Microbiota attached to and encapsulated by granulomas dissected from tilapia spleen: A case report

Abstract Granulomatous diseases are caused mainly by bacterial infections that the fish immune system cannot eliminate; consequently, pathogens are encapsulated, forming a complex structure where immune cells participate in containing the damage. The granuloma is covered by a collagenous layer that may be an attractive surface for external bacteria, leading to the potential development of a microbiota associated with the outer side of granulomas. Therefore, this study aimed to describe the microbiota attached to and encapsulated by granulomas dissected from tilapia spleen obtained from a commercial farm. Tissue samples with an advanced granulomatosis stage were analyzed by amplifying and sequencing the V4 variable region of the 16S rRNA gene. More than 20 phyla were detected attached to granulomas; however, only five represented 94% of the relative abundance, and the genus Gordonia was the most abundant with 21.9%. This bacterium has been previously identified in the tilapia gut and has also been reported as the etiologic agent of granulomatous disease in humans and terrestrial animals. This is the first time it has been detected in fish granulomas. Finally, these results provide the first evidence of microbiota attached to granulomatous lesions while demonstrating that these can provide a surface that can harbour highly diverse microbial communities.