The immunomodulatory effects of yeast glycoprotein in sea cucumber (Apostichopus japonicus) against skin ulceration syndrome induced by Vibrio alginolyticus.

The antibacterial efficacy of yeast glycoprotein (YG) against Vibrio alginolyticus (VA) and its potential benefits for sea cucumber (Apostichopus japonicus) health and growth were investigated. A total of 844 proteins were identified in YG, with the top 30 ranked by iBAQ (intensity-based Absolute Quantification, a metric reflecting protein abundance) values for further analysis. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of YG against V. alginolyticus were determined as 50 mg/mL and 25 mg/mL, respectively. Oxford cup assays revealed concentration-dependent antibacterial effects, with inhibition zones ranging from 6.03 mm to 14.86 mm. Transmission electron microscopy (TEM) observations demonstrated that YG treatment disrupted bacterial cell walls, causing membrane damage and leakage of intracellular contents. Enzyme activity assays further confirmed YG-induced bacterial lysis, as indicated by increased alkaline phosphatase (AKP) levels. Additionally, YG significantly reduced bacterial surface hydrophobicity, impairing adhesion capacity. Feeding trials demonstrated that dietary supplementation with YG enhanced immune enzyme activities, including lysozyme, catalase, and nitric oxide synthase, in sea cucumber coelomic fluid. Growth performance parameters such as specific growth rate (SGR) and feed conversion efficiency (FCE) improved significantly with YG supplementation. In a bacterial challenge experiment, cumulative survival rates increased to 53.33 % in the YG-treated groups compared to the positive control. Intestinal microbiota analysis showed that YG supplementation modified gut microbial composition, reducing species diversity. Histopathological observations indicated reduced tissue damage in YG-fed sea cucumbers post-infection. These findings suggest that YG exhibits strong antibacterial activity and immunomodulatory effects, making it a promising dietary supplement for enhancing disease resistance and growth performance in sea cucumbers.