated with infections of the reproductive tract and respiratory system in poultry. To assess the current prevalence and resistance profile of these bacteria in Poland, we collected and investigated 63 strains of Gallibacterium from diseased domestic poultry flocks including geese, laying hens, breeding hens and an ornamental hen. Detailed characterization of the isolates included the analysis of phenotypic antimicrobial resistance profiles and biofilm formation ability. Furthermore, the genetic background of 40 selected isolates regarding the presence of virulence and antimicrobial resistance genes and mobile genetic elements was determined. All investigated isolates were multidrug resistant, most prominently to β-lactams, fluoroquinolones, sulfonamides and macrolides. A total of 48 different resistance profiles were detected. Of all isolates, 50.8% formed a strong biofilm, where strains isolated from geese appeared to be better at biofilm formation than strains isolated from laying and breeding hens. Single-nucleotide polymorphism genotyping revealed that G. anatis bv. haemolytica strains are restricted in host and geographical distribution, and the geese isolates showed greater phylogenetic similarity. Whole genome sequencing enabled identification of 25 different antimicrobial resistance determinants. The most common resistance genes were tetB, blaROB-1, and blaTEM-1 which may be located on mobile genetic elements. All isolates possessed the toxin gene gtxA, and the fimbrial gene flfA was identified in 95% of strains. Our results indicated that all G. anatis bv. haemolytica isolates showed multidrug resistant phenotypes. Strains isolated from geese were characterized by the highest percentage of isolates resistant to selected antimicrobials, probably reflecting host-related adaptations.

Prevalence, Identification and Antibiotic Resistance of Gallibacterium anatis Isolates from Chickens in Poland

The Gram-negative bacterium Gallibacterium anatis is part of the normal avian respiratory, intestinal and reproductive tract microflora and can be transmitted horizontally and vertically. With the coexistence of other relevant factors, G. anatis becomes an opportunistic pathogen, economically damaging to the poultry industry. This bacterium's prevalence and molecular epidemiology were investigated, and the antimicrobial treatment options for G. anatis infection in chicken flocks in Poland were assessed. Tracheal samples from 182 flocks were collected between April 2022 and March 2023. The bacterial prevalence was determined by PCR targeting the gyrB gene and 16-23S rRNA. Gallibacterium anatis was identified by matrix-assisted laser desorption/ionisation-time-of-flight mass spectrometry (MALDI-TOF) after culturing and PCR amplification. Isolates' susceptibility to 11 antimicrobials was assessed with a disc diffusion test. Isolates were also tested for gyrB, GtxA and flfA virulence genes and blaROB, aphA, tetB and tetH antibiotic resistance genes by PCR. Forty-one flocks (22.5%) were positive through PCR. Antibiotic resistance was most frequently observed against tilmicosin, tylosin, enrofloxacin, amoxicillin, tetracycline and doxycycline. Multiple resistance to at least eight antibiotics occurred in 20% of isolates and to at least four in 100%. The occurrence of gyrB was noted in 100%, GtxA was detected in 89%, and flfA was found in 14% of positive samples. The tetB gene was present in 61.0% of positive samples, tetH was in 36.0%, aphA was in 16.7%, and blaROB was in 5.6%. Significant differences were found in G. anatis isolates related to the presence of the virulence genes GtxA and gyrB and the presence of resistance genes (p < 0.05) associated with resistance to tetracyclines, β-lactams and aminoglycosides. The continued rise in the resistance of G. anatis to a broadening range of antibiotics is a major problem for the poultry industry worldwide, as well as for public health. The findings of this study may expand the knowledge of the pathogenicity of G. anatis in poultry.

Sequence Analysis, Antibiogram Profile, Virulence and Antibiotic Resistance Genes of XDR and MDR Gallibacterium anatis Isolated from Layer Chickens in Egypt

Background

Gallibacterium anatis is incriminated frequently in severe economic losses and mortalities in the poultry industry. This study aimed to detect the prevalence of G. anatis in layer chickens, sequence analysis, the antibiogram profiles, and PCR screening of virulence determinants and antibiotic resistance genes.

Methods

Accordingly, 300 samples (tracheal swabs, ovary and oviduct, and lung) were randomly collected from 100 diseased layer chickens from private commercial layer farms at Elsharkia Governorate, Egypt. The bacteriological examination was carried out. The retrieved isolates were tested for 16S rRNA-23S rRNA gene sequencing, antibiogram profiling, PCR screening of virulence (gtxA, fifA, and gyrB), and antibiotic resistance genes (bla_ROB, aphA1, tetB, and tetH).

