Preclinical studies demonstrated that noninvasive methods that alter gut microbiota are capable of inducing antidepressant like effects in rat models of depression (Desbonnet et al, 2010). Findings in human populations showed differences in gut flora of depressed individuals when compared to healthy controls, including higher levels of Bacteroidetes, Proteobacteria and Actinobacteria, and a negative correlation between Faecalibacterium and the severity of their depression (Jiang et al, et al 2015). Other studies point to a beneficial effect of probiotics on mood and anxiety in normal volunteers (Messaoudi et al, 2011; Benton et al, 2007), as well as in individuals with Irritable Bowel Syndrome (O' Mahoney et al, 2005) and Chronic Fatigue Syndrome (Rao et al, 2009). However, there have been no systematic studies to date reporting on the impact of probiotics on mood in individuals with major depression.