Cooperation in the dark: signalling and collective action in quorum-sensing bacteria

The study of quorum-sensing bacteria has revealed a widespread mechanism of coordinating bacterial gene expression with cell density. By monitoring a constitutively produced signal molecule, individual bacteria call limit the ir expression of group-beneficial phenotypes to cell densities that guarant ee an effective group outcome. In this paper, we attempt to move away from a commonly expressed view that these impressive feats of coordination are e xamples of multicellularity in prokaryotic populations. Here, re we look mo re closely at the individual conflict underlying this cooperation, illustra ting that, even under significant levels of genetic conflict, signalling an d resultant cooperative behaviour can stably exist. A predictive two-trait model of signal strength and of the extent of cooperation is developed as a function of relatedness (reflecting multiplicity of infection) and basic p opulation demographic parameters. The model predicts that the strength of q uorum signalling will increase as conflict (multiplicity of infecting strai ns) increases, as individuals attempt to coax more cooperative contribution s from their competitors, leading to a devaluation of the signal as an indi cator of density. Conversely, as genetic conflict increases, the model pred icts that the threshold density for cooperation will increase and the subse quent strength of group cooperation will be depressed.