Hematoxylin and eosin staining
The partial heart, liver, spleen, lung and small intestine tissues of each group were removed and fixed with 10% formalin, were then dehydrated, and embedded in paraffin. To evaluate the pathological changes, the paraffin-embedded tissues were sliced into 5 μm thin sections to conduct hematoxylin and eosin staining, and then observed by a light microscope.
Viscera index measurement
After APEC infection for 24 h, the chickens of each group were euthanatized and obtain the weight. The complete heart, liver, spleen, lung, kidney and bursa of fabricius were harvested and weighed. The viscera index was calculated as each viscera weight divided by chicken weight.
16S rDNA