Results

The prevalence of G. anatis was 25% in the examined diseased layer chickens. The sequence analyses emphasized that the tested strains derived from a common ancestor and exhibited a notable genetic similarity with other G. anatis strains from USA, China, and Denmark. The isolated G. anatis strains were highly resistant to sulfamethoxazole-trimethoprim, oxytetracycline, penicillin, ampicillin, kanamycin, neomycin, and erythromycin. The PCR revealed that the retrieved G. anatis strains carried gtxA, gyrB, and fifA virulence genes with a prevalence of 100%, 100%, and 38.3%, respectively. Approximately 30.1% of the retrieved G. anatis isolates were XDR to six antimicrobial classes and harbored bla_ROB, aphA1, and tetB resistance genes. Moreover, 20.5% of the isolated G. anatis strains were MDR to three different classes and carried bla_ROB and tetH resistance genes.

Conclusion

Briefly, this study emphasized the existence of XDR and MDR G. anatis strains in poultry. Florfenicol and norfloxacin displayed a promising antimicrobial effect against the emerging XDR and MDR G. anatis in poultry. The emergence of XDR and MDR G. anatis is considered a public health alarm.

A putative siderophore receptor of Gallibacterium anatis 12656-12 under Fur control also binds hemoglobin

Pasteurellaceae family members obtain iron directly from host proteins or through siderophore-dependent mechanisms. Although Gallibacterum anatis expresses different virulence factors, its response to growth under iron restriction is unknown. G. anatis cultured in the presence of 2,2′-dipyridyl, up-expressed an approximately 65 kDa protein and repressed the expression of a 70 kDa protein. MALDI-TOF analysis of those proteins indicated homology with CirA (65 kDa), a protein involved in iron-siderophore acquisition in Mannheimia succinoproducens and a TonB-dependent receptor (70 kDa protein), a protein that binds chicken hemoglobin; however, G. anatis siderophore production was not detected by chromo azurol S (CAS)-BHI agar determination. This putative G. anatis siderophore receptor is under Fur control, but not the hemoglobin binding protein, as observed in G. anatis 12656-12 fur mutant (Ω fur 126.13) grown in the presence or not of 2,2′-dipyridyl. The addition of FeCl₃ to the culture medium diminished the growth and biofilm production in approximately 30% and 35%, respectively, in the wild-type strain, but the growth of Ω fur 126.13 strain was not affected and biofilm production increased in 35%. G. anatis Ω fur 126.13 presented lower virulence when it was inoculated to 35-day-old chickens in comparison to the wild-type strain. The induction of more than one iron uptake mechanism could benefit pathogenic microorganisms such as Gallibacterium.

Respiratory and Reproductive Impairment of Commercial Layer Chickens After Experimental Infection with Gallibacterium anatis Biovar haemolytica

The prevalence of Gallibacterium anatis in poultry production has increased over the last two decades. However, only a few studies have explored the pathogenicity of this bacterium in commercial layer chickens. This trial studied the aspects of the pathogenicity of a Gallibacterium anatis biovar haemolytica local Egyptian isolate (previously registered as strain B14 with GenBank accession no. KJ026147). We used 500 base pairs of a 16S ribosomal RNA gene and the 16S-23S ribosomal RNA intergenic spacer, partial sequence in an experimental infection trial in commercial White Shaver layer chickens aged 19 wk. The hens were divided into three groups of 40 birds each. The hens in Groups 1 and 2 were experimentally infected through the intranasal (IN) and intravenous (IV) routes, respectively, with a dose of 0.2 ml/bird containing 1.2 × 109 colony-forming units/ml. In contrast, Group 3 was kept as a noninfected control group. Both IN and IV infections resulted in a delayed egg laying for 1 wk and a significant (P ≤ 0.05) drop in egg production by 7.81% and 10.28% compared with the control group over 7 wk. Severe lesions in the form of hemorrhagic pneumonia, catarrhal tracheitis, ovarian follicle and oviductal regression, and septicemia were evident on necropsy, demonstrating the pathogenicity of G. anatis as a primary pathogen.

Immune Suppression Induced by Gallibacterium anatis GtxA During Interaction with Chicken Macrophage-Like HD11 Cells

The RTX toxin GtxA expressed by Gallibacterium anatis biovar haemolytica has been proposed a major virulence factor during disease manifestations in the natural host, the chicken. To better understand the role of GtxA in the pathogenesis of G. anatis, we compared the GtxA expressing wildtype strain with its isogenic ∆gtxA mutant that was unable to express GtxA during exposure to chicken macrophage-like HD11 cells. From adhesion and invasion assays, we showed that GtxA appears to promote adhesion and invasion of HD11 cells. By using quantitative RT-PCR, we also demonstrated that the G. anatis expressing GtxA induced a mainly anti-inflammatory (IL-10) host cell response as opposed to the pro-inflammatory (IL-1β, IL-6 and TNF-α) response induced by the GtxA deletion mutant. Interestingly, these results, at least partly, resemble recent responses observed from spleen tissue of chickens infected with the same two bacterial strains. The effect of the GtxA toxin on the type of cell death was less clear. While GtxA clearly induced cell death, our efforts to characterize whether this was due to primarily necrosis or apoptosis through expression analysis of a broad range of apoptosis genes did not reveal clear answers.

