There are approximately 38 trillion bacterial cells in the average human body, compared to approximately 30 trillion human cells. You are, by cell count, more microbe than person. This fact, established definitively by researchers at the Weizmann Institute in 2016, has not fully penetrated public understanding, perhaps because its implications are genuinely unsettling.
The Gut-Brain Axis
The most surprising finding in microbiome research over the past decade is the gut-brain axis — the bidirectional communication network between the enteric nervous system of the gut and the central nervous system of the brain. Gut bacteria produce neurotransmitters, including approximately 90% of the body's serotonin. They influence immune system development. They communicate with the vagus nerve. They are, in ways that neuroscientists are still working to understand, participants in cognition.
"We have been treating the microbiome as a parasite, or at best a passenger. It is neither. It is part of us in a way that challenges the concept of individual biological identity."
What Disrupts It
The human microbiome is highly sensitive to disruption — by antibiotics, by diet changes, by stress, by altered sleep patterns. The widespread overuse of antibiotics in both medicine and agriculture has, researchers now believe, fundamentally altered the microbiome composition of populations in industrialized countries relative to pre-antibiotic baselines. The effects of this alteration are not fully understood. The process of understanding them is just beginning.