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

Comparative Genomics Analyses Support the Reclassification of Bisgaard Taxon 40 as Mergibacter gen. nov., With Mergibacter septicus sp. nov. as Type Species: Novel Insights Into the Phylogeny and Virulence Factors of a Pasteurellaceae Family Member Associated With Mortality Events in Seabirds

The Pasteurellaceae family has been associated with fatal diseases in numerous avian species. Several new taxa within this family, including Bisgaard taxon 40, have been recently described in wild birds, but their genomic characteristics and pathogenicity are not well understood. We isolated Bisgaard taxon 40 from four species of seabirds, including one sampled during a mass, multi-species mortality event in Florida, United States. Here, we present a comprehensive phenotypic and genetic characterization of Bisgaard taxon 40 and comparative genomic analysis with reference strains from the Pasteurellaceae family, aiming at determining its phylogenetic position, antimicrobial susceptibility profile, and identifying putative virulence factors. In silico multilocus sequence-based and whole-genome-based phylogenetic analysis clustered all Bisgaard taxon 40 strains together on a distinct branch separated from the other members of the Pasteurellaceae family, indicating that Bisgaard taxon 40 could represent a new genus. These findings were further supported by protein similarity analyses using the concatenation of 31 conserved proteins and other taxonomic approaches such as the percentage of conserved protein test. Additionally, several putative virulence factors were identified, including those associated with adhesion (capsule, ompA, ompH) and colonization (exbD, fur, galU, galE, lpxA, lpxC, and kdsA) of the host and a cytolethal distending toxin (cdt), which may have played a role in disease development leading to the mortality event. Considerably low minimum inhibitory concentrations (MICs) were found for all the drugs tested, in concordance with the absence of antimicrobial resistance genes in these genomes. The novel findings of this study highlight genomic and phenotypic characteristics of this bacterium, providing insights into genome evolution and pathogenicity. We propose a reclassification of these organisms within the Pasteurellaceae family, designated as Mergibacter gen. nov., with Mergibacter septicus sp. nov. as the type species. The type strain is Mergibacter septicus A25201^T (=DSM 112696).

Reclassification of [Haemophilus] haemoglobinophilus as Canicola haemoglobinophilus gen. nov., comb. nov. including Bisgaard taxon 35

[Haemophilus] haemoglobinophilus and the unpublished Bisgaard taxon 35 are associated with respiratory and urogenital tract infections in dogs. A total of 21 strains including the type strain of [Haemophilus] haemoglobinophilus were included in the investigation. Strains of [Haemophilus] haemoglobinophilus and taxon 35 formed a monophyletic group demonstrating at least 97.8 and 96.5% similarities within the group based upon 16S rRNA and rpoB gene sequence comparisons, respectively. Glaesserella australis was the most closely related species to [Haemophilus] haemoglobinophilus and taxon 35 with 96.1 % 16S rRNA gene sequence similarity which is slightly higher than the 95 % separating most genera of the family Pasteurellaceae. However, the conserved protein sequence phylogeny documented a unique position of [Haemophilus] haemoglobinophilus with only 81 % identity to the most closely related species, genomospecies 1 of the genus Rodentibacter which is lower than the 85 % separating most genera of the family Pasteurellaceae. The conserved protein sequence identity to Haemophilus influenzae, the type species of the genus, was 77%, demonstrating that [Haemophilus] haemoglobinophilus is not properly classified as a member of the genus Haemophilus. On the basis of the phylogenetic comparisons, the taxa [Haemophilus] haemoglobinophilus and taxon 35 are proposed to be included with a novel genus Canicola with one species, Canicola haemoglobinophilus which is reclassified from [Haemophilus] haemoglobinophilus. Phenotypic characters obtained with isolates genetically approved to represent Canicola haemoglobinophilus were in accordance with those of the members of the family Pasteurellaceae, and the novel genus can be separated from most of the existing genera by a positive catalase reaction, lack of V-factor requirement for growth, lack of haemolysis of blood agar and negative Voges-Proskauer and urease tests. The novel genus cannot be separated by biochemical and physiological characteristics alone from the genera Aggregatibacter, Avibacterium, Frederiksenia and Spirabiliibacterium. However, MALDI-TOF mass spectroscopy and also RpoB amino acid signatures allowed a clear separation from these taxa, supporting the existence of a novel genus. The DNA G+C content is 37.0-37.8 mol% for the genus, based on the whole genomic sequences. The type strain of Canicola haemoglobinophilus is CCUG 3714^T (=ATCC 19416^T=NCTC 1659^T) isolated in 1901 from the prepuce of a dog in Germany.

Genomic Islands in the Full-Genome Sequence of an NAD-Hemin-Independent Avibacterium paragallinarum Strain Isolated from Peru

Here, we report the full-genome sequence of an NAD-hemin-independent Avibacterium paragallinarum serovar C-2 strain, FARPER-174, isolated from layer hens in Peru. This genome contained 12 potential genomic islands that include ribosomal protein-coding genes, a nadR gene, hemocin-coding genes, sequences of fagos, an rtx operon, and drug resistance genes.

The Cytolethal Distending Toxin Contributes to Microbial Virulence and Disease Pathogenesis by Acting As a Tri-Perditious Toxin

This review summarizes the current status and recent advances in our understanding of the role that the cytolethal distending toxin (Cdt) plays as a virulence factor in promoting disease by toxin-producing pathogens. A major focus of this review is on the relationship between structure and function of the individual subunits that comprise the AB₂ Cdt holotoxin. In particular, we concentrate on the molecular mechanisms that characterize this toxin and which account for the ability of Cdt to intoxicate multiple cell types by utilizing a ubiquitous binding partner on the cell membrane. Furthermore, we propose a paradigm shift for the molecular mode of action by which the active Cdt subunit, CdtB, is able to block a key signaling cascade and thereby lead to outcomes based upon programming and the role of the phosphatidylinositol 3-kinase (PI-3K) in a variety of cells. Based upon the collective Cdt literature, we now propose that Cdt is a unique and potent virulence factor capable of acting as a tri-perditious toxin that impairs host defenses by: (1) disrupting epithelial barriers; (2) suppressing acquired immunity; (3) promoting pro-inflammatory responses. Thus, Cdt plays a key role in facilitating the early stages of infection and the later stages of disease progression by contributing to persistence and impairing host elimination.