But if the depression lingers and is accompanied by other markers like insomnia, fatigue, brain fog, or weight change, it can be classified as major depressive disorder (MDD). But there is another contributing factor: gut microbes. Gut-brain axisThe gut-brain axis has been boggling minds for more than a decade now, and gut microbes that can improve mood are called psychobiotics. The research is clear: As microbes ferment in our gut, they produce thousands of substances called metabolites. In fact, we can transfer gut microbes from depressed people into mice and make the mice depressed, too.