There is a huge need to search for new treatment options and potential biomarkers of therapeutic response to antidepressant treatment. Depression and metabolic syndrome often coexist while pathophysiological overlap, including microbiota changes, may play a role. The aim of the study is to assess the effect of probiotic supplementation on symptoms of depression, anxiety and stress, metabolic parameters, inflammation and oxidative stress markers, and faecal microbiota in adult patients with depressive disorders depending on the co-occurance of MetS. The trial will be a four-arm, parallel group, prospective, randomized, double-blind, controlled design that will include 200 participants and will last 20 weeks. The probiotic preparation will contain Lactobacillus helveticus Rosell®-52, Bifidobacterium longum Rosell®-175. We will assess the level of depression, anxiety and stress, quality of life, blood pressure, body mass index and waist circumference, white blood cells count, serum levels of C-reactive protein, HDL cholesterol, triglicerides, fasting glucose, faecal microbiota composition and the level of some faecal microbiota metabolites, as well as inflammation markers and oxidative stress parameters in serum. The trial may establish a safe and easy-to-use treatment option as an adjunct in a subpopulation of depressive patients only partially responsive to pharmacologic treatment. (ClinicalTrials.gov identifier: ).