“…The gut microbiota (gut flora) plays a critical role in preserving host health by releasing various metabolites to stimulate hosts’ neural, endocrine, and immune systems [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. However, microbial functions can be changed markedly by multiple factors, such as lifestyle (living environments), nutrition and health condition (pathophysiological disorders), life-stage (age), and medical substances in humans [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. It can be further affected by management issues such as rearing conditions (restriction, crowding, heat, cold environments), breeding programs (selected for a special production indicator), and stress-related factors (transportation and weaning) in farm animals [ 16 , 17 , 18 , 19 ].…”