This project is aimed at investigating for the first time if probiotic supplementation improves the effect of antidepressants on depressive symptoms (i.e. better and faster remission) in patients with severe MDD. Furthermore, this study will also test if probiotic supplementation modulates immune signalling and inflammatory processes, HPA axis responses, neurogenesis, the release of appetite-regulating hormones, the composition of gut microbiota and brain perfusion, structure and activation and if these changes are associated with the probiotic-induced effect on depressive symptoms.