https://www.clinicalnutritionjournal.com/article/S0261-5614(18)30161-4/fulltext?rss=yes

“One hundred and ten depressed patients were randomly assigned to receive the probiotic (Lactobacillus helveticus and Bifidobacterium longum), prebiotic (galactooligosaccharide) or placebo for 8 weeks.”—authors
They were better after the treatment compared to placebo, P<0.05. (Hmmmm, is that good enough?)
“Overall, 8 weeks of probiotic supplements to subjects with MDD resulted in an improvement in BDI score compared with placebo whereas no significant effect of prebiotic supplementation was seen.

Study was registered in IRCT.ir under IRCT2015092924271N1.”—authors