AvxA, a composite serine-protease-RTX toxin of Avibacterium paragallinarum

Avibacterium paragallinarum, the Causative Agent of Infectious Coryza: A Comprehensive Review

Infectious coryza (IC) caused by Avibacterium paragallinarum (AP) has risen in importance as a poultry disease over the past several years because of its increased incidence in commercial poultry in both Europe and the United States. Because of this rise in importance, more attention has been focused on diagnosis, isolation, and surveillance of this bacterial pathogen. As a result, new knowledge has been produced and published. This review was compiled with the main purpose of summarizing and presenting the updated knowledge available about AP. However, the new knowledge can only be understood in the context of previously known facts about the disease. Therefore, this review has been organized in two major parts. The first part is a review of the established knowledge about AP, followed by recent updates. In the first part, we summarize the established well-known as well as some of the less-known facts and literature about AP. The second section focuses on specifics of the latest IC outbreaks in commercial poultry in northern latitudes, particularly in Europe and in North America. Additionally, we reviewed the current geographical distribution of the disease in Asia, South America, and Africa. The crises created by emerging or re-emerging disease outbreaks ignite interest in understanding the disease and pathogen in order to combat it properly. This results in new knowledge that improves the understanding of the disease features, leading to improved disease prevention, control, and eradication. Although knowledge about AP has advanced, knowledge gaps about the disease still persist. Therefore, this review concludes with summarizing the current knowledge gaps as well as potential areas for future research.

Identification and characterization of biosynthetic loci of lipooligosaccharide and capsular polysaccharide in Avibacterium paragallinarum

Infectious coryza is an acute respiratory disease in chickens caused by Avibacterium paragallinarum. Lipooligosaccharides (LOSs) and capsular polysaccharides are important components of Av. paragallinarum. Herein, we identified that gene cluster L6 and two genes waaF, waaQ were associated with LOS synthesis, and two genes acbD and ccbF1 were involved in capsular synthesis. Mutant and complementary strains of these genes were generated by natural transformation. Wild-type strains produced LOS that yielded an upper and lower band. In comparison, ΔwaaQ and ΔwaaF yielded a truncated lower band and lacked the upper band, while ΔL6 did not exhibit the upper band, and the lower band was identical to that of the wild-type strain. The survival rates of wild-type strain, ΔwaaF, ΔwaaQ, and ΔL6 in chicken serum were 4.89 % ± 0.27 %, 0.0013 % ± 0.0002 %, 0.43 % ± 0.05 %, and 3.1 % ± 0.35 %, respectively. Notably, the resistances of ΔwaaF, ΔwaaQ, and ΔL6 to chicken serum were significantly lower than that of parent strain. By contrast, the survival rate of the ΔacbD strain was 55.17 % ± 0.61 %, and its resistance to chicken serum was significantly higher than that of the wild-type strain (p < 0.001). Deletion of the waaF, waaQ, L6, acbD, and ccbF1 genes resulted in enhanced formation of biofilm without altering immunogenicity in chickens. The ΔwaaF, ΔwaaQ, and ΔccbF1 strains exhibited heightened susceptibility to fowlicidin-2. Furthermore, ΔwaaF, ΔacbD, and ΔccbF1 strains shown a decrease in pathogenicity (p < 0.05). These results are valuable for advancing research on the pathogenesis of Av. paragallinarum.

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.

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.

A sequence-based two-level method for the prediction of type I secreted RTX proteins

Many Gram-negative bacteria use the type I secretion system (T1SS) to translocate a wide range of substrates (type I secreted RTX proteins, T1SRPs) from the cytoplasm across the inner and outer membrane in one step to the extracellular space. Since T1SRPs play an important role in pathogen-host interactions, identifying them is crucial for a full understanding of the pathogenic mechanism of T1SS. However, experimental identification is often time-consuming and expensive. In the post-genomic era, it becomes imperative to predict new T1SRPs using information from the amino acid sequence alone when new proteins are being identified in a high-throughput mode. In this study, we report a two-level method for the first attempt to identify T1SRPs using sequence-derived features and the random forest (RF) algorithm. At the full-length sequence level, the results show that the unique feature of T1SRPs is the presence of variable numbers of the calcium-binding RTX repeats. These RTX repeats have a strong predictive power and so T1SRPs can be well distinguished from non-T1SRPs. At another level, different from that of the secretion signal, we find that a sequence segment located at the last 20-30 C-terminal amino acids may contain important signal information for T1SRP secretion because obvious differences were shown between the corresponding positions of T1SRPs and non-T1SRPs in terms of amino acid and secondary structure compositions. Using five-fold cross-validation, overall accuracies of 97% at the full-length sequence level and 89% at the secretion signal level were achieved through feature evaluation and optimization. Benchmarking on an independent dataset, our method could correctly predict 63 and 66 of 74 T1SRPs at the full-length sequence and secretion signal levels, respectively. We believe that this study will be useful in elucidating the secretion mechanism of T1SS and facilitating hypothesis-driven experimental design and validation.

Identification and functional analysis of the cytolethal distending toxin gene from Avibacterium paragallinarum

Avibacterium paragallinarum is the causative agent of infectious coryza, an important respiratory disease of chickens. Cytolethal distending toxins (CDTs) are a family of protein cytotoxins that cause cell cycle arrest and apoptosis in eukaryotic cells. Whole-genome sequencing analysis showed that Av. paragallinarum contains cdtABC genes. Filter-sterilized lysates prepared from Av. paragallinarum or from recombinant Escherichia coli expressing cdtABC genes exhibited CDT activity on HeLa cells and chicken embryo fibroblast (DF-1) cells. In vitro DNase assays showed that purified recombinant CdtB has DNase activity. Polymerase chain reaction and sequencing analysis revealed that the cdtABC genes are present in all strains of Av. paragallinarum examined in this study. This is the first report of the identification and functional analysis of cdtABC genes from Av. paragallinarum. The gene products of cdtABC genes may be involved in the pathogenesis of the disease caused by Av. paragallinarum.

Draft Genome Sequence of the Virulent Avibacterium paragallinarum Serotype A Strain JF4211 and Identification of Two Toxins

Avibacterium paragallinarum is an important pathogen of chicken livestock causing infectious coryza. Here, we report the draft genome sequence of the virulent A. paragallinarum serotype A strain JF4211 (2.8 Mbp and G+C content of 41%) and the two toxin operons discovered from the annotation of the genome.