Bacteria flourish in nearly every place on Earth imaginable including in and on humans. They also reproduce and therefore evolve much more quickly than we do, so understanding their evolution is vital to every aspect of our lives.
A surprisingly common strategy for bacteria - controlling everything from virulence to production of useful resources such as cellulase - is a form of communication called quorum sensing. Bacterial cells using quorum sensing consistently produce a signal molecule and then detect the concentration of that molecule in the surrounding area. The bacteria generally have a signal-molecule density threshold, called a quorum, that triggers them to start performing a new behavior. The behaviors controlled by quorum sensing are usually only useful if there are enough organisms doing them at the same time. Producing public goods such as light or enzymes that can digest nutrients are common examples of such behaviors. Therefore, quorum sensing is an essential behavior enabling bacteria to respond to their environment.
However, researchers know relatively little about the specific costs and benefits that make quorum sensing a beneficial mechanism for controlling a given trait. This information is important because it can indicate how we might be able to manipulate bacterial genes or environmental factors to change the quorum-sensing controlled behavior, which would let us directly intervene in human illnesses.
I recently read an interesting paper by Heilmann et al., “Why do bacteria regulate public goods by quorum sensing?—How the shapes of cost and benefit functions determine the form of optimal regulation”, which uses mathematical modeling to untangle the selective pressures on using quorum sensing to control public-goods altruism. Here’s a quick summary.
What was done
Heilmann et al. derived mathematical equations to describe the fitness of bacteria that use quorum sensing to control the production of a public good (a resource that must be exported out of the cell and therefore is shared among neighboring organisms). They focused on the benefit of the public good as the population size of the bacteria changes and found that the best strategy for production would be either to continuously increase production as population size increases (“continuous”) or to not produce anything until a tipping point in the population size (“discontinuous”). Which of these categories the optimal strategy falls into depends on how beneficial the public good is at smaller concentrations. If the benefit of the public good is sigmoidal so that it initially decelerates as the concentration increases, then the cost of producing the public good at small concentrations (the cost is a linear function) outweighs the benefit. It is only when the concentration of public good hits a critical amount that there is a net benefit, leading to a discontinuous strategy of production being best. If, instead, the benefit of the public good accelerates initially, a continuous production strategy is best.
What it means
This work provides a theoretical framework to test the many quorum-sensing-controlled public goods strategies we already know about in natural systems. Because the optimal public goods strategy depends on the shape of the benefits curve, we can use this framework to determine what strategy various bacteria are likely to use and then manipulate the benefits they are receiving to our own gain.
What recent papers have you found interesting?