It will test, in a proof-of-concept study, the antidepressant efficacy of adjunctive administration of probiotics in bipolar depression and evaluate potential inflammatory and metabolic markers associated with response to treatment. With this objective, 84 individuals with bipolar disorder type I or type II presenting subsyndromal depressive symptoms or a major depressive episode will be recruited. The study design lasts 12 weeks and is expected to last two years.