Yet another topic of particular interest to me (yes, I know…I am interested in pretty much everything!) is the gut-brain axis. I’m always looking for the latest on how one affects the other, as I reckon that this line of research is eventually going to lead to cures for illnesses ranging from autism to Parkinson’s to Alzheimer’s to so-called “mental illnesses,” and more.
An article was just published in the very prestigious journal, Nature, that beautifully illustrates how powerful is the effect of the gut biome on the brain.[i] Just the title is compelling enough: “A neurotransmitter produced by but bacteria modulates host sensory behavior.” (For all you parents of children with autism…just give that a think for an extra minute!) Researchers at Brandeis University used the roundworm worm, C. elegans (my regular readers know this animal is frequently used in research such as this): bacteria are actually this worm’s primary food source. They studied how worms fed different strains of bacteria then reacted to a molecule called octanol, which is secreted by some bacteria, and which is usually avoided in high concentrations by the worm. (They use their olfactory (smell) system to sense the presence of the octanol.)
Those worms fed the bacteria Providencia alcalifaciens were less likely to avoid octanol than worms fed other bacteria, and in fact, if these bacteria were present in the gut of a worm, it was more likely to move toward octanol. The presence of this particular bacteria made the smell of octanol more appealing. This suggested to the researchers that something secreted by the bacteria was influencing the worms’ behavior. The next step, of course, was to figure out just what that substance might be.
It turned out that the brain chemical tyramine, produced by this gut bacteria, was the key. In worms, tyramine is converted to something called octopamine, which, in turn, “…targets a receptor on sensory neurons that controls avoidance behavior.”[ii] That is, a substance produced by gut bacteria negated the worms’ natural tendency to avoid something. To test this, the scientists created genetically engineered worms that could not produce tyramine themselves. When fed Providencia alcalifaciens, however, they still did not avoid octanol, meaning that outside sources (i.e. bacteria in the form of food) could still drive behavior.
More than that, worms fed on Providencia alcalifaciens preferred to eat that kind of bacteria over any other kind. The tyramine produced by the bacteria was also the driving factor in this behavior. The bacteria did an excellent job of manipulating their host to perpetuate their own survival.
The scientists point out that other commensal bacteria have been shown to affect feeding decisions in fruit flies, including dictating food preferences, and in mammals, gut bacteria are known to “modulate satiety responses” (i.e. feeling hungry versus feeling full) although the mechanism of action is still unknown. Still, if you think about that in light of our current obesity epidemic, it would explain an awful lot. Our altered gut flora (i.e. biome depletion) could well be playing a huge role in directing people to eat more than they need or to eat foods that are good for the gut bacteria but not necessarily their hosts. The researchers conclude by summing up their findings: “Our results describe a pathway by which neurotransmitters produced by natural commensal bacteria direct host sensory behavioral decisions by supplementing or compensating for the activity of key host biosynthetic enzymes, thereby altering the fitness of both host and microorganisms.” So think about that the next time you reach for that deep fried Twinkie. Is it you craving it or is it your microorganisms?
Who or what is really in control here?! (cue Twilight Zone music)