Dietary Escherichia coli Nissle 1917 Modulates Gut Microbiota and Inflammatory Cytokines in Hybrid Grouper in a Recirculating Aquarium System

Probiotics are widely studied as antibiotic alternatives in commercial aquaculture, yet their effects on fish maintained under long-term aquarium conditions remain poorly understood. This study addressed this gap by evaluating dietary Escherichia coli Nissle 1917 (EcN) supplementation on gut microbiota and inflammatory cytokine expression in hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂) from a recirculating aquarium system. In this study, hybrid grouper were maintained in triplicate tanks under long-term aquarium environments, and fed a basal diet with 1 × 108 CFU/g EcN (SS group) or a control diet (CS group) for 28 consecutive days. Based on 16S rRNA high-throughput sequencing and qPCR, the intestinal microbiota and expression levels of IL-4, TNF-α, and IL-1β were measured. At the phylum level, the relative abundance of Firmicutes increased from 15.63% (CS) to 66.70% (SS), while Proteobacteria decreased from 76.77% to 30.61%. At the genus level, Exiguobacterium became the dominant taxon in the SS group. Furthermore, EcN supplementation significantly upregulated IL-4 expression and downregulated TNF-α and IL-1β expression. EcN supplementation significantly altered gut microbiota composition, with marked changes in community structure and notable shifts in dominant taxa. Thus, this study provides one of the investigations into EcN-mediated restructuring of intestinal bacterial communities and modulation of host immune transcriptional responses in hybrid grouper maintained under controlled aquarium settings. These findings offer a foundation for designing microbiome-targeted interventions in captive marine fish systems.