Overview of the experimental procedure for bacterial reconstitution. During the experiment, all mice (including SPF mice) were housed in a Trexler-type flexible film isolator in a standard germ- free state. For generation of 0WexGF (mice, i.e., GF mice with bacterial reconstitution at the time of delivery), pregnant GF mice were housed with SPF female mice 1 day before delivery and only male pups were retrieved after weaning. To generate 5WexGF mice (i.e., GF mice with bacterial reconstitution at 5 weeks old), since 5 weeks, GF male mice began to cohabit with SPF female mice of the same age. Male mice in all groups were sacrificed at 9 weeks of age.