Abstract: The aim of this study was to evaluate the effects of Astragalus polysaccharide (APS), saponins (AS) and probiotics on the resistance to E. coli infection in broilers. A total of 200 1-day-old white-feathered broiler chickens were randomly divided into blank group, test group, positive group and model group, with 50 chickens in each group. The experiment lasted for 42 d. From 1 to 21 days, chicks in the blank group and model group were fed a basic diet and each of them was given 0.2 mL of normal saline every day, while chicks in the test group were fed a basic diet containing APS (1 g·kg^-1) and AS (10 mg·kg^-1), and each of them was given 0.2 mL of probiotic complex bacterial solution (Streptococcus alactolyticus∶Lactobacillus plantae∶Bacillus subtilis=1∶1∶1, the concentration of bacterial liquid was 1×10⁹ CFU·mL^-1), and chicks in the positive group were fed a basal diet containing 50 mg·kg^-1 doxycycline hydrochloride soluble powder every day. At the 21st day, chickens in test group, positive group and model group each were given 0.2 mL Escherichia coli O₇₈ bacterial solution (6×10⁸ CFU·mL^-1). Duodenum and jejunum tissue structure, sIgA, inflammatory factors, and oxidation markers were detected at days 0, 1, 7, 14, and 21 of infection (i.e., days 21, 22, 28, 35, and 42 of the trial period), as well as bacterial count in cecum contents. The results were showed as follows: 1) The number of E. coli in model group (except for the 0 day of infection) was significantly higher than that in blank group, test group and positive group (P<0.05), and the number of chicken lactic acid bacteria in test group was significantly higher than that in blank group, positive group and model group (P<0.05). 2) After 1 and 7 days of infection, the sIgA content in duodenum and jejunum tissues of test group and positive group was significantly higher than that of blank group and model group (P<0.05); After 14 days of infection, the sIgA content of jejunum tissue in test group was significantly higher than that in blank group and model group (P<0.05). 3) When infected for 1 and 7 d, the TNF-α content of duodenum and jejunum in the test group and the positive group was significantly lower than that in the model group (P<0.05); When infected for 7 days, the content of duodenum and jejunum IL-10 in the test group and the positive group was significantly higher than that in the model group (P<0.05); After 14 and 21 days of infection, the content of IL-10 in jejunal tissue in the test group and the positive group was significantly higher than that in the model group (P<0.05). 4) When infected for 1, 7, 14 and 21 days, the MPO activity of duodenal and jejunal tissues in the test group and the positive group was significantly higher than that in the blank group (P<0.05).When infected for 1 d, the SOD of jejunal tissues in the test group and the positive group was significantly higher than that in the model group (P<0.05), and when infected after 7 and 21 days, the T-AOC of duodenal and jejunal tissues in the test group and the positive group was significantly higher than that in the model group (P<0.05). In conclusion, under the condition of the current experiment, the combination of APS, AS and probiotic complex can enhance the ability of chicks to resist E. coli infection, and have the effect of replacing antibiotics.

Key words: Astragalus polysaccharides, Astragalus saponins, probiotics, broiler intestines, protective effect