Papers, Scissors, Rock: A bacterial strategy game


As the old saying goes: "There's nothing new under the Sun" and indeed it turns out that Nature also invented "Paper, Scissors, Rock" many millennia before the first kids ever settled a schoolyard dispute using this game. In a recent PNAS article, researchers from the University of Washington, Seattle describe the survival dynamics of a community of three strains of bacteria that compete for resources. In addition to a healthy, wild type strain, there is a second strain that produces the bacterial toxin colicin as well as a bunch of other proteins that allow it to survive its own toxic strategy (without which it would be shooting itself in the proverbial foot). A third strain has evolved resistance to the colicin protein produced by the second strain, by acquiring mutations that prevent colicin from binding to its own proteins.

However, in keeping with the maxim that there's no such thing as a free lunch, these latter two strains of bacteria pay a price for their competitive strategies. There is a significant cost in energy and resources for the bacteria that produces colicin and all of the accompanying proteins that enable it to survive its own offensive strategy. The colicin-resistant strain also pays a price insofar as the resistance mutations it needs to survive its competitor's colicin strategy, compromise to some degree, the functions of certain vital proteins that it also needs for its general well-being.

The scenario we have then is that the colicin producing strain can outperform the wild type bacteria in the absence of the colicin resistant strain. The colicin resistant strain can outperform the colicin producing strain when the wild type strain is not around. The wild type strain can outperform the colicin resistant strain when the colicin producing strain is not around. ... the bacterial equivalent of the "Paper, Scissors, Rock" game.

So what happens when all three strains are present at once? A very interesting dynamic of "survival of the weakest" emerges.

If one of the strains really starts to dominate the strain it grows better than, this actually favors its other competitor. If the toxin producing strain starts to dominate the wild type strain, this favors the growth of the toxin resistant strain. If the toxin resistant strain starts to dominate the toxin producing strain, this favors the growth of the wild type strain. If the wild type strain starts to dominate the toxin resistant strain, this favors the growth of the toxin producing strain.

"Paper, Scissors, Rock"

When the authors of the study cultured these three strains of bacteria in their experiments, this scenario is indeed what they observed in the laboratory. When all three strains were present in the same culture, the researchers consistently observed over many experiments, that no single strain emerges on top - the result of an interesting competitive scenario that actually encourages restraint amongst the players. The authors point out that this kind of competitive dynamic has also driven the evolution of behavioral traits in the natural world, such as mating.

While not exactly the same situation, this scenario kind of reminds me of that famous scene in the movie "A Beautiful Mind" in which John Nash, the great luminary in the field of game theory, has an epiphany in which he imagines himself and his colleagues, all competing for the attentions of the same pretty blonde. Sometimes it might be very much in everybody's best interests if disputes could be settled over a game of "Paper, Scissors, Rock" instead of a barroom brawl, a showdown with law enforcement or even a war.

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