Effects of feed supplementation with Effective Microorganisms (EM bacteria) on growth, gut morphology and microbiota, immune response, and Edwardsiella tarda resistance in Anguilla japonica glass eel

The Japanese eel ( Anguilla japonica ) is a high-value aquaculture species facing severe population decline and rearing challenges during the glass eel nursery stage. The use of functional paste feeds with probiotic additives, Effective Microorganisms (EM bacteria), offer promising solutions for improving survival, growth, and biosecurity during this critical phase. This study investigated the effects of dietary supplementation with EM bacteria on the growth performance, intestinal morphology, immune responses, gastrointestinal microbiota composition, and disease resistance of Japanese glass eels. Japanese glass eels were fed paste feed supplemented with 0% (control group), 0.5%, 1%, 2%, or 5% EM bacteria for 12 weeks. The Japanese glass eel among 1% and 2% EM bacteria supplied groups showed significantly higher in final body weight, percent weight gain, specific growth rate, and condition factor, compared to the control group. Histological analysis revealed well-developed midgut villi in the 1% and 2% groups, while abnormal intestinal structures were observed in the control, 0.5%, and 5% groups. Immune-related gene expression analysis indicated increased expression of Lysozyme in the 2% and 5% groups, and lower expression of pro-inflammatory, oxidative stress-related genes and heat shock protein (IL-6, SOD, POD, HSP70, HSP90) in the 1% and 2% groups. Following the Edwardsiella tarda challenge test, 2% group demonstrated the highest survival rate (75%), significantly outperforming the control and other treatment groups. Alpha and beta diversity analyses of intestinal microbiota indicated that 2% supplementation significantly enhanced microbial diversity and altered community composition, promoting beneficial genera, and show significantly higher alpha diversity indices and further confirmed by the PCoA and UniFrac heatmap. In contrast, the 5% group exhibited reduced microbial diversity and an imbalanced microbiota. These findings suggest that 1–2% EM supplementation in paste feed optimally enhances gut health, immunity, and disease resistance in Japanese glass eel aquaculture.