Ca. Similichlamydia in Epitheliocystis Co-infection of Gilthead Seabream Gills: Unique Morphological Features of a Deep Branching Chlamydial Family

Abstract

The Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) bacterial superphylum constitutes a broad range of organisms with an intriguing array of ultrastructural morphologies, including intracellular membranes and compartments and their corresponding complex genomes encoding these forms. The phylum Chlamydiae are all obligate intracellular bacteria and, although much is already known of their genomes from various families and how these regulate the various morphological forms, we know remarkably little about what is likely the deepest rooting clade of this phylum, which has only been found to contain pathogens of marine and fresh water vertebrates. The disease they are associated with is called epitheliocystis; however, analyses of the causative agents is hindered by an inability to cultivate them for refined in vitro experimentation. For this reason, we have developed tools to analyse both the genomes and the ultrastructures of bacteria causing this disease, directly from infected tissues. Here we present structural data for a member of the family Ca. Similichlamydiaceae from this deep-rooted clade, which we have identified using molecular tools, in epitheliocystis lesions of gilthead seabream (Sparus aurata) in Greece. We present evidence that the chlamydial inclusions appear to develop in a perinuclear location, similar to other members of the phylum and that a chlamydial developmental cycle is present, with chlamydial forms similar to reticular bodies (RBs) and elementary bodies (EBs) detected. Division of the RBs appeared to follow a budding process, and larger RBs with multiple condensed nucleoids were detected using both transmission electron microscopy (TEM) and by focused-ion beam, scanning electron microscopy (FIB-SEM). As model hosts, fish offer many advantages for investigation, and we hope by these efforts to encourage others to explore the biology of fish pathogens from the PVC.

Environmental Metagenomic Assemblies Reveal Seven New Highly Divergent Chlamydial Lineages and Hallmarks of a Conserved Intracellular Lifestyle

“…All described Chlamydiae exhibit a similar biphasic developmental cycle, including the most deeply branching clades (Abdelrahman and Belland, 2005 ; Steigen et al, 2013 ; Seth-Smith et al, 2017 ). The transcription factor Euo targets more than 100 promoters in W. chondrophila , and may be a key player in cell cycle regulation (Domman and Horn, 2015 ; de Barsy et al, 2016 ).…”

“…Although the genome of this strain is partially covered, peptidoglycan synthesis genes are located in various regions along the chromosome, which reinforces the likely absence of these genes from S. epinepheli . Indeed, this bacterium divides through a budding process (Seth-Smith et al, 2017 ), which suggests that the family Parilichlamydiaceae may present a significantly different division mechanism than other chlamydial lineages.…”

“…They all encode a type III secretion system, homologs of known effector proteins and transporters involved in nutrient uptake. In addition, members of the Parilichlamydiaceae , the most deeply branching lineage in the current phylogeny, are fish pathogens that share the typical biphasic life cycle of other Chlamydiae (Steigen et al, 2013 ; Stride et al, 2013 ; Seth-Smith et al, 2017 ). Despite these similarities, some central mechanisms such as cell division might significantly differ in distantly-related clades such as S. epinepheli .…”

A Review on Chlamydial Diseases in Animals: Still a Challenge for Pathologists?

“…169 Complicating the differential diagnosis of epitheliocystis for pathologists is the recent realization that other bacteria from completely different bacterial phyla including beta-and gamma-proteobacteria 80,152 may cause similar diseases as CRB. Many of these alternative agents may have subtle differences in their morphology compared to chlamydial agents so microscopic differentiation may be possible, 153 however, this will need to be supported by PCR and in situ hybridization for confirmatory diagnosis at this stage.…”

The Paradigms They Are a-Changin’: past, present and future of PVC bacteria research

“…Initially containing only pathogenic species, environmental or amoeba symbiotic organisms have been recently described [reviewed in Horn 2008 ]. Because those are non-pathogenic but share fundamental cell biology processes with the pathogenic ones, they form excellent model systems to decipher the molecular bases of this historical plague to society (Seth-Smith et al 2017 ; König et al 2017 ).…”