Abstract
Background: Cetobacterium somerae, a symbiotic microorganism resident in various fish intestines, is recognized for its beneficial effects on fish gut health. However, the mechanisms underlying the effects of C. somerae on gut health remain unclear. In this experiment, we investigated the influence of C. somerae (CGMCC No.28843) on the growth performance, intestinal digestive and absorptive capacity, and intestinal structural integrity of juvenile grass carp (Ctenopharyngodon idella) and explored its potential mechanisms. Methods: A cohort of 2,160 juvenile grass carp with an initial mean body weight of 11.30 ± 0.01 g were randomly allocated into 6 treatment groups, each comprising 6 replicates (60 fish per replicate). The experimental diets were supplemented with C. somerae at graded levels of 0.00 (control), 0.68 × 10⁹, 1.35 × 10⁹, 2.04 × 10⁹, 2.70 × 10⁹, and 3.40 × 10⁹ cells/kg feed. Following a 10-week experimental period, biological samples were collected for subsequent analyses. Results: Dietary supplementation with C. somerae at 1.35 × 10⁹ cells/kg significantly enhanced growth performance, intestinal development, and nutrient retention rate in juvenile grass carp (P < 0.05). The treatment resulted in increased intestinal acetic acid concentration and enhanced activities of digestive enzymes and brush border enzymes (P < 0.05). Furthermore, it reduced intestinal permeability (P < 0.05), preserved tight junctions (TJ) ultrastructural integrity, and increased the expression of TJ and adherens junctions (AJ) biomarkers at both protein and transcriptional levels (P < 0.05). Mechanistically, these effects may be correlated with enhanced antioxidant capacity and coordinated modulation of the RhoA/ROCK, Sirt1, and PI3K/AKT signaling pathways. The appropriate supplementation levels, based on weight gain rate, feed conversion ratio, the activity of serum diamine oxidase and the content of lipopolysaccharide, were 1.27 × 10⁹, 1.27 × 10⁹, 1.34 × 10⁹ and 1.34 × 10⁹ cells/kg, respectively. Conclusions: C. somerae improved intestinal digestive and absorptive capacity of juvenile grass carp, maintained intestinal structural integrity, and thus promoted their growth and development. This work demonstrates the potential of C. somerae as a probiotic for aquatic animals and provides a theoretical basis for its utilization in aquaculture.