The role of GtxA during Gallibacterium anatis infection of primary chicken oviduct epithelial cells

Gallibacterium anatis (G. anatis), one of the major pathogens causing reproductive tract disorders in laying hens, leads to a reduction in egg production and increased mortality, caused by either single or mixed infections with other pathogens. As a specific virulence factor of G. anatis, the role of GtxA in layers' salpingitis remains unclear. In this study, we explored the effect of GtxA on G. anatis infection by comparing wild strain Yu-PDS-RZ-1-SLG (RZ) and its GtxA deleted counterpart RZΔgtxA in primary chicken oviduct epithelial cells (COEC). Their adherence, invasion, cytoxicity, and ability to induce apoptosis and and cytokine secretion were evaluated and the cytotoxicity and cytokine secretion of the recombinant GtxA protein and its N-terminal adenylate cyclase and C-terminal RTX hemolysin domain were also analyzed. We found that the adhesion ability of RZΔgtxA was significantly lower than that of parental strain RZ, and its toxicity to COEC was weakened; Meanwhile, apoptosis was inhibited and the expression of IL-6, IL-2, TNF-α and IFN-γ were dramatically reduced in COEC infected by RZΔgtxA. In contrast, the recombinant protein GtxA inhibited the proliferation of oviduct cells and induced obvious cytotoxicity, and the expression of IL-6, TNF-α and IFN-γ were up-regulated in COEC interacted with recombinant proteins. Our study indicates that GtxA promotes G. anatis adherence to cells, changes cells permeability and expression of inflammatory factors, resulting in cell damage and apoptosis.

First Report of Isolation of Gallibacterium anatis from Layer Chickens in Morocco with Decrease in Laying Performance

Gallibacterium is a genus of the family of Pasteurellaceae. It is well known as a commensal inhabitant of the respiratory and reproductive tract of healthy chickens. But in the last years, Gallibacterium anatis is increasingly reported in field cases with a decrease in laying performance due to infections of the reproductive tract. The aim of the present study was to investigate the implication of G. anatis infection in layer flocks facing a decrease in laying performance in Morocco. Birds were received from five different laying hen farms in two regions in Morocco showing a drop of egg production. Necropsy revealed 46.1 % (24/52) of sampled birds showed variable lesions in ovaries, salpinx, and trachea. In fact, 24 birds were affected by salpingitis, 18 by oophoritis, and 11 birds by atrophy of ovaries. Furthermore, tracheitis was observed in 24 birds. Bacteriological investigation was done from different organs, and G. anatis was found in ovaries (n = 20), trachea (n = 17), and cloaca (n = 3). Identification was based on growth morphology, Gram staining, and biochemical properties. Additionally, polymerase chain reaction test using specific primers for the genus identification was carried out. All isolates showed bands of 925 bp specific for G. anatis expressing the virulent toxin GtxA. Antibiotic resistance testing was performed and revealed that isolates were sensitive to enrofloxacin, florfenicol, and gentamycin but resistant to ampicillin, erythromycin, oxytetracycline, and sulfamethoxazole-trimethoprim. The present study is the first report of G. anatis in Morocco, demonstrating the need for further epidemiologic investigations as well as in regard to antibiotic resistance development.

Etiology, epidemiology, pathology, and advances in diagnosis, vaccine development, and treatment of Gallibacterium anatis infection in poultry: a review

Gallibacterium anatis is a Gram-negative bacterium of the Pasteurellaceae family that resides normally in the respiratory and reproductive tracts in poultry. It is a major cause of oophoritis, salpingitis, and peritonitis, decreases egg production and mortality in hens thereby severely affecting animal welfare and overall productivity by poultry industries across Europe, Asia, America, and Africa. In addition, it has the ability to infect wider host range including domesticated and free-ranging avian hosts as well as mammalian hosts such as cattle, pigs and human. Evaluating the common virulence factors including outer membrane vesicles, fimbriae, capsule, metalloproteases, biofilm formation, hemagglutinin, and determining novel factors such as the RTX–like toxin GtxA, elongation factor-Tu, and clustered regularly interspaced short palindromic repeats (CRISPR) has pathobiological, diagnostic, prophylactic, and therapeutic significance. Treating this bacterial pathogen with traditional antimicrobial drugs is discouraged owing to the emergence of widespread multidrug resistance, whereas the efficacy of preventing this disease by classical vaccines is limited due to its antigenic diversity. It will be necessary to acquire in-depth knowledge on important virulence factors, pathogenesis and, concerns of rising antibiotic resistance, improvised treatment regimes, and novel vaccine candidates to effectively tackle this pathogen. This review substantially describes the etio-epidemiological aspects of G. anatis infection in poultry, and updates the recent development in understanding the pathogenesis, organism evolution and therapeutic and prophylactic approaches to counter G. anatis infection for safeguarding the welfare and health of poultry.

