Certain species of bifidobacteria are highly prevalent and abundant in the infant gut. Their presence in the infant gut is at least partly due to their ability to metabolize human milk oligosaccharides (HMOs) and has been associated with a range of health benefits. Not all bifidobacteria can metabolize (all) HMOs and those that can may follow different metabolic pathways. According to Prof. Douwe van Sinderen weaning may impose metabolic substrate switching in some bifidobacterial species yet may also impact either positively or negatively on their prevalence and abundance thereby explaining so-called infant-specific and adult-specific bifidobacterial species.