Probiotics are defined as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host,” and bifidobacteria and lactobacilli are most commonly used as probiotics in food and medicine globally. Research suggests that bifidobacteria have co-evolved with hominids for over 15 million years, and we have collectively named the species that are characteristic of the human intestinal tract as “Human-Residential Bifidobacteria (HRB)”. Accumulating evidence shows that HRB plays an extremely important role in human health. However, the mechanisms behind the probiotic effect of bifidobacteria remain unclear, because probiotics research has historically focused on the human (host) side (e.g., functional evaluation through clinical trials) and research from the bacterial side is limited. To address this research gap, our department will promote research from the probiotic side. Specifically, we will elucidate the molecular mechanisms underlying symbiosis and coevolution between HRB and humans.