Numbers-wise, microbes including bacteria, viruses, and fungi outnumber human cells by about ten to one. The vast majority of these microbes are harmless or even beneficial, with just a small handful known to cause problems. One algae virus that was previously believed to be harmless in humans has been discovered to cause a modest reduction in brain power. Robert Yolken of Johns Hopkins University School of Medicine, Stanley Division of Developmental Neurovirology served as lead author of the paper, which was published in the Proceedings of the National Academy of Science.
Previous research by the team for an unrelated study found that the virus, Acanthocystis turfacea Chlorella virus 1 (ATCV-1), that typically infects freshwater green algae can be found in the throats of otherwise healthy humans. While some viruses mutate and can infect different species, such as avian flu evolving and infecting humans, this is a very rare example of a virus that is able to infect a completely different kingdom.
ATCV-1 was found in 40 of the 92 participants from that initial trial. Further study revealed that those who were infected by the virus scored 7 to 9 points lower on attention and visual processing tests, respectively, than those who were not infected. While this is not a massive difference, it is statistically significant.
“This is a striking example showing that the ‘innocuous’ microorganisms we carry can affect behavior and cognition,” Yolken said in a press release. “Many physiological differences between person A and person B are encoded in the set of genes each inherits from parents, yet some of these differences are fueled by the various microorganisms we harbor and the way they interact with our genes.”
The team went on to infect a group of healthy mice with the virus, which seemed to dramatically affect attention span and cognition. The infected group took more time solving mazes than their uninfected counterparts, and they did not pay as much attention to new pathways presented or new objects placed within the environment that should have been of interest.
Brain scans confirmed that ATCV-1 was the culprit behind the sapped attention by causing changes in expression to nearly 1300 genes in the hippocampus, the area responsible for storing memories and helping bodies be properly oriented in the environment. The virus altered how the hippocampus responds to the neurotransmitter dopamine, which is widely used throughout the brain and integral to the immune system. Though the virus has not been discovered to infect the brain directly, it could act on the connection with the immune system.
Though mice are not on par cognitively with humans, the fact that both groups showed attention deficit was of interest to the researchers. The team also isn’t sure exactly how the virus is producing these cognitive changes or how it initially came to infect humans. Further studies are required.
“The similarity of our findings in mice and humans underscores the common mechanisms that many microbes use to affect cognitive function in both animals and people,” added co-author Mikhail Pletnikov. “This commonality is precisely what allows us to study the pathologies that these microorganisms fuel and do so in a controlled systematic way.”
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