There are trillions of bacteria living in our guts and there's growing evidence that they can have a major impact on our mental well-being. So could we soon see a food supplement that can treat depression? The science behind this so-called gut-brain axis and whether we can manipulate it isn't yet conclusive, but there are plenty of believers. We speak to a woman who says her irritable bowel syndrome and anxiety disappeared after she started taking kefir, a fermented milk drink. She was so convinced by its impact that she now runs a south London business making and selling it. There are already food supplements out there targeting anxiety and depression, but are they getting ahead of the science? We speak to one of the major probiotics manufacturers and hear from a leading scientist who says unproven product claims could be dangerous for mental health patients and that they shouldn't be seen as a silver bullet. Plus, how human are we really? We find out just how many strings our microbes are pulling. (Image: Head outline with food representing the brain. Credit: Getty Images)