Preliminary Study On The Mechanism Of Integrate Prevention And Control Of Streptococcosis In Tilapia By Compound Probiotics

In this paper,tilapia was used as the research object.By adding several different combinations of probiotics in the feed,the effects of compound probiotics on the tilapia probiotics mechanism and disease resistance were preliminarily explored,and provided theoretical basis for the application of compound probiotics in the comprehensive prevention and control of streptococcosis in tilapia.The healthy tilapia with initial body weight of(99.17±1.80g)were randomly divided into five groups,45 in each group.The basic feed group,Bacillussubtilis-Bacilluslicheniformisgroup,Bacillussubtilis-Enterococcus faecalis group,Bacillus licheniformis-Enterococcus faecalis group,Bacillus subtilis-Bacillus licheniformis-Enterococcus faecalis group were set up,and the samples were taken after 42 days of continuous feeding.The effects of compound probiotics on growth performance,digestive enzyme activity,intestinal mucosal structure,immune function,antioxidant capacity and intestinal flora of tilapia were studied in order to explore its probiotic mechanism.The results showed that the weight gain rate and specific growth rate of tilapia were significantly increased in Bacillus subtilis-Bacillus licheniformis group,Bacillus subtilis-Enterococcus faecalis group,Bacillus licheniformis-Enterococcus faecalis group,Bacillus subtilis-Bacillus licheniformis-Enterococcus faecalis group(P < 0.01).Compared with the basic feed group,the feed coefficient of tilapia added compound probiotics decreased by 28.01%,32.38%,30.11% and 30.67%,respectively,and there was no significant difference between the groups(P>0.05).The activities of trypsin,amylase and lipase in the intestine and hepatopancreas of tilapia were significantly increased in the experimental groups with compound probiotics(P < 0.01).The intestinal mucosa structure of each experimental group with compound probiotics was more complete and clearer than that of the control group.The intestinal villi were well developed and arranged neatly and tightly compared with the control group.The length,width and thickness of intestinal villus of tilapia in each experimental group were higher than those in the basic feed group(P < 0.01).The spleen index,immune index and antioxidant index of tilapia in each experimental group were higher than those in the basic feed group.The results of 16 Sr DNA high throughput sequencing showed that the intestinal microorganisms of tilapia mainly consisted of Fusobacteria,Proteobacteria,Bacteroides,Actinobacteria,Cyanobacteria,and Firmicutes.Compared with the control group,the relative abundance of Proteobacteria,Bacteroidetes,Actinomycetes,and Firmicutes in the experimental groups added with compound probiotics was significantly higher than that in the control group.The number of OTU,Chao1 value,Ace value,Shannon index and Simpson index of the control group were lower than those of the experimental group with compound probiotics.The results showed that the growth performance of tilapia could be improved by adding compound probiotics into the feed,the digestive enzymes activities in the intestine and hepatopancreas of tilapia could be effectively improved,and the intestinal mucosal morphology of tilapia could be improved,the immune and antioxidant functions of tilapia could be enhanced,the intestinal flora structure could be regulated,and the probiotic effect of the Bacillus subtilis-Enterococcus faecalis group(group C)and Bacillus licheniformis-Enterococcus faecalis group(group D)is superior to other groups.In order to verify the effect of compound probiotics on the prevention and control of tilapia against Streptococcus agalactiae,the tilapia was artificially infected with Streptococcus agalactiae after the42-day culture experiment.The results showed that the protection rates of tilapia in the experimental groups with compound probiotics were as follows: Bacillus subtilis-Enterococcus faecalis group(40%)> Bacillus licheniformis-Enterococcus faecalis group(33.33%)= Bacillus subtilis-Bacillus licheniformis-Enterococcus faecalis group(33.33%)>Bacillus subtilis-Bacillus licheniformis group(20%)>basic feed group.The activities of LZM,AKP,T-SOD,POD and T-AOC in serum of tilapia in each experimental group were higher than those in the basic feed group.At the phylum classification level,the relative abundance of Fusobacteria in the basic feed group,Bacillus subtilis-Bacillus licheniformis group,Bacillus subtilis-Enterococcus faecalis group,Bacillus licheniformis-Enterococcus faecalis group,Bacillus subtilis-Bacillus licheniformis-Enterococcus faecalis group were 23.33%,46.27%,37.95%,43.56%,57.29%,respectively.The relative abundance of Actinobacteria was 20.08%,2.2%,6.65%,2.63%,and 4.58%,respectively;the relative abundance of Firmicutes was 5.88%,8.27%,20.80%,8.04%,and 2.27%;The relative abundance of Firmicutes in the Bacillus subtilis-Enterococcus faecalis group was significantly higher than that in the other groups.The relative abundance of the Fusobacteria in the Bacillus subtilis-Bacillus licheniformis-Enterococcus faecalis group was significantly higher than that in the other groups.In conclusion,the results showed that the addition of compound probiotics in feed could regulate the composition of intestinal flora of tilapia,improve the immune and antioxidant functions of tilapia,and enhance the disease resistance of tilapia,so as to improve the resistance of tilapia to Streptococcus agalactis,and the Bacillussubtilis-Enterococcus faecalis group had the best disease resistance effect.