This past weekend, the New York Times ran an article addressing the connection between the microbiome and mood. Evidence is increasing that resident microbes can affect host systems such as the immune system, the digestive system, and, more and more, the nervous system. However, the human microbiome is very complex, with over 1000 different species (and that’s just bacteria!). There are a number of hard-to-control variables when conducting human microbiome studies, such as diet and other potentially confounding factors. These can complicate drawing strong conclusions in human microbiome studies.