The Influence Of Bacillus Subtilis On Glucose Tolerance Of Nile Tilapia

Fish is the main economic species of aquaculture and an important source of high quality protein for the human body.Aquaculture,as one of the important food supply channels for human beings,has made rapid progress in recent years.However,with the continuous expansion of aquaculture,aquaculture also faces many problems,such as the shortage of fishmeal and fish oil resources,the rising cost of farming,and the environmental pollution caused by the low efficiency of fish use.Solving the series of problems that arise during fish farming is imminent.Polysaccharide represented by starch is an important part of aquatic feed.It has a wide range of sources and low prices.If we can improve the utilization of starch by fish,it can effectively alleviate the problems of rising fish culture costs caused by shortage of fishmeal and fish oil resources,and environmental pollution caused by low fish utilization efficiency.However,the utilization rate of carbohydrates in feed is low.Therefore,improving the utilization efficiency of carbohydrates in fish is one of the important research hotspots in aquaculture.Studies in higher animals have shown that gut microbes play an important role in the utilization of host carbohydrates,but in aquatic animals,related research has not yet been carried out.In this study,a strain of Bacillus subtilis,which can efficiently utilize starch to produce acetic acid,was isolated from the intestinal tract of tilapia by starch medium.The similarity of SU1 to Bacillus toyonensis BCT-7112 was 99%.Because SU1 has better starch utilization ability in vitro,this study mainly explores the effect of SU1 added to feed into tilapia on the tolerance of tilapia glucose,and preliminary research on its mechanism.In order to investigate whether SU1 can successfully colonize the fish gut,this study first screened SU1 with a medium supplemented with rifampicin,which was labeled with rifampicin resistance.SU1 bacteria were then added to the feed at 109 CFU/g dough to determine the colonization of SU1 in tilapia.In order to find out whether SU1 bacteria can affect the feed utilization efficiency and glucose tolerance of tilapia,three treatment groups were set up in this study: control group(MC group),high carbohydrate group(HC group),high carbohydrate plus bacteria group.(HC+LB group),an 8-week culture experiment was carried out to explore fish growth,glucose metabolism,and intestinal health.The results showed that SU1 had no significant effect on the growth of tilapia,but SU1 could increase the glucose tolerance level of tilapia.The Ussing Chamber technique combined with the relative expression of the intestinal tight junction protein gene for intestinal permeability showed that SU1 can improve intestinal barrier damage caused by high carbohydrate feed in tilapia.Further studies showed that the high carbohydrate group(HC+LB group)had higher levels of hepatic gluconeogenesis than the higher carbohydrate group(HC group)(P<0.05).Isotopic tracer glucose experiments showed that SU1 addition may affect the conversion of glucose to proteins and lipids,thereby increasing the glucose tolerance level of tilapia.The levels of insulin-related intestinal hormones GLP-1(glucagon-like peptide-1)and IGF-1(insulin-like growth factor 1)in serum were measured.The results showed that the high carbohydrate plus bacteria group(HC+LB group)intestinal hormone GLP-1 and IGF-1 levels were significantly higher than the carbohydrate group(HC group)(P<0.05).The detection of insulin receptor IR in the intestine showed that the expression of intestinal insulin receptor in the high carbohydrate group(HC+LB group)was significantly increased(P<0.01).By analyzing the data of the high carbohydrate plus bacteria group(HC+LB group)and the high carbohydrate group(HC group),we speculated that B.subtilis SU1 may increase tilapia glucose tolerance through three pathways.1.SU1 bacteria increased the expression of insulin receptor IR in the intestinal tilapia,and increased the glucose tolerance of tilapia through the insulin receptor pathway.2.SU1 bacteria repaired intestinal damage caused by high-starch feed of tilapia and increased glucose tolerance of tilapia;3.SU1 bacteria promoted the secretion of insulin-related hormones GLP-1 and IGF-1,and increased glucose tolerance in tilapia.However,the specific signaling pathway mechanism is still unclear.In this study,a strain of Bacillus subtilis SU1,which can efficiently utilize starch,was isolated from the digestive tract of fish,and its mechanism of improving glucose tolerance of tilapia was discussed.It provides a new way to study the efficiency of carbohydrate utilization in fish.The technical route and research platform involved in the study also laid the foundation for the subsequent development of aquaculture probiotics.