Hydrostatic Filtration Enables Large-Scale Production of Outer Membrane Vesicles That Effectively Protect Chickens against Gallibacterium anatis

Gallibacterium anatis is a Gram-negative opportunistic avian pathogen representing an emerging threat to poultry meat and egg production worldwide. To date, no vaccine able to effectively prevent the morbidity associated with G. anatis infections has been developed yet. Our group previously reported that inoculation of different combinations of G. anatis outer membrane vesicles (OMVs), FlfA and GtxA-N proteins is effective in preventing lesions caused by G. anatis infections in layer chickens. Here we report the testing of the efficacy as vaccine prototypes of G. anatis OMVs isolated by hydrostatic filtration, a simple technique that allows the cost-effective isolation of high yields of OMVs. Layer chickens were immunized with OMVs alone or in combination with FlfA and/or GtxA-N proteins. Subsequent challenge with a heterologous G. anatis strain showed that immunization with OMVs alone could significantly reduce the lesions following a G. anatis infection. A second study was carried out to characterize the dose-response (0.25, 2.5 and 25 µg) relationship of G. anatis OMVs as immunogens, showing that 2.5 μg of OMVs represent the optimal dose to elicit protection in the immunized animals after a similar challenge. Additionally, administration of ≥2.5 μg of G. anatis OMVs induced specific IgY titers and possibly vertical transfer of immunity.

GtxA is a virulence factor that promotes a Th2-like response during Gallibacterium anatis infection in laying hens

GtxA, a leukotoxic RTX-toxin, has been proposed a main virulence factor of Gallibacterium anatis. To evaluate the impact of GtxA during infection, we experimentally infected laying hens with a G. anatis wild-type (WT) strain and its isogenic gtxA deletion mutant (ΔgtxA), respectively, and monitored the birds during a 6 day period. Birds inoculated with ΔgtxA had significantly reduced gross lesions and microscopic changes compared to the birds inoculated with the WT strain. To assess the host response further, we quantified the expression of pro-inflammatory cytokines and apoptosis genes by RT-qPCR. In the ovarian tissue, the expression levels of IL-4 and TNF-α were significantly lower in the ΔgtxA group compared to the WT group, while IL-6 and IL-10 levels appeared similar in the two groups. In the spleen tissue of ΔgtxA infected chickens, IL-4 expression was also lower compared to the WT infected chickens. The results indicated that GtxA plays a key role in an acute cytokine-mediated Th2-like response against G. anatis infection in the ovary tissue. The pro-inflammatory response in the ovary tissue of birds inoculated with ΔgtxA mutant was thus significantly lower than the wild-type response. This was, at least partly, supported by the apoptosis gene expression levels, which were significantly higher in the ΔgtxA mutant compared to the wild-type infected chickens. In conclusion, GtxA clearly plays an important role in the pathogenesis of G. anatis infections in laying hens. Further investigations into the specific factors regulating the host response is however needed to provide a more complete understanding of the bacteria-host interaction.

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

Exotoxins play a central role in the pathologies caused by most major bacterial animal pathogens. The large variety of vertebrate and invertebrate hosts in the animal kingdom is reflected by a large variety of bacterial pathogens and toxins. The group of repeats in the structural toxin (RTX) toxins is particularly abundant among bacterial pathogens of animals. Many of these toxins are described as hemolysins due to their capacity to lyse erythrocytes in vitro. Hemolysis by RTX toxins is due to the formation of cation-selective pores in the cell membrane and serves as an important marker for virulence in bacterial diagnostics. However, their physiologic relevant targets are leukocytes expressing β2 integrins, which act as specific receptors for RTX toxins. For various RTX toxins, the binding to the CD18 moiety of β₂ integrins has been shown to be host specific, reflecting the molecular basis of the host range of RTX toxins expressed by bacterial pathogens. Due to the key role of RTX toxins in the pathogenesis of many bacteria, antibodies directed against specific RTX toxins protect against disease, hence, making RTX toxins valuable targets in vaccine research and development. Due to their specificity, several structural genes encoding for RTX toxins have proven to be essential in modern diagnostic applications in veterinary medicine.

