This study was undertaken to isolate active secondary metabolites from immunostimulatory Bacillus Licheniformis XY-52 and evaluate their activities at 0%, 0.5%, 1.0%, and 2.0% doses supplementation with feed on immune response in common carp at weeks 1, 2, and 3. By applying chromatography techniques and successive recrystallization, two purified metabolites were obtained and identified by spectral data (mass spectrometry and nuclear magnetic resonance) as: Cyclo-(Phe-Tyr) and Cyclo-(Phe-Gly). The results revealed that humoral innate immune parameters (lysozyme activity, phagocytic activity and bactericidal activity) were significantly (P < 0.05) increased after feeding on the two active compounds-supplemented diet. Furthermore, administration of the two active compounds significantly (P < 0.05) up regulated IL-1β, Type 1 IFN, IFN g2b, IL10 and TNF-α gene expression. Heat shock protein 70 (HSP70) gene expression was significantly (P < 0.05) lower as compared to control group at the end of trial. Common carp fed with the two compounds had higher survival rates (69.3%) compared to the controls (32.0%) after challenged with the pathogen, Aeromonas hydrophila. The present study indicates that the two isolated active compounds could positively influence immune response and enhance disease resistance of common carp against A. hydrophila infection.
