Wolves, Bacteria and Cheaters

This post was chosen as an Editor's Selection for ResearchBlogging.orgThroughout the natural world species have discovered the benefits of cooperating to achieve common goals. Wolves hunt together in packs, many birds seasonally migrate in flocks and no bee will hesitate to work or give its life for the colony. But when cooperation becomes the norm, cheating can become a fruitful strategy on its own. Let’s consider wolves giving chase to a moose. Some wolves might lunge upon the beast as it is fleeing, sinking their teeth into its flesh and holding on to the moose in an effort to slow it down. This is not without risk for these wolves, a 700 kg moose charging through the woods can cause some serious damage. Other wolves take less risk, they just give chase and wait until others have killed the prey. We can consider these more careful wolves as a kind of cheaters: the daring wolves invest more energy and take more risks, but the benefits are shared by the entire pack.

All these subtle dynamics have their analogs in the smaller world of microbes. When some species of bacteria are faced with hostile environments (like the human body where they are under threat of the immune system), they form a tough physical barrier (a biofilm) that protects them from external threats. Every individual that participates, invests in the form of building materials and signalling molecules to communally create the biofilm. The potential group that benefits is much greater though, since individuals that don’t participate in the construction of the biofilm, still enjoy greater protection when the biofilm is there. If you want to read more about biofilms and cheating, check out these blogposts by Lab Rat and Iddo Friedberg.

Top: a pack of wolves chases a bull moose. Not all wolves invest equally, some run along 'herding' the moose, without attacking it. Source and amazing story here. Bottom: Pseudomonas Aeruginosa can cause infections. The biofilms that they form protect them from the immune system. Not all Pseudomonas invest equally in the construction of these biofilms. Source.

Strategies of ‘cheating’ are only effective up to a point, a population that entirely consists of cheaters is unlikely to be sustainable. Population dynamics between cheaters and cooperators are really interesting and much easier to study in prokaryotes than in animals. In the case of prokaryotes, the genes and molecules involved in the cooperative behaviour are more easily identified and manipulated. Artificially creating a cheating a bacterium is sometimes as simple as knocking out a single gene! Natalie Jiricny and colleagues took a more comprehensive approach in a study recently published in the Journal of Evolutionary Biology. They created an entire range of cheaters, so that it became possible to compare the relative success of big cheats, small cheats and hard-working wild types.

The bacterial species used in this study is the hardy Pseudomonas Aeruginosa. These bacteria can be found all over the world and can cause serious infections in immuno-compromised people. During such an infection, the host is actively withholding iron, which is essential for the growth of Pseudomonas. To this end, Pseudomonas secrete specialized molecules that can scavenge iron (siderophores). Making such a molecule is costly, and a bacterium has no guarantee that the iron that is scavenged by the siderophores that it produced, get returned to it. Once released, such siderophores can be considered a public good: everyone can use them, regardless of whether who made it.

The beautiful thing about this study is that the results are really straightforward to explain. First up is the relative success of cheating mutants versus the cooperators when they were cultured all by themselves. Unsurprisingly, the cheaters did worse in this set-up: the amount of siderophores produced is positively correlated with the growth of the population (first graph). If noone is scavenging iron, cheaters are at a real disadvantage! Now, if mutant cheaters are grown together with the cooperators, the amount of siderophores that the mutants produce is positively correlated with the population growth of the cooperators (graph 2)! This means that the wild type cooperators benefit more from small cheats than from big cheats. Even if the small cheats produce less siderophores than the cooperators, the cooperators still enjoy some benefits from the siderophores that they do produce. For the mutants the story became clear in an experiment where iron was rare. In this scenario, the bigger cheaters leeching off cooperators perform better than the smaller cheats (graph 3). Investing in scavenging those rare iron molecules just doesn’t make sense if others are already doing it for you!

The success of cheating seems to be dependent on the social environment. If there are no cooperators around, cheaters don’t have much to gain. But if everyone is producing heaps of siderophores, cheating can be discomfortingly rewarding! By using different grades of cheating, Jiricny and colleagues have shown the inherent vulnerability of cooperative systems. This work also might have clinical implications, since the Pseudomonas that infect patients suffering from cystic fibrosis seem to lose social functions over time, possibly because social cheaters emerge and overtake the cooperators. Non-conventional antibiotics that target the slower growing cheaters might be more effective in treating Pseudomonas infections.

JIRICNY, N., DIGGLE, S., WEST, S., EVANS, B., BALLANTYNE, G., ROSS-GILLESPIE, A., & GRIFFIN, A. (2010). Fitness correlates with the extent of cheating in a bacterium Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2010.01939.x

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