The human microbiome consists of about 100 trillion bacteria and microbial cells—give or take a few trillion—that outnumber human cells something like ten to one. They can significantly affect human physiology. Changes in one’s microbiome can trigger changes in many cellular activities that can be beneficial or contribute to disease. The results of a small study published in Nature revealed that bacteria living in the gastrointestinal system are surprisingly responsive to change in what a person eats. Moreover, these changes in one’s microbiome can happen incredibly fast—within three or four days of a big shift in what you eat. Lawrence David, assistant professor at the Duke Institute for Genome Sciences and Policy and one of the study’s authors, evaluated ten participants: some with plant-based diet (avoiding animal products) and others with an animal-based diet (eating milk, cheese, and meat). In the subjects eating animal products the researchers saw a significant increase in Bilophila wadsworthia—reportedly the third most common anaerobic bacteria recovered from patients with perforated and gangrenous appendicitis.