Trillions of microbes live in our gastrointestinal tracts, and all of those cells have their own genomes and proteins, many of which can affect our bodies. Scientists are learning more about the various ways that gut microbes can influence our brains and behavior. New research has drawn attention to viruses that live in the gut microbiome, sometimes called the gut viriome. Many of those viruses only infect bacterial cells; those viruses are called bacteriophages or phages. Reporting in Cell Host & Microbe, a team of scientists showed that bacteriophages called Caudovirales and Microviridae are prominent members of the gut viriome, and they are linked to changes in memory tests and executive functions. Higher levels of a phage called Microviridae were associated with an impairment in executive functions, while higher levels of Caudovirales have been associated with improvements in memory. These trends held true in mouse and fruit fly models as well as humans.
In this study, scientists enrolled a group of 942 people, and the volunteers reported what they ate to the researchers. Samples were also taken to assess the gut microbiome, which can be done by assessing the DNA in fecal samples, then identifying the microbial species the samples contain. This effort revealed that people carrying higher levels of Caudovirales were better at verbal memory and executive processes. But if there were higher levels of Microviridae in a person's gut, their executive abilities were not as good.
This research also revealed that people who consumed more dairy products had higher levels of Caudovirales in their gut microbiome. Previous work by other scientists has suggested that people who eat more dairy perform better on cognitive tests, which seems to confirm these findings.
The researchers tested these results in a mouse model, transplanting microbiota from humans into the rodents' intestines. Mice were subjected to a novel object recognition test, and the scientists also found that there was an increase in the activity of genes in the prefrontal cortex that promote memory functions.
When mice received more Caudovirales, their cognitive performance was improved compared to other mice, and they displayed "significant improvements in spatial memory and emotional memory," said study leader Dr. Rafael Maldonado of Pompeu Fabra University.
When fruit flies were given lactococcal Siphoviridae bacteriophages, which are a type of Caudovirales, their memory test scores improved, and the activity of genes involved in memory was again promoted.
This research has revealed more about one aspect of relationship between the microbiome and the brain, the so-called gut-brain axis, but there is much more work to be done, especially considering that there are many more species of bacteriophages in the gut, and their impact may be mediated through the bacteria they infect. The study has suggested, however, that changes to the diet could improve cognition and memory in people.