Effects of dietary probiotic supplementation on the growth,gut health and disease resistance of juvenile Nile tilapia(Oreochromis niloticus)

This study investigated the effects of the Streptococcus agalactiae antagonizing probiotics Bacillus cereu NY5 and Bacillus subtilis as feed additives for Nile tilapia in terms of growth performance,intestinz health and re sistance to S.agalactiae.A total of 720 apparently healthy juvenile Nile tilapia(0.20±0.05 g were randomly divided into 4 equal groups with 3 replicates for each group.Fish were fed a basal die(control check group,CK group) supplemented with B.subtilis(1 x 108 CFU/g feed,BS group),B.cereu NY5(1 x 10～8 CFU/g feed,BC group),and B.subtilis+B.cereus NY5(0.5 x 10～8 CFU/g feed of each probioti BS+BC group) for 6 wk,and the probiotic supplementation groups were then fed the basal diet for 1 w to investigate the gut microbial community.The results of this study showed that BS+BC and B treatments significantly increased weight gain(WG),feed conversion ratio(FCR) and S.agalactia resistance in Nile tilapia(P <0.05).Gut microvilli length and density and c-type lysozyme(lyzc) gen expression were significantly increased by probiotic supplementation(P <0.05).The results of high throughput sequencing showed that the B.cereus NY5 and B.subtilis+B.cereus NY5-supplementei feed resulted in a significant improvement in tilapia autochthonous gut bacterial communities ani had a stimulation effect on a variety of potential probiotics after 6 wk of feeding.After cessation c probiotic administration for 1 wk,the gut bacteria of the fish in the BS+BC and BC groups had mino changes and maintained a stable state.Consequently,it was inferred that,as a feed supplement,B.cereu NY5 and the mixture of B.subtilis and B.cereus NY5 at 1 x 10～8 CFU/g feed were able to promote growtl and disease resistance,which may be associated with the supplement's effects on gut immune statu intestinal morphology,and intestinal microbial community composition.