Cheaters never prosper, not even bacteria

We all know that cheating is wrong, but some people do it anyway. I know I had my fair share of classmates growing up who would try to copy my homework (perils of being a nerd). Big or small, cheating has consequences. If you’re caught peeking at someone’s answers while taking a test, you’ll probably get a zero.

The existence of consequences for cheating makes sense in the context of social cooperation— rules and laws facilitate our happy coexistence, and thus need to be enforced. It turns out that the phenomenon of social cooperation isn’t unique to humans. Biologists have also observed that animals living in groups can have complex social structures that are regulated by rules to prevent cheating, and “police” who enforce the rules. In ant colonies, for example, worker ants are policed (link) by occasional violent attacks to prevent individual workers from cheating. In this case, cheating means a single worker ant reproducing more than would be good for the colony as a whole. By policing these cheaters, the overall harmony of the colony is maintained.

While scientists have long known that animals exhibit cooperative behavior, a recent study published in Proceedings of the National Academy of Sciences showed that bacteria do the same thing. A team led by Dr. Peter Greenberg from the University of Washington studied social behavior in a species of bacteria called Pseudomonas aeruginosa. All social groups need a means of communication. Humans use language, among other things, to communicate with one another. In P. aeruginosa (and, in fact, all bacteria), communication is mediated by a process called quorum sensing.

In a legislative body, quorum refers to the minimum number of members that must be present at a meeting to make its proceedings valid. Bacteria are able to sense quorum, or how many bacteria of the same species around them, by sending out molecules that can then be recognized by special receptors on other bacteria. If you’re an individual bacterium, these receptors tell how many of your buddies are nearby by detecting the concentration of these quorum sensing molecules. When quorum sensing is intact, the entire population of bacteria benefits by being able to coordinate their behavior. If there’s a certain valuable resource around, like a nutrient, the whole population benefits if the nutrient is equally shared. In order to make sure they share equally with each other, the bacteria need to be able to sense how many other bacteria are around, which they accomplish through quorum sensing.

Unfortunately, this system is subject to cheating. After all, these nutrients are really yummy to the bacteria, and they don’t always want to share. It’s like if you brought a bunch of cupcakes to school to celebrate your birthday, but then realized that the cupcakes are delicious and you’d actually just rather sit at home and eat them all yourself. P. aeruginosa is able to cheat when it loses the ability for quorum sensing. This occurs by random a mutation event, but subsequently allows an individual bacterium to take up nutrients and reproduce independently of its buddies. This is great for the cheater (the bacterium that loses the ability for quorum sensing), but bad for the population as a whole, so the population needs a way of controlling this cheating behavior.

 It turns out that cooperating P. aeuroginosa, those that don’t cheat and maintain the capacity for quorum sensing, police the activity of the cheaters by producing a toxic molecule called cyanide. The same gene that mediates quorum sensing is also implicated in the detoxification of cyanide. So cheaters are punished for their behavior by losing the ability to break down cyanide produced by cooperators. The cheaters die, the cooperators prosper, and all is right with the world.