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Epitheliocystis agents in sea bream Sparus aurata: morphological evidence for two distinct chlamydia-like
1Departament de Biologia, Animal, Biologia Vegetal i d'Ecologia, Facultat de Veterinària, Universitat Autònoma de Barcelona, Bellaterra, Spain. ibiae@blues.uab.es
Abstract:
The morphology of membrane-bound intracellular inclusions, or 'cysts', of epitheliocystis from sea bream Sparus aurata is described. Inclusions under the light microscope appear either granular or amorphous. Granular inclusions do not elicit a proliferative host reaction and contain the 3 distinctive developmental stages of chlamydial organisms: the highly pleomorphic reproductive form or reticulate body, the condensing form or intermediate body and the infective non-dividing rather uniform elementary body. Amorphous inclusions may elicit a proliferative host reaction and contain prokaryotic organisms which differ morphologically from those reported within granular cysts. More or less elongated electron-lucent organisms divide by fission to give rise to electron-dense non-dividing small cells with a dense nucleoid. Vacuolated and non-vacuolated small cells are reported. The morphology and developmental cycle of sea bream epitheliocystis agents would support their chlamydial nature; however, the immunohistochemical study conducted on gill samples which carried both inclusions failed to demonstrate the expression of lipopolysaccharide (LPS) chlamydial antigen. The different stages of the 2 distinct developmental cycles described in the present study are compared with electron microscope observations of epitheliocystis organisms reported from different host species. The hypothesis that epitheliocystis infection in the sea bream might be caused by a unique highly pleomorphic chlamydia-like agent, the life history of which includes 2 entirely different developmental cycles, is discussed.
Insights
Epitheliocystis in sea bream presents two distinct cyst morphologies, granular and amorphous, with different developmental stages. Despite morphological similarities to chlamydia, a key chlamydial antigen was not detected, suggesting a unique pathogen.
Area of Science:
- Ichthyopathology
- Microbiology
- Cell Biology
Background:
- Epitheliocystis is a common gill disease in farmed fish, including sea bream (Sparus aurata).
- The causative agents are historically difficult to classify, with some exhibiting chlamydial characteristics.
Purpose of the Study:
- To describe the morphology and developmental cycle of epitheliocystis in sea bream.
- To investigate the potential chlamydial nature of the causative agent.
- To compare findings with other reported epitheliocystis infections.
Main Methods:
- Light and electron microscopy were used to examine intracellular inclusions (cysts) in sea bream gills.
- Immunohistochemical staining was performed to detect chlamydial lipopolysaccharide (LPS) antigen.
Main Results:
- Two distinct inclusion types were observed: granular and amorphous.
- Granular inclusions contained three stages resembling chlamydial developmental forms (reticulate body, intermediate body, elementary body).
- Amorphous inclusions contained prokaryotic organisms with a different morphology and developmental cycle, dividing by fission.
Conclusions:
- The morphology of granular inclusions supports a chlamydial or chlamydia-like nature of the agent.
- The failure to detect chlamydial LPS antigen in either inclusion type challenges a definitive chlamydial classification.
- Sea bream epitheliocystis may be caused by a unique pleomorphic agent with two distinct life cycle stages.