Specific Chicken Egg Yolk Antibody Improves the Protective Response against Gallibacterium anatis Infection

Gallibacterium anatis is a pathogen associated with peritonitis and salpingitis in chickens and other avian species. Novel safety prevention strategies are urgently needed because of widespread multidrug resistance and antigenic diversity. The objective of this study was to produce a specific chicken egg yolk antibody and evaluate its protective response against a G. anatis infection model in 4-week-old chicks. Enzyme-linked immunosorbent assays showed that hens immunized with the recombinant N terminus of Gallibacterium toxin A (GtxA-N) had significantly increased antibody titers against GtxA-N in serum and egg yolk IgY. Western blotting showed that IgY antibody had specificity against GtxA-N in the egg yolks of immunized hens. The growth of G. anatis in brain heart infusion (BHI) broth and agar was significantly inhibited by the GtxA-N-specific IgY antibody. The protective effects of the specific IgY antibody were evaluated in G. anatis-infected chicks after intramuscular injection (10 mg/ml). The anti-GtxA-N antibody titers in the sera of G. anatis-challenged chicks following an injection of specific IgY antibody were significantly higher than those of the control and the nonspecific IgY groups, but lower lesion scores for the peritoneum, liver, and duodenum were found after specific IgY antibody treatment. The results from this study suggest that the GtxA-N-specific IgY antibody could potentially improve the protective response against G. anatis infection in chicks.

Vaccination with outer membrane vesicles and the fimbrial protein FlfA offers improved protection against lesions following challenge with Gallibacterium anatis

Gallibacterium anatis is an opportunistic poultry pathogen belonging to the Pasteurellaceae family. It has been shown to cause oophoritis, salpingitis and peritonitis in hens, as well as being associated with reduced semen quality in cockerels. Widespread multidrug resistance and substantial antigenic variation among strains of Gallibacterium anatis is a major constraint to treatment with antimicrobials and prevention of infection by vaccination. Novel vaccine strategies targeting G. anatis are therefore necessary. Outer membrane vesicles (OMVs) are nanosized vesicles formed from the outer membrane of Gram-negative bacteria. These vesicles have shown promising potential as both adjuvants and as vaccine candidates against numerous bacterial species. A high vesiculating mutant of G. anatis (G. anatis ΔtolR) has previously been made, enabling production of OMVs in large scale. In this study, we elucidated the potential of G. anatis ΔtolR OMVs as adjuvant for the conserved antigens GtxA-N (the N-terminal part of the RTX like toxin Gallibacterium toxin A) and FlfA (F17-like fimbria), as well as evaluated if combinations of OMVs together with antigens could facilitate cross-protective immunity against three different strains of G. anatis. We showed that ΔtolR OMVs function as an adjuvant for GtxA-N by inducing antigen specific antibody production. However, OMVs in combination with GtxA-N failed to induce protection against lesions after challenge infection. In contrast, vaccination with OMVs in combination with FlfA protected against lesions, especially in the salpinx, caused by two diverse strains of G. anatis, thereby indicating a cross-protective potential. No protection against the third G. anatis strain 7990 could be obtained in any of the experimental settings. In conclusion, ΔtolR OMVs and FlfA could serve as potential future vaccine components againt G. anatis.

Detection of Gallibacterium anatis by TaqMan fluorescent quantitative PCR

To better understand the prevalence of Gallibacterium anatis in different poultry species, a rapid and accurate method was developed to detect G. anatis using a TaqMan fluorescent quantitative polymerase chain reaction (qPCR). Specific primers and a TaqMan probe were designed based on the reference gtxA gene sequence. The qPCR standard curve showed a good linear relationship, and the method showed good reproducibility, sensitivity, and specificity, indicating its suitability for G. anatis identification and quantitative analysis. A comparison of the detection results in 160 clinical swab samples showed that the detection rate (54.4%) of the qPCR for G. anatis was better than that of two conventional methods: gyrB gene-based qPCR for G. anatis (51.9%) and culture-based identification (34.4%). G. anatis was detected in layer chicken (77.3%), Silkie chicken (72.7%), and duck (27.1%) with relatively high detection rates, whereas dove (8.8%) and quail (3.0%) showed lower detection rates, indicating the different prevalence of G. anatis in different fowl species.

