The purpose of this study is to determine the safety and effectiveness of a nutritional supplement, the probiotic bacteria GanedenBC30. Probiotics are live microorganisms which when taken in large enough amounts are supposed to provide a health benefit. The investigators would like to see whether this supplement can be used as a treatment for Irritable Bowel Syndrome (IBS) and Major Depressive Disorder (MDD). Participants in the study will have both IBS and MDD. In order to find out if GanedenBC30 is effective in treating IBS and MDD, the investigators will compare it to a placebo. About 32 people will take part in this research study. About half of these 32 people will take part at Charles River Medical Associates and half will take part at Burlington Medical Associates. Participants will be recruited from among patients already being seen at Charles River Medical Associates or Burlington Medical Associates for their primary care. The study will last for two months, during which time participants will make eight study visits and will take either Ganeden BC30 or placebo. The investigators hypothesize that subjects treated with the probiotic will have significantly better outcomes with regard to depression and IBS symptoms compared to those who receive placebo.