Probiotics regulate the intestinal microbiome to promote growth in juvenile Salmo trutta fario.

Probiotics improve aquaculture growth and immunity, but their dosage effects on gut microbes remain unclear. This study evaluated the impact of different levels of Effective Microorganisms (EM) probiotics on growth performance and intestinal microbiota of Juvenile Salmo trutta fario. Dietary EM supplementation significantly enhanced growth, with the medium-dose group (0.5% EM) exhibiting the best performance. Dietary EM supplementation significantly enhanced growth, with the medium-dose group (0.5% EM) showing the best performance. EM influenced hepatic antioxidants, with the medium-dose group similar to the control, and altered serum biochemistry, particularly in medium and high-dose groups, while most hematological parameters remained unchanged. Furthermore, 0.5% EM reduced beta diversity and inter-individual variation in the gut microbiota, suggesting enhanced microbial stability. EM supplementation altered the relative abundance of beneficial genera, such as Lactobacillus, Bifidobacterium, and Faecalibacterium, in a dose-dependent manner. Co-occurrence network analysis revealed several probiotic genera as potential keystone taxa, suggesting their crucial roles in maintaining the stability and function of the intestinal microbiota. Overall, EM exhibited clear benefits for growth and development, with 0.5% showing superior effects over 0.1% and 1%, emphasizing the importance of probiotic dosage for fish health and providing a basis for optimizing EM application in Salmo trutta aquaculture.