Despite considerable preclinical evidence, clinical trials assessing the effects of probiotics on individuals with major depressive disorder (MDD) are scarce. This study aimed to provide further evidence of the acceptability, tolerability and putative efficacy of probiotics in this patient group and to improve our understanding of the underlying mechanisms of action.

This double-blind randomised placebo-controlled pilot and mechanistic trial investigated the effects of an 8-week adjunctive multi-strain probiotic intervention in adults with MDD taking antidepressants. Psychiatric data and stool and blood samples were collected at baseline, week 4 and week 8. A computer-based emotion recognition task was also administered. Stool samples from 25 matched healthy controls were also obtained.

49 participants, randomised to probiotic (n = 24) or placebo (n = 25), were included in intent-to-treat analyses. Standardised effect sizes (SES) from linear mixed models demonstrated that the probiotic group attained greater improvements in depressive (HAMD week 4: SES [95%CI] = 0.70[0.01, 0.98]; IDS week 8: SES [95%CI] = 0.64 [0.03, 0.87]) and anxiety symptoms (HAMA week 4: SES [95%CI] = 0.67 [0.00, 0.95]; week 8: SES [95%CI] = 0.79 [0.06, 1.05]), compared to the placebo group. Attrition was 8% (n = 3 placebo, n = 1 probiotic), adherence was 97.2% and there were no serious adverse reactions. The probiotic modified the composition of the faecal microbiota by normalising richness and diversity towards healthy control levels. The probiotic also increased levels of specific taxa, including Bacillaceae (FDR p < 0.05), which correlated with reductions in anxiety scores (FDR p < 0.05). There was no impact of treatment on levels of inflammatory cytokines (CRP, TNFα, IL-1β, IL-6, IL-17) or BDNF. However, probiotics showed a tendency to increase positive affective bias and improved the accuracy of recognition of all emotions, except sadness.

Compared to placebo, the probiotic group had greater improvement in depressive and anxiety scores, from as early as 4 weeks. The acceptability, tolerability and estimated effect sizes on key clinical outcomes are promising and encourage further investigation of this probiotic as add-on treatment in MDD. The beneficial effects of probiotics in this patient group may be partially mediated by modification of the composition of the gut microbiota and improvement of affective biases, inherent to depressive disorders.