behavior.
We human beings may think of ourselves as a highly evolved species of conscious individuals, but we are all far less human than most of us appreciate. Scientists have long recognized that the bacterial cells inhabiting our skin and gut outnumber human cells by ten-to-one. Indeed, Princeton University scientist Bonnie Bassler compared the approximately 30,000 human genes found in the average human to the more than 3 million bacterial genes inhabiting us, concluding that we are at most one percent
human. We are only beginning to understand the sort of impact our bacterial passengers have on our daily lives.
Moreover, these bacteria have been implicated in the development of neurological and behavioral disorders. For example, gut bacteria may have an influence on the body’s use of
vitamin B6, which in turn has profound effects on the health of nerve and muscle cells. They modulate immune tolerance and, because of this, they may have an influence on
autoimmune diseases, such as multiple sclerosis. They have been shown to influence
anxiety-related behavior, although there is
controversy regarding whether gut bacteria exacerbate or ameliorate
stress related anxiety responses. In
autism and other pervasive developmental disorders, there are reports that the specific bacterial species present in the gut are altered and that
gastrointestinalproblems exacerbate behavioral symptoms. A newly developed biochemical test for autism is based, in part, upon the end products of bacterial
metabolism.
But this new study is the first to extensively evaluate the influence of gut bacteria on the biochemistry and development of the brain. The scientists raised mice lacking normal gut microflora, then compared their behavior, brain chemistry and brain development to mice having normal gut bacteria. The microbe-free
animals were more active and, in specific behavioral tests, were less anxious than microbe-colonized mice. In one test of anxiety, animals were given the choice of staying in the relative safety of a dark box, or of venturing into a lighted box. Bacteria-free animals spent significantly more time in the light box than their bacterially colonized littermates. Similarly, in another test of anxiety, animals were given the choice of venturing out on an elevated and unprotected bar to explore their environment, or remain in the relative safety of a similar bar protected by enclosing walls. Once again, the microbe-free animals proved themselves bolder than their colonized kin.
Pettersson’s team next asked whether the influence of gut microbes on the brain was reversible and, since the gut is colonized by microbes soon after birth, whether there was evidence that gut microbes influenced the development of the brain. They found that colonizing an adult germ-free animal with normal gut bacteria had no effect on their behavior. However, if germ free animals were colonized early in life, these effects could be reversed. This suggests that there is a critical period in the development of the brain when the bacteria are influential.
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