2.2. Bacterial strains
The bacterial strain used in this study (Enterotoxigenic E. coli K88 C83907) was purchased from the China Institute of Veterinary Drugs Control (Beijing, China) and preserved by the National Engineering Laboratory of Bio-feed Safety and Pollution Prevention (Hangzhou, China). The challenge strain harbored the genes for enterotoxin STb, LT and the genes for fimbria F4ac detected as described by Gao (2014).