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You Are the Germs You Eat aka How Human Culture Shapes Evolution

Posted by admin at 10:01 PM Friday, April 16, 2010

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Your gut is filled with trillions of bacteria that feed on the remains of your every meal(1). These bacteria, however, are not simply freeloaders that take advantage of a cozy habitat where food is always abundant; they serve many important functions. Among these functions are the salvaging some nutrients which our bodies cannot digest, production of vitamin K and maintaining the proper functioning of the immune system(2).

Life is not easy for an intestinal resident bacterium. There is intense competition for the available resources, in this case what is left of the food we eat after our stomach and intestinal enzymes have broken down most of the complex compounds. Therefore, bacteria that can better break down what is left will survive and multiply better. For example, humans cannot break down the polysaccharides in the plants which we feed on (think “dietary fibers”), so these compounds are fair game for the intestinal flora. Therefore, one would think that the resident bacteria (those that have won, for now, the natural selection contest) have accumulated a lot of these “glycosidases” (enzymes that break down carbohydrates), and indeed this is what research has shown(3). So the bacteria that live in our gut are proficient at breaking down our dietary remains, which makes a lot of sense from an evolutionary perspective.

The question that one can ask is how these bacteria have evolved to have so many genes for breaking down specifically the carbs that we eat and can’t digest (especially since not all of us eat the same carbs)? Well, one general way bacteria evolve is by a mechanism called “lateral gene transfer”, which basically means bacteria can share DNA with each other. Lateral gene transfer is responsible for specifically hard to control infections: the disease pathogens get the genes for antibiotic resistance from other bacteria, process which makes them harder to kill. It is also possible that lateral gene transfer is responsible for the accumulation of glycosidase genes in gut bacteria.

On April 8, 2010, Hehemann et al.(4) reported a rather interesting example of such genetic transfer. The team was studying a glycosidase known as porphyranase, which breaks down the polysaccharide porphyran. Porphyran is found mainly in a type of marine red algae named Porphyra (hence its name). Though the researchers started studying this enzyme in a marine bacterium that shares an ecosystem with the algae, they soon discovered it was also found in a gut bacterium (Bacteroides plebeius). Now the real twist: the gut bacteria only possess porphyranase in Japanese individuals, not in Americans(3, 5). The probable reason for this: people in Japan eat seaweed in large amounts, while people in the USA do not. The fascinating conclusion is that bacteria in our diets help shape the bacteria in our gut. The different eating habits of humans across the world contribute to them having different intestinal floras, a discovery which might have big implications in the current global context. The food we eat today in the developed world is significantly “cleaner” that the food that has shaped our evolution. Also, global trade allows us to eat foods that were previously unavailable to us because of geography. All these mean that the ecosystem of our intestines is changing faster than ever before, but also that we are depriving our residents of the marine bacteria they used to see very often. This could have major implications for our health, though at this point what those implications are is anyone’s guess. What better way to end this discussion than the scientist’s favorite phrase: More research is required to…

  • 1. L. Dethlefsen, M. McFall-Ngai, D. A. Relman, Nature 449, 811 (2007).
  • 2. F. Guarner, J.-R. Malagelada, The Lancet 361, 512 (2003).
  • 3. P. J. Turnbaugh et al., Nature 457, 480 (2009).
  • 4. J.-H. Hehemann et al., Nature 464, 908 (2010).
  • 5. K. Kurokawa et al., DNA Res 14, 169 (January 1, 2007, 2007).

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