Effects of enterococcus faecium postbiotics on growth, nonspecific immunity, antioxidant capacity, and intestinal health in Micropterus salmoides

Largemouth bass (Micropterus salmonoides), as an important characteristic freshwater aquaculture species, is highly favored for its delicate flesh, rapid growth, and absence of intermuscular bones. However, high-density intensive farming has led to frequent disease outbreaks, and the extensive use of antibiotics has resulted in the emergence of antimicrobial resistance and drug residues. Postbiotics, as non-viable microbial components with probiotic functions, have the potential to improve host health, enhance immunity, and protect against pathogens, making them a research hotspot as potential antibiotic alternatives. This study investigated the effects of feeding live Enterococcus faecalis (MSEF22) and its postbiotic components (HK-EF) on the growth, immunity, antioxidant capacity, intestinal barrier integrity, and gut microbiota of Micropterus salmoides. Fish with an initial body weight of (30.78 ± 0.57) g were randomly assigned to three groups: a control group (basal diet + PBS), an MSEF22 group (basal diet + E.faecalis, 1×10⁷ CFU/g), and an HK-EF group (basal diet + inactivated E.faecalis, 1×10⁷ CFU/g). A 42-day feeding trial was conducted. Results showed that both MSEF22 and HK-EF significantly increased the weight gain rate (WGR) and specific growth rate (SGR). Serum biochemical analysis revealed significant increases in alkaline phosphatase (AKP) and acid phosphatase (ACP) levels in fish fed with MSEF22 or HK-EF. Histological examination of intestinal tissues showed more intact and smoother intestinal epithelial villi, along with a significant increase in goblet cell numbers in both treatment groups. Gut microbiota analysis indicated that the Chao1 index of the MSEF22 group differed markedly from the control group. HK-EF supplementation significantly increased the abundance of Cetobacterium, while both MSEF22 and HK-EF significantly reduced the abundance of Mycoplasma, a known pathogen. Moreover, MSEF22 and HK-EF enhanced the interactions of Cetobacterium with other gut microbes, promoting a stable microbial community, as demonstrated by a co-occurrence network analysis. In conclusion, feeding MSEF22 and HK-EF improved the growth performance, non-specific immune function, and antioxidant capacity of M.salmoides, while enriching beneficial gut bacteria and maintaining intestinal health. These findings suggest that postbiotic metabolites of E.faecalis have promising potential for application in intensive aquaculture.