The microbes in our body make up 100 trillion cells, ten fold the number of human cells. They encode 100-fold more genes than our own genome and weigh three pounds-the same as our brain. To nautre, we are more microbial than we are human. We could sculpt a unique image of ourself without human cells or genes, from the gut habitat in the foiled tube of our intestines, colon, skin, and fingertips. ¹³

Over the past decade we have witnessed substantial developments in studying this gut habitat and microorganisms at the bacterial and genetic level.²¹ From this, we've learned gut microbiota are involved in the development and function of host immune functions, the regulation of gut mobility, maintaining intestinal homeostasis, and nutrient absorption. Colonization of the gut microbiota begins at birth, and after a year infants develop a complex microbiome.

A growing body of research has begun to look at the imapct of the gut microbiota on brain and behavior. One area of focus involves the interactions between commensal bacterial organisms and the nervous system, which emerged the concept of the gut-brain-axis. The following pages seek to better understand how the microbiome and brain talk to each-other, and the pathways that signal this communication. A central question I've kept with me throughout this project is: where do the gut-brain interactions currently reside in neuroscientific research, and how can it progress forward? I hope the following pages serve to better understand this question, and leave you with more along the way.