“Ultrastructural insights into a Candidatus Parilichlamydia sp. infection of gill goblet cells in Greater amberjack (7108)”

Ultrastructural insights into a Candidatus Parilichlamydia sp. infection of gill goblet cells in Greater amberjack.

Introduction

The Candidatus family Parilichlamydiaceae, currently including two proposed genera Ca. Similichlamydia and Ca. Parilichlamydia has received increasing attention due to its evolutionary significance revealed by their deeply branching position in the Chlamydial taxonomic trees. Despite the considerable interest in this chlamydial family and the increasing number of bacteria associated with it, our understanding of these pathogens and their biology remains limited.

In a previous study, we identified a Ca. Parilichlamydia species in Greater Amberjack (Seriola dumerili) farmed in Greece, showing 98.64% similarity to Ca. Parilichlamydia carangidicola and associated with a chronic, non-proliferative intracellular infection of mucous cells on the afferent edge of the gill filaments. Building on these findings, the present study uses electron microscopy to examine infected mucous cells in detail and characterize the host tissue response.

Methods

Gill samples were obtained from a one-year monitoring study of greater amberjack cultured in Crete (Greece). Samples displaying distinctive infection of mucous cells were selected and processed for transmission electron microscopy. Sections were cut from the embedded gills with cross sections of the trailing/afferent edge and of the filament tip, stained with lead citrate and uranyl acetate and observed in the transmission electron microscope JEOL JEM-2100 (University of Crete).

Results & Conclusions

Electron microscopy revealed distinct developmental forms of Ca. Parilichlamydia in mucous cells of Greater Amberjack gills, with Intermediate Bodies resembling those of Ca. Similichlamydia and Elementary Bodies similar to C. trachomatis. A previously undescribed inclusion membrane type was observed, characterized by extensive interdigitations likely formed through fusion with cytoplasmic vesicles. The presence of multivesicular bodies further suggests complex intracellular remodeling.

Immune cell involvement was evident, including macrophage engagement with infected cells, a potential reservoir role of granular cells, and localized phagoptosis, indicating an active host immune response. Together, these findings provide new insights into the ultrastructural features and host interactions of Parilichlamydiaceae, advancing our understanding of chlamydial infections in fish and laying the groundwork for future studies on their pathogenic mechanisms and evolutionary context.

1. Cascarano, Maria Chiara, University of Crete, Hellenic Centre for Marine Research, Presenter
2. Papadogiorgaki, Sevasti, University of Crete, Author
3. Katharios, Pantelis, Hellenic Centre for Marine Research, Author