Enhancement of vibriosis resistance in juvenile Penaeus vannamei by supplementation of diets with different yeast products

Penaeus vannamei were reared on five different experimental diets containing: (1) Saccharomyces cerevisiae at 1%; (2) beta-glucan, extracted from S. cerevisiae, at 0.1%; (3) Phaffia rhodozyma at 1%; (4) experimental yeast (HPPR1) at 1% and; (5) a control diet. Wet weight, survival (5) and feed conversion ratio were monitored over a 7-week period. Results indicate that, while there was no significant difference in weight gain on the different diets (P = 0.196), survival of animals reared on the diets containing S. cerevisiae, P, rhodozyma and the experimental yeast was significantly higher (P = 0.006) than the animals reared on the diet containing beta-glucan, and higher, though not significantly, than those reared on the control diet. Biomass was found to be statistically different (P = 0.042), the animals reared on the P. rhodozyma diet displaying greater biomass than the animals on the beta-glucan diet. Feed conversion ratio was not different for any of the diets (P = 0.233). At the end of the growth trial, the ability of the shrimp to clear bacteria from the hemolymph was tested by immersing the animals in a viable cell suspension of Vibrio harveyi strain BP05. Twenty-seven hours post immersion, the animals fed the S. cerevisiae, P. rhodozyma HPPR1 and control diet had effectively cleared the bacteria from the hemolymph and were not statistically different from nonchallenged control shrimp, while the animals fed the glucan diet still displayed elevated numbers in the order of 10(3) CFU/ml (P = 0.002). Additionally, mortalities were noted for these animals during the immersion challenge, unlike for the animals on the other treatments. Determination of phenoloxidase activity of the animals showed a significant difference among the five treatments with phenoloxidase activity for the Phaffia-treated animals being significantly lower (P = 0.003) than any of the other diets except the beta-glucan diet. These results indicate that even though no clear immunostimulatory effect could be found for the different treatments, it appears that all three yeasts, and especially the Phaffia diet, had a positive effect on the animals, leading to better survival. The animals on the glucan diet showed poor performance overall, indicating that not only did this treatment not aid in increasing resistance of the animals to infection, but on the contrary, these animals showed reduced performance when compared to the animals fed the control diet. (C) 1999 Elsevier Science B.V. All rights reserved.