Effects of dietary supplementation of intestinal autochthonous bacteria on the innate immunity and disease resistance of grass carp (Ctenopharyngodon idellus)

Intestinal microorganisms of aquatic animals have a crucial impact on the host and some may have the potential to be used as probiotics. In this work, three strains of autochthonous bacteria were isolated from the intestinal tract of the healthy grass carp (Ctenopharyngodon idellus). The effects of single or combined use of dietary Shewanella xiamenensis A-1, S. xiamenensis A-2, and Aeromonas veronii A-7 were assessed on innate immunity and disease resistance of grass carp. The grass carp were fed 28 days by five experimental diets: i) one control diet with no probiotics, ii) three diets supplemented with 1 x 10(8) cell g(-1) of intestinal bacteria A-1 (group A1), A-2 (group A2) and A-7 (group B), and iii) mixed diet (group MIX) comprising of bacteria A 1, A-2 and A 7 dosed at 1 x 10(8) cell g(-1), at a ratio of 4: 2: 1, which was set according to their quantity ratio in our study. At the end of the 28-day feeding experiment, the survival of grass carp against Aeromonas hydrophila was challenged for 14 days. Results showed that responses of several non-specific immune parameters of groups fed probiotics were enhanced; i.e. all probiotic groups led to significant enhancement of both respiratory burst, phagocytic and lysozyme activities than that of control activity. The complement C3, total serum proteins, albumin and globulin levels were also significantly improved on day 14 of feeding regime with probiotics compared to the control. Moreover, the expression of four immune-related genes (IL-8, IL-1 beta, lysozyme-C, and TNF-alpha genes) was significantly up-regulated in grass carp fed probiotic diets compared to control group on days 7, 14, and 28. The cumulative mortality of grass carp, experimentally challenged with A. hydrophila, was reduced in groups fed probiotics, i.e. A-1 = 46.67%, A-2 = 33.33%, B = 53.33% and mixed = 26.67%, respectively, than that of controls (80%) after 14 days of post-infection. Overall, the results of this study indicated the potential of using the three isolated autochthonous bacteria (single or combined) to improve immunity and disease resistance of grass carp. (C) 2015 Elsevier B.V. All rights reserved.