Amazingly, scientists from the Karolinska Institute in Sweden and the Genome Institute of Singapore led by Sven Pettersson recently reported in the Proceedings of the National Academy of Sciences Jan 31, 2011, that normal gut flora, the bacteria that inhabit our intestines, the normal "flora" of our gut, have a significant impact on brain development and adult behavior.
As to how much of "us" is "human", Princeton scientist Bonnie Bassler compared our 30,000 genes to the 3,000,000 bacterial genes inhabiting our skin and gut, demonstrating that "we" are only 1% human, genetically. It has long been known that the number of bacterial cells on our skin and in our gut is 10X the number of cells "we" have. We seem to be little more than a complex transport and maintenance "host" system for bacteria.
These bacteria have a great effect on us, as well. Gut bacteria affect our immune tolerance, may have an influence on autoimmune diseases and even how we use vitamin B6 with resultant effects on the health of nerve and muscle cells. They also effect anxiety-related behavior, although it's not yet clear whether it is positive or negative. The gut bacteria also directly effect gastrointestinal problems and make behavioral symptoms worse. A newly developed test for autism is based on products of bacterial metabolism.
Petersson, et. al.'s work was the first to demonstrate that gut bacteria affected the biochemistry and development of the brain. Mice lacking normal gut microflora were more active and less anxious than mice with normal gut bacteria levels. In one experiment, mice were given the choice of staying in the safety of a dark box or of venturing into a lighted box. Mice w/o gut bacteria spent significantly more time in the light box than their littermates with normal gut bacteria levels. The mice were then given the choice of venturing out on an elevated and unprotected bar, or remaining on a bar protected by enclosing walls. Once again, the bacteria-free mice were bolder than their "normal" littermates.
Pettersson’s team next looked at whether the behavioral influence of gut microbes on the brain could be reversed. Since the gut is naturally colonized by bacteria soon after birth, they looked at colonizing adult bacteria-free animals with normal gut bacteria - there was no effect on their behavior. However, if very young bacteria-free animals were colonized, these effects could be reversed, demonstrating that there is a critical period in the development of the brain when the bacteria are influential.
What is going on? How does this effect happen? Two genes tied to anxiety -- NGF1-A and BDNF -- were at lower levels in certain brain regions in gut bacteria-free animals. Several neurotransmitters, including dopamine, serotonin and noradrenaline, associated with planning, movement coordination and responses to new stimuli were elevated in related brain regions, while there were no effects in brain regions involved in memory or executive function.
Pettersson’s team found roughly 40 genes affected by the presence of gut bacteria, demonstrating an amazing ability of gut bacteria to influence whether brain cells turn specific genes on or off. Specifically how they accomplish this feat is still unclear, although there are several possibilities including actually controlling chemical signals released from the gut into the bloodstream which then go to the brain.
The truly amazing conclusion is that animal behaviors and how the brains of young animals develop are likely strongly influenced by their gut bacteria.
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