Pathogenesis of experimental vibriosis in blue mussel (Mytilus edulis) larvae based on accurate positioning of GFP-tagged Vibrio strains and histopathological and ultrastructural changes of the host

Abstract

Bacteria, especially Vibrio spp., are largely responsible for larval diseases in shellfish aquaculture. Despite the severe impact of bacterial diseases on larval health, only a few direct studies have characterized the pathogenesis of vibriosis in bivalve larvae. To clarify the onset and advancement of vibriosis in blue mussel Mytilus edulis larvae, two known larval pathogens Vibrio splendidus ME9 and Vibrio anguillarum NB10 were tagged with green fluorescence protein (GFP), and combined with synchronous histopathological analyses, we identified a new type of pathogenesis of experimental vibriosis in bivalve larvae. The GFP-tagged Vibrio strains were first filtered by larvae through the velum and entered into the larval stomach through the esophagus. Later, they proliferated rapidly in the style sac and digestive glands, inducing necrosis of the digestive organs in the dorsal region during the first 24 h post challenge. Necrosis continued with esophagus disruption and velar deformation. Ultimately, the loss of distinguishable internal structures further disrupted the associated regulatory functions and rapidly induced larval death. It was confirmed later by ultrastructural observations that the Vibrio strains grew along the digestive tract and disrupted the digestive epithelium as a portal of entry. Moreover, we obtained the first evidence that the microvilli and cilia of the gastrointestinal epithelial cells were most sensitive to the vibriosis since the earliest disruptions of the larvae occurred in these positions, as shown by ultrastructural observations. The progression of the infection pathway as described in this study will facilitate further research that aims at the analysis of the gastrointestinal morphology after the introduction of anti-Vibrio therapy in the culture system.