Many bacteria live at high density in complex communities containing members of the same and different species. Within these communities, bacterial cells can strongly affect the growth and survival of neighbouring cells, which are social traits in an evolutionary sense. For example, microbes secrete compounds that promote the growth of neighbouring cells, such as enzymes that break down complex proteins into nutrient sources. Other secretions, such as toxins, inhibit the growth of surrounding cells.  Understanding these interactions and