The effects of different hydrolysis methods on peptidoglycan(PG) were assessed in terms of their impact on the innate immunity and disease resistance of Pacific white shrimp,Litopenaeus vannamei.PG derived from Bifidobacterium thermophilum was prepared in the laboratory and processed with lysozyme and protease under varying conditions to produce several different PG preparations.A standard shrimp feed was mixed with 0.05% PG preparations to produce a number of experimental diets for shrimp.The composition,concentration,and molecular weight ranges of the soluble PG were analyzed.Serum phenoloxidase and acid phosphatase activity in the shrimp were determined on Days 6-31 of the experiment.The protective activity of the PG preparations was evaluated by exposing shrimp to white spot syndrome virus(WSSV).Data on the composition of the PG preparations indicated that preparations hydrolyzed with lysozyme for 72 h had more low-molecular-weight PG than those treated for 24 h,and hydrolysis by protease enhanced efficiency of hydrolysis compared to lysozyme.SDS-PAGE showed changes in the molecular weight of the soluble PG produced by the different hydrolysis methods.Measurements of serum phenoloxidase and acid phosphatase activity levels in the shrimp indicated that the PG preparations processed with enzymes were superior to the preparation which had not undergone hydrolysis in enhancing the activity of the two serum enzymes.In addition,the preparation containing more low-molecular-weight PG enhanced the resistance of the shrimp to WSSV,whereas no increased resistance was observed for preparations containing less low-molecular-weight PG.These findings suggest that the immunity-enhancing activity of PG is related to its molecular weight and that increasing the quantity of low-molecular-weight PG can fortify the effect of immunity enhancement. The effects of different hydrolysis methods on peptidoglycan (PG) were assessed in terms of their impact on the innate immunity and disease resistance of Pacific white shrimp, Litopenaeus vannamei. PG derived from Bifidobacterium thermophilum was prepared in the laboratory and processed with lysozyme and protease under varying conditions to produce several different PG preparations. A standard shrimp feed was mixed with 0.05% PG preparations to produce a number of experimental diets for shrimp. The composition, concentration, and molecular weight ranges of the soluble PG were analyzed.Serum phenoloxidase and acid phosphatase activity in the shrimp were determined on Days 6-31 of the experiment. The protective activity of the PG preparations was evaluated by exposing shrimp to white spot syndrome virus (WSSV). Data on the composition of the PG preparations indicated that preparations hydrolyzed with lysozyme for 72 h had more low-molecular-weight PG than those treated for 24 h, and hydrolysis by p rotease enhanced efficiency of hydrolysis compared to lysozyme. SDS-PAGE showed changes in the molecular weight of the soluble PG produced by the different hydrolysis methods. Measurements of serum phenoloxidase and acid phosphatase activity levels in the shrimp indicated that the PG preparations processed with enzymes were superior to the preparation which had not undergone hydrolysis in enhancing the activity of the two serum enzymes. In addition, the preparation containing the more low-molecular-weight PG enhanced the resistance of the shrimp to WSSV, and the no increased resistance was observed for preparations containing less low-molecular-weight PG. These findings suggest that the immunity-enhancing activity of PG is related to its molecular weight and that increasing the quantity of low-molecular-weight PG can fortify the effect of immunity enhancement.