The human gut microbiota includes species that play an important part in gut homeostasis. For example, bacterial species that produce the metabolites butyrate and lactate, such as Bifidobacterium and Lactobacillus, have been implicated in the maintenance of the colonic epithelium. Research efforts are focusing on understanding the mechanism underlying those beneficial effects. Kweon and colleagues report that lactic-acid-producing bacteria support intestinal epithelial cell regeneration. Bacterial-derived lactate promotes the expansion of intestinal stem cells (ISCs) and epithelial differentiation in vivo and ex vivo. Moreover, a Lactobacillus plantarum strain lacking lactate dehydrogenase activity failed to elicit ISC proliferation in mice. Finally, pretreatment with lactic-acid-producing bacteria or lactate protected mice from gut injury induced by radiation and drug treatment.