Comparative effects of four immunostimulants on immunity, antioxidant capacity and intestinal microbiota of juvenile Coreius guichenoti: A preliminary screening

The inefficient artificial breeding of the endangered largemouth bronze gudgeon ( Coreius guichenoti ) is partly attributed to compromised immunity and poor hepatic-intestinal health, yet no nutritional immunostimulants have been evaluated in this species. This study evaluated four immunostimulants—curcumin (CU), astaxanthin (AX), chitosan (CH), and Bacillus coagulans CICC 20138 (BC)—on immune function, antioxidant capacity, and intestinal microbiota of C. guichenoti . Six hundred juveniles (8.03 ± 0.31 g) were allocated to five groups (including control, CO) and fed diets supplemented with 200 mg/kg of each immunostimulant, based on previously reported effective levels in other fish species, for 8 weeks. Compared to CO, all immunostimulant groups showed elevated plasma immunoglobulin M, albumin, complement C3/C4, and lysozyme levels. Hepatic glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT) activities increased in AX and BC groups, with reduced malondialdehyde (MDA). Intestinal antioxidant enzyme activities (GPX, SOD, CAT) were highest in BC, while MDA decreased in all supplemented groups. BC-fed fish exhibited reduced hepatic vacuolation and improved hepatocyte integrity. Intestinal microbiota analysis revealed higher Chao1 and Faith_pd diversity indices in BC, alongside a marked downregulation of the "enterobacterial common antigen biosynthesis" pathway. This suppression correlated with reduced pathogenic genera ( Plesiomonas , Shigella ) compared to CO. Collectively, B. coagulans CICC 20138 enhanced immunity, attenuated oxidative stress, mitigated hepatointestinal damage, and suppressed pathogen-associated metabolic pathways. These results identify B. coagulans CICC 20138 as a promising immunostimulant for C. guichenoti , warranting further dose-response optimization.