Lactulose selectively stimulates members of the gut microbiota, as determined by multi-modal activity-based sorting

There is much interest in the development of dietary supplements that selectively promote the growth of beneficial gut bacteria. The selectivity of many candidate prebiotics has, however, not been thoroughly investigated. Here, we evaluated stimulation of the human gut microbiota by the disaccharide lactulose using an <i>ex vivo</i> multimodal activity-based cell sorting approach. Incubation of human donor stool with lactulose resulted in growth or stimulation of a restricted diversity of bacterial genera, most prominently <i>Bifidobacterium</i>, <i>Collinsella</i>, and <i>Lactococcus</i>. Physiological analysis of lactulose-responsive strains isolated by Raman activated cell sorting revealed that most were capable of lactulose degradation. Among these isolates, <i>Lactococcus lactis</i> could not degrade lactulose, but its growth was boosted by co-cultivation with lactulose degraders. This suggests that inter-species facilitation contributes to the lactulose degradation niche. Moreover, we observed that lactulose selectively activates metabolically important taxa, including health-associated genera such as <i>Faecalibacterium</i> and <i>Gemmiger</i>^1,2,3, potentially indicating broader functional effects beyond compositional changes. These results provide novel insights into the physiology and ecology of lactulose utilization by the human gut microbiota and underscore the potential of lactulose as a prebiotic dietary supplement.