Animals and experimental design
1-day-old male hyline-brown healthy chickens were provided by Jilin Academy of Agricultural Sciences (Changchun, Jilin, China). The whole experiment mainly included seven groups: control group (CON), Baicalin treatment alone group (BAI), broad-spectrum antibiotic group (Abx), APEC infection group (APEC), broad-spectrum antibiotic + APEC group (Abx+APEC), Baicalin + APEC group (BAI+APEC), Baicalin+ broad-spectrum antibiotic + APEC group (BAI + Abx + APEC), and each group included 10 chickens (n=10). The microbiota-deplete chickens were established with broad-spectrum antibiotic (ampicillin, neomycin, metronidazole and vancomycin) in their drinking water for two weeks. Baicalin was dissolved in a solvent mixture containing DMSO at a dose of 200 mg/kg was administrated by oral gavage 1 week before APEC infection. The model of chicken colibacillosis was induced by intratracheal inoculation with 2×109CFU APEC-O78 for 24 h. chickens had free access to feed and water during the entire rearing period. The chickens were reared in an environmentally controlled room: The temperature was maintained at 35 °C for the first 2 days, and then decreased gradually to 22 °C (45% humidity) under a 16/8 light/dark cycle.