Immunogenic and protective efficacy of recombinant protein GtxA-N against Gallibacterium anatis challenge in chickens

Gallibacterium anatis is a major cause of reproductive tract infections in chickens. Here, we aimed to evaluate the efficacy of the recombinant protein GtxA-N at protecting hens, by addressing three objectives; (i) evaluating the antibody response following immunization (ii) scoring and comparing lesions, following challenge with G. anatis, in immunized and non-immunized hens and (iii) investigating if the anti-GtxA-N antibody titre in individual hens correlated with the observed lesions. Two consecutive experiments were performed in hens. In the first experiment hens were immunized with GtxA-N on day 0 and day 14, infected with G. anatis on day 28 and euthanized on day 56. The GtxA-N antibody response was assessed in pooled serum samples throughout the experiment, using an indirect enzyme-linked immunosorbent assay (ELISA). In the second experiment the GtxA-N antibody titres were assessed in individual hens before and after immunization. Subsequently, the hens were inoculated with G. anatis and finally all hens where euthanized and submitted for post mortem examination 48 h after inoculation. Immunization elicited strong antibody responses that lasted at least 8 weeks (P < .0001). The individual antibody titres observed in response to immunization varied considerably among hens (range: 174,100-281,500). Lesion scores following G. anatis infection were significantly lower in immunized hens compared to non-immunized hens (P = .004). Within the immunized group, no correlation was found between the individual antibody titres and the lesion scores. This study clearly demonstrated GtxA-N as a vaccine antigen able of inducing protective immunity against G. anatis.

Bacterial determinants of importance in the virulence of Gallibacterium anatis in poultry

Gallibacterium anatis, a member of the Pasteurellaceae family, constitute a part of the normal micro-flora of the upper respiratory tract and the lower genital tract in chickens. However, increasing evidence indicate that G. anatis is also associated with a wide range of pathological changes, particularly in the reproductive organs, which leads to decreased egg production, lowered animal welfare and increased mortality. As a recently defined opportunistic pathogen limited focus has been placed on the pathogenesis and putative virulence factors permitting G. anatis to cause disease. One of the most studied virulence determinants is a large RTX-like toxin (GtxA), which has been demonstrated to induce a strong leukotoxic effect on avian macrophages. A number of fimbria of different sizes and shapes has been described. Particularly fimbriae belonging to the F17-like family appears to be common in a diverse selection of G. anatis strains. Mutants lacking the FlfA fimbria were severely attenuated in experimentally infected chickens. Additional characteristics including the ability to express capsular material possibly involved in serum resistance; secretion of metalloproteases capable of degrading immunoglobulins, and hemagglutinins, which may promote biofilm formation are all factors likely linked to the virulence of G. anatis. A major advantage for the study of how G. anatis interact with its host is the ability to perform biologically relevant experimental infections where natural routes of exposure allows reproduction of lesions observed during spontaneous infections. This review summarizes the current understanding of the G. anatis pathogenesis and discusses the contribution of the established and putative virulence factors described for this bacterium to date.

Chaperone-usher fimbriae in a diverse selection of Gallibacterium genomes

Background

Fimbriae are bacterial cell surface organelles involved in the pathogenesis of many bacterial species, including Gallibacterium anatis, in which a F17-like fimbriae of the chaperone-usher (CU) family was recently shown to be an important virulence factor and vaccine candidate. To reveal the distribution and variability of CU fimbriae 22 genomes of the avian host-restricted bacteria Gallibacterium spp. were investigated. Fimbrial clusters were classified using phylogeny-based and conserved domain (CD) distribution-based approaches. To characterize the fimbriae in depth evolutionary analysis and in vitro expression of the most prevalent fimbrial clusters was performed.

Results

Overall 48 CU fimbriae were identified in the genomes of the examined Gallibacterium isolates. All fimbriae were assigned to γ4 clade of the CU fimbriae of Gram-negative bacteria and were organized in four-gene clusters encoding a putative major fimbrial subunit, a chaperone, an usher and a fimbrial adhesin. Five fimbrial clusters (Flf-Flf4) and eight conserved domain groups were defined to accommodate the identified fimbriae. Although, the number of different fimbrial clusters in individual Gallibacterium genomes was low, there was substantial amino acid sequence variability in the major fimbrial subunit and the adhesin proteins. The distribution of CDs among fimbrial clusters, analysis of their flanking regions, and evolutionary comparison of the strains revealed that Gallibacterium fimbrial clusters likely underwent evolutionary divergence resulting in highly host adapted and antigenically variable fimbriae. In vitro, only the fimbrial subunit FlfA was expressed in most Gallibacterium strains encoding this protein. The absence or scarce expression of the two other common fimbrial subunits (Flf1A and Flf3A) indicates that their expression may require other in vitro or in vivo conditions.

Conclusions

This is the first approach establishing a systematic fimbria classification system within Gallibacterium spp., which indicates a species-wide distribution of γ₄ CU fimbriae among a diverse collection of Gallibacterium isolates. The expression of only one out of up to three fimbriae present in the individual genomes in vitro suggests that fimbriae expression in Gallibacterium is highly regulated. This information is important for future attempts to understand the role of Gallibacterium fimbriae in pathogenesis, and may prove useful for improved control of Gallibacterium infections in chickens.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1093) contains supplementary material, which is available to authorized users.

