Gut Feeling: Understanding the Mechanisms Underlying the Antidepressant Properties of Probiotics

Inclusion criteria for MDD participants: * aged 18-55; * currently in a depressive episode according to DSM-5 criteria, who are currently on antidepressant treatment but remain symptomatic (HAM-D17 score \>13); * on stable treatment regimen of an approved treatment for at least 6 weeks; * non-smokers; * for participants willing to take part in the neuroimaging component: right-handed, no contraindications, non-claustrophobic, weight \<126kg; Exclusion criteria for MDD participants: * eating disorder, bipolar disorder, schizophrenia or psychotic symptoms; * substance dependence in the past year, except for caffeine; * active suicidal ideation; * use of probiotic supplements in the past 2 weeks, or regular use of a probiotic; * use of antibiotics in the past 12 weeks; * history of allergic reaction to any of the components of BioKult; * history of history of a systemic medical illness; * current presence of significant GI problems or disease or history of major GI surgery; * pregnancy or breastfeeding; * following a dietary regimen unrepresentative of the general population (e.g. fasting or following a specific diet); * regular/current use of antacids, proton pump inhibitors, laxatives, antidiarrheals; Inclusion criteria for Healthy Volunteers: * Aged 18-55; * No current or historic presence of depression, other psychiatric disorder or substance dependence * No history of a systemic medical illness; * No family history of psychiatric disorder; * Non-smoker; * Not used probiotic supplements in the past 2 weeks, nor regular use of a probiotic; * Not used antibiotics in the past 12 weeks; * No current presence of gastrointestinal disease, or history of major GI surgery; * No reported regular/current use of antacids, proton pump inhibitors, laxatives, antidiarrheals; * pregnancy or breastfeeding; * not currently following a dietary regimen or dietary restrictions unrepresentative of the general population (e.g. fasting or following a specific diet);