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Sterile mice
Large human studies mostly show that something is associated with something else, without showing cause or effect. Mice raised sterile, with no microbes of their own, get around that. Researchers create microbes in the lab or take them from mice that are genetically anxious, put them into the sterile mice, and watch for changes in mood and attention span.
The mood and attention span change. The condition is transmissible, which shows the microbes have a direct effect rather than being secondary.
The human data
The human data lines up with it. Virtually all depressed or anxious people have deranged microbiomes and produce abnormal chemicals. One of those chemicals is serotonin, which is made by gut microbes and helps keep the neurochemical balance that stops people from getting very depressed or very anxious.
How are germ-free mice raised?
Pups are delivered by cesarean section into sterile conditions and kept in sealed plastic isolators with sterilized air, water, food, and bedding. James Reyniers pioneered the approach at the University of Notre Dame in the 1930s and 1940s. Without microbes, the animals' immune tissue and intestinal lining develop abnormally.
How did microbe swaps change mouse personality?
In a 2011 paper in Gastroenterology, Premysl Bercik, Stephen Collins, and colleagues at McMaster University swapped gut microbes between two mouse strains, one naturally timid and one bold. Each strain took on the behavior of its donor. The timid mice became more exploratory, and the bold mice became more cautious.
Has depression transferred from humans to animals?
In a 2016 paper in the Journal of Psychiatric Research, John Cryan, Ted Dinan, and colleagues at University College Cork gave rats gut microbes from depressed patients. The rats developed depression-like behavior, including reduced interest in pleasurable things and signs of anxiety. Rats given microbes from healthy people did not.
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