In silico prediction of Gallibacterium anatis pan-immunogens

The Gram-negative bacterium Gallibacterium anatis is a major cause of salpingitis and peritonitis in commercial egg-layers, leading to reduced egg production and increased mortality. Unfortunately, widespread multidrug resistance and antigenic diversity makes it difficult to control infections and novel prevention strategies are urgently needed. In this study, a pan-genomic reverse vaccinology (RV) approach was used to identify potential vaccine candidates. Firstly, the genomes of 10 selected Gallibacterium strains were analyzed and proteins selected on the following criteria; predicted surface-exposure or secretion, none or one transmembrane helix (TMH), and presence in six or more of the 10 genomes. In total, 42 proteins were selected. The genes encoding 27 of these proteins were successfully cloned in Escherichia coli and the proteins expressed and purified. To reduce the number of vaccine candidates for in vivo testing, each of the purified recombinant proteins was screened by ELISA for their ability to elicit a significant serological response with serum from chickens that had been infected with G. anatis. Additionally, an in silico prediction of the protective potential was carried out based on a protein property prediction method. Of the 27 proteins, two novel putative immunogens were identified; Gab_1309 and Gab_2312. Moreover, three previously characterized virulence factors; GtxA, FlfA and Gab_2156, were identified. Thus, by combining the pan-genomic RV approach with subsequent in vitro and in silico screening, we have narrowed down the pan-proteome of G. anatis to five vaccine candidates. Importantly, preliminary immunization trials indicated an in vivo protective potential of GtxA-N, FlfA and Gab_1309.

Assessing pathogenicity of Gallibacterium anatis in a natural infection model: the respiratory and reproductive tracts of chickens are targets for bacterial colonization

Two separate bird trials were performed to establish a reliable route of infection for Gallibacterium anatis in chickens, comparing intranasal (i.n.) and intravenous (i.v.) applications. Additionally, three mutually divergent isolates from three geographical locations, as shown by MALDI-TOF-MS and partial rpoB gene sequence analysis, were compared. In the first trial, birds were infected with one of the selected isolates by the i.v. or i.n. route. Subsequently, birds were killed 3, 12 and 24 h post infection following i.v. infection while at 3, 7 and 10 days post infection (dpi) in the case of i.n. infection along with birds of the control group. As a result, i.n. infection showed prominent and consistent bacterial tissue distribution in different organs persisting until 10 dpi, which was a striking contrast to the i.v. infection route. Likewise, histopathology revealed mild to severe tracheal lesions following i.n. infection. The second trial was set up to confirm both the achieved results and the robustness of i.n. infection but with an extended observation period, until 28 dpi In agreement with the preceding trial, identical results for bacteriological and histopathological examinations were obtained with persistency of bacteria until 28 dpi Comparing the three different isolates from Mexico, China and Austria, the Mexican isolate showed a somewhat higher pathogenicity than the other strains. Consequently, pathogenesis of G. anatis strains was studied in chickens elucidating i.n. infection as the most reliable route characterized by a long-lasting bacteraemia, targeting the respiratory and reproductive tract.

The fimbrial protein FlfA from Gallibacterium anatis is a virulence factor and vaccine candidate

The Gram-negative bacterium Gallibacterium anatis is a major cause of salpingitis and peritonitis in egg-laying chickens, leading to decreased egg production worldwide. Widespread multidrug resistance largely prevents treatment of this organism using traditional antimicrobial agents, while antigenic diversity hampers disease prevention by classical vaccines. Thus, insight into its pathogenesis and knowledge about important virulence factors is urgently required. A key event during the colonization and invasion of mucosal surfaces is adherence, and recently, at least three F17-like fimbrial gene clusters were identified in the genomes of several G. anatis strains. The objective of this study was to characterize the putative F17-like fimbrial subunit protein FlfA from G. anatis 12656-12 and determine its importance for virulence. In vitro expression and surface exposure of FlfA was demonstrated by flow cytometry and immunofluorescence microscopy. The predicted function of FlfA as a fimbrial subunit protein was confirmed by immunogold electron microscopy. An flfA deletion mutant (ΔflfA) was generated in G. anatis 12656-12, and importantly, this mutant was significantly attenuated in the natural chicken host. Furthermore, protection against G. anatis 12656-12 could be induced by immunizing chickens with recombinant FlfA. Finally, in vitro expression of FlfA homologs was observed in a genetically diverse set of G. anatis strains, suggesting the potential of FlfA as a serotype-independent vaccine candidate This is the first study describing a fimbrial subunit protein of G. anatis with a clear potential as a vaccine antigen.

Genome analysis and phylogenetic relatedness of Gallibacterium anatis strains from poultry

Peritonitis is the major disease problem of laying hens in commercial table egg and parent stock operations. Despite its importance, the etiology and pathogenesis of this disease have not been completely clarified. Although avian pathogenic Escherichia coli (APEC) isolates have been incriminated as the causative agent of laying hen peritonitis, Gallibacterium anatis are frequently isolated from peritonitis lesions. Despite recent studies suggesting a role for G. anatis in the pathogenesis of peritonitis, little is known about the organism’s virulence mechanisms, genomic composition and population dynamics. Here, we compared the genome sequences of three G. anatis isolates in an effort to understand its virulence mechanisms and identify novel antigenic traits. A multilocus sequence typing method was also established for G. anatis and used to characterize the genotypic relatedness of 71 isolates from commercial laying hens in Iowa and 18 international reference isolates. Genomic comparisons suggest that G. anatis is a highly diverse bacterial species, with some strains possessing previously described and potential virulence factors, but with a core genome containing several antigenic candidates. Multilocus sequence typing effectively distinguished 82 sequence types and several clonal complexes of G. anatis, and some clones seemed to predominate among G. anatis populations from commercial layers in Iowa. Biofilm formation and resistance to antimicrobial agents was also observed in several clades. Overall, the genomic diversity of G. anatis suggests that multiple lineages exist with differing pathogenic potential towards birds.

Natural transformation of Gallibacterium anatis

Gallibacterium anatis is a pathogen of poultry. Very little is known about its genetics and pathogenesis. To enable the study of gene function in G. anatis, we have established methods for transformation and targeted mutagenesis. The genus Gallibacterium belongs to the Pasteurellaceae, a group with several naturally transformable members, including Haemophilus influenzae. Bioinformatics analysis identified G. anatis homologs of the H. influenzae competence genes, and natural competence was induced in G. anatis by the procedure established for H. influenzae: transfer from rich medium to the starvation medium M-IV. This procedure gave reproducibly high transformation frequencies with G. anatis chromosomal DNA and with linearized plasmid DNA carrying G. anatis sequences. Both DNA types integrated into the G. anatis chromosome by homologous recombination. Targeted mutagenesis gave transformation frequencies of >2 × 10(-4) transformants CFU(-1). Transformation was also efficient with circular plasmid containing no G. anatis DNA; this resulted in the establishment of a self-replicating plasmid. Nine diverse G. anatis strains were found to be naturally transformable by this procedure, suggesting that natural competence is common and the M-IV transformation procedure widely applicable for this species. The G. anatis genome is only slightly enriched for the uptake signal sequences identified in other pasteurellaceaen genomes, but G. anatis did preferentially take up its own DNA over that of Escherichia coli. Transformation by electroporation was not effective for chromosomal integration but could be used to introduce self-replicating plasmids. The findings described here provide important tools for the genetic manipulation of G. anatis.

Complete genome sequence of Gallibacterium anatis strain UMN179, isolated from a laying hen with peritonitis

Gallibacterium anatis is a member of the normal flora of avian hosts and an important causative agent of peritonitis and salpingitis in laying hens. Here we report the availability of the first completed G. anatis genome sequence of strain UMN179, isolated from an Iowa laying hen with peritonitis.

The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae

RTX toxins are bacterial pore-forming toxins that are particularly abundant among pathogenic species of Pasteurellaceae, in which they play a major role in virulence. RTX toxins of several primary pathogens of the family of Pasteurellaceae are directly involved in causing necrotic lesions in the target organs. Many RTX toxins are known as haemolysins because they lyse erythrocytes in vitro, an effect that is non-specific, but which serves as a useful marker in bacteriological identification and as an easily measurable signal in vitro in experimental studies. More recent studies have shown that the specific targets of most RTX toxins are leukocytes, with RTX toxins binding to the corresponding β-subunit (CD18) of β2 integrins and then exerting cytotoxic activity. After uptake by the target cell, at sub-lytic concentrations, some RTX toxins are transported to mitochondria and induce apoptosis. For several RTX toxins the binding to CD18 has been shown to be host specific and this seems to be the basis for the host range specificity of these RTX toxins. Observations on two very closely related species of the Pasteurellaceae family, Actinobacillus suis, a porcine pathogen particularly affecting suckling pigs, and Actinobacillus equuli subsp. haemolytica, which causes pyosepticaemia in new-born foals (sleepy foal disease), have revealed that they express different RTX toxins, named ApxI/II and Aqx, respectively. These RTX toxins are specifically cytotoxic for porcine and equine leukocytes, respectively. Furthermore, the ApxI and Aqx toxins of these species, when expressed in an isogenetic background in Escherichia coli, are specifically cytotoxic for leukocytes of their respective hosts. These data indicate the determinative role of RTX toxins in host specificity of pathogenic species of Pasteurellaceae.