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Thursday, March 24, 2016

Microbes can play games with the mind

The bacteria in our guts may help decide who gets anxiety and depression


The 22 men took the same pill for four weeks. When interviewed, they said they felt less daily stress and their memories were sharper. The brain benefits were subtle, but the results, reported at last year’s annual meeting of the Society for Neuroscience, got attention. That’s because the pills were not a precise chemical formula synthesized by the pharmaceutical industry.
The capsules were brimming with bacteria.
In the ultimate PR turnaround, once-dreaded bacteria are being welcomed as health heroes. People gobble them up in probiotic yogurts, swallow pills packed with billions of bugs and recoil from hand sanitizers. Helping us nurture the microbial gardens in and on our bodies has become big business, judging by grocery store shelves.
These bacteria are possibly working at more than just keeping our bodies healthy: They may be changing our minds. Recent studies have begun turning up tantalizing hints about how the bacteria living in the gut can alter the way the brain works. These findings raise a question with profound implications for mental health: Can we soothe our brains by cultivating our bacteria?
By tinkering with the gut’s bacterial residents, scientists have changed the behavior of lab animals and small numbers of people. Microbial meddling has turned anxious mice bold and shy mice social. Rats inoculated with bacteria from depressed people develop signs of depression themselves. And small studies of people suggest that eating specific kinds of bacteria may change brain activity and ease anxiety. Because gut bacteria can make the very chemicals that brain cells use to communicate, the idea makes a certain amount of sense.
Though preliminary, such results suggest that the right bacteria in your gut could brighten mood and perhaps even combat pernicious mental disorders including anxiety and depression. The wrong microbes, however, might lead in a darker direction.
Open channels

Although the communication lines aren’t fully understood, bacteria in the gut and cells in the brain may stay in touch in several ways. Signals can move along the vagus nerve or be carried by chemical messengers, such as serotonin, and by molecules that travel via the immune system. 

Studying germ-free mice
Bacteria in the gut may help brains develop, based on studies from mice born and raised without bacteria. These mice are different from normal mice in several key brain areas.

Striatum: In mice without bacteria, the flux of the neural messengers dopamine and serotonin is altered in the striatum, a brain area involved in movement and emotional responses. New connections may form more readily in the striatum too. These changes may cause bacteria-free animals to move and explore abnormally.

Hippocampus: Involved in memory and navigation, the hippocampi of germ-free mice have reduced levels of molecules that sense serotonin and the growth factor BDNF. These mice display memory problems.

Amygdala: Germ-free mice have changes in the levels of serotonin, BDNF and other signaling molecules in the amygdala, a brain structure involved in emotions. These alterations might contribute to an increase in risk-taking behavior.

Hypothalamus: The brain’s stress responder, the hypothalamus, shows boosts in corticotropin-releasing factor and adrenocorticotropic hormone in germ-free mice. The changes might be related to the animals’ heightened stress responses.

SOURCE: S.M. COLLINS, M. SURETTE AND P. BERCIK/NAT. REV. MICROBIOL. 2012

Cecile G. Tamura