The majority of depressed patients remain symptomatic despite treatment with currently available antidepressants, and a significant proportion do not respond at all. Thus, the development of new effective antidepressant interventions remains a significant unmet public health need. Given the accumulating evidence for a relationship between the gastrointestinal microbiota and the central nervous system functioning and pathophysiology, including that of affective disorders, it is cogent to measure the impact of probiotic interventions in individuals with refractory depression. The present study proposes to examine the gut microbiota, inflammatory milieu and impact of probiotics on mood in a population of individuals who have been refractory to treatment with standard antidepressant agents.