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A new organ
Thousands of species live in the colon and together weigh about the same as the brain, several pounds. Their discovery over the last ten years is like finding a new organ.
The immune system
The chemicals they make are key for the immune system. Most of the immune system sits in the gut, where immune cells talk to the microbes all the time through those chemicals, and the immune system in turn helps fight aging, cancer, infections, and allergies.
Mood and appetite
Microbes also produce chemicals that affect the brain and help determine whether a person feels happy or sad. They help regulate appetite and the feeling of being full, and they supply the B vitamins and neurochemicals such as serotonin, which is key to how antidepressants work. How the body responds to painkillers, antidepressants, and food depends on them.
Why does the gut hold so much immune tissue?
The gut lining is a large surface in constant contact with food, microbes, and whatever else arrives. Gut-associated lymphoid tissue, including clusters called Peyer's patches, is often described as holding around 70 percent of the body's immune cells. Immune cells there learn to tolerate harmless bacteria and food while attacking pathogens. In a 2005 paper in Cell, Sarkis Mazmanian showed that a single molecule from the bacterium Bacteroides fragilis could correct immune defects in germ-free mice.
How do gut microbes help make serotonin?
About 90 percent of the body's serotonin is made in the gut by enterochromaffin cells. In a 2015 paper in Cell, Jessica Yano and Elaine Hsiao showed that certain spore-forming bacteria signal those cells to produce more of it. Germ-free mice had lower gut serotonin, which rose after colonization with microbes. Serotonin made in the gut does not cross into the brain, which makes its own.
How do gut microbes signal the brain?
The routes include the vagus nerve, immune signaling, and microbial chemicals such as short-chain fatty acids carried in the blood. In a 2011 paper in PNAS, John Bienenstock, John Cryan, and colleagues found that mice fed Lactobacillus rhamnosus showed less anxiety-like behavior and altered GABA receptors in the brain. The effects disappeared when the vagus nerve was cut.
How was the human microbiome mapped?
The US National Institutes of Health launched the Human Microbiome Project in 2007, and its main results appeared in Nature in 2012. The project sampled 242 healthy adults at five body sites. Most gut bacteria cannot be grown in a lab dish, so researchers read their DNA, especially the gene for 16S ribosomal RNA, to identify them.
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