This curious 2006 article has an interview with one researcher in one column and an interview with another researcher in another column. Their results differed.
Pro probiotic. “Children with [infectious acute diarrhea] who took Lactobacillus [various strains and species, in nutritional supplement form, not in yogurt form] had a shorter duration of diarrhea (on average 0.7 days shorter) than those who took placebo. Also, they had fewer episodes of diarrhea, i.e. fewer stools, on the second day of treatment than those in the placebo group. Interestingly, the children who took higher doses of Lactobacillus had shorter duration of diarrhea, and it seems that a daily dose of at least 10 billion viable bacteria is necessary to have a beneficial effect.”
Anti probiotic. “I published a big study looking at Lactobacillus GG in kids with Crohn’s disease who were already doing fairly well on medication. We put them on the probiotic or a placebo for two years. We followed them for two years and looked for whether the probiotic group had a lower rate of relapse and whether there were any differences between the two groups. We didn’t find any differences.”