Impact of <scp> <i>Streptococcus iniae</i> </scp> Infection on Gut Microbiome Diversity and Immune Response in Four‐Finger Threadfin ( <scp> <i>Eleutheronema tetradactylum</i> </scp> )

Streptococcus iniae is one of the key etiological agents of Streptococcosis and is a significant pathogen affecting four-finger Threadfin (Eleutheronema tetradactylum), leading to severe clinical manifestations, such as ascitic fluid accumulation in the abdominal cavity and intestinal haemorrhages. Understanding the interplay between fish infections, gut microbiota dysbiosis and immune responses is critical for effective management in aquaculture. This study investigates the impact of the acute infection caused by S. iniae on the gut microbiome composition and immune response of Threadfin fish. Following experimental infection, we observed significant differences in taxonomic richness and diversity between healthy and infected fish during the acute phase of the disease. The gut microbiota was primarily dominated by Proteobacteria, Firmicutes, Actinobacteria and Bacteroidetes. Notably, infection led to a marked decline in microbiota richness and diversity in the gut, accompanied by an increase in the relative abundance of pathogenic genera such as Streptococcus and Vibrio. Immune modulation, as evidenced by altered gene expression of selected immune parameters, was observed in tissue samples following clinical infection. Also, histological examinations revealed significant disruptions to intestinal villi architecture in infected fish. These findings suggest that S. iniae alters the gut microbiota composition, which may compromise immune function and elevate the risk of intestinal inflammation. This research underscores the importance of monitoring gut microbiota dynamics as a potential diagnostic tool for managing bacterial infections in aquaculture settings.