Abstract: The purpose of this study was to explore the effects of probiotics on chickens' resistance to Escherichia coli infections,and to provide a basis for the development of microecological agents.Based on the Oxford Cup test and the bacterial adhesion inhibition test,three kinds of probiotics and their equal proportion mixed bacterial liquid with good effect of inhibiting Escherichia coli in vitro were obtained initially:Lactobacillus plantarum ZN-3,Lactobacillus rhamnosus QC,Clostridium butyricum HYCB and mixed bacteria preparation X.The 7-day-old SPF chicks were continuously orally fed with probiotics for 30 days and infected with Escherichia coli XT-13 once a day from the 26th to 30th day,and the survival rate and weight change were recorded.Through the same oral and infection protocols,the chicks were necropsied after infection with XT-13,and the results of necropsy and pathology were analyzed,and the amount of bacteria in the liver and intestines was detected by immunohistochemistry.In addition,in order to analyze the effect of oral administration of mixed probiotic preparations on cytokines in chickens infected with Escherichia coli,non-lethal doses of XT-13 were selected to infect chickens,and the levels of interleukin-10 (IL-10),interleukin-17 (IL-17) in serum and avian defensin-β1 (AvBD1) in the liver were measured on the 4th,8th and 14th days after infection.At the same time,the bactericidal ability of the serum was tested on the 14th day after infection.Finally,the broad-spectrum protection ability of probiotics to O78 serotype Escherichia coli AV006 infected chickens was evaluated.The results showed that the antibacterial effect of the supernatant of each probiotic fermentation broth was obvious,and the mixing of probiotics could reduce the adhesion rate of XT-13 to DF-1 cells (P<0.05).The weight gain effect of mixed probiotics was better than that of single probiotics,which had a good protective effect on chickens infected with XT-13.The survival rate was 87.50%.The mixed probiotics significantly reduced the load of XT-13 in liver and large intestine,alleviated the pathological damage,and regulated the levels of IL-10 and IL-17 in serum and the levels of AvBD1 in the liver,while improving the bactericidal ability of the serum.In addition,the broad-spectrum protection test showed that the mixed probiotics reduced the mortality of chickens infected with O78 serotype Escherichia coli.Therefore,the mixed preparation of Lactobacillus plantarum ZN-3,Lactobacillus rhamnosus QC and Clostridium butyricum HYCB could promote chick weight gain,reduce bacterial load,reduce tissue lesions,regulate the host's immune response,and had a good prevention effect on avian colibacillosis,which had a great potential to develop into a probiotic preparation against avian colibacillosis.

Key words: probiotics; avian colibacillosis; pathology; broad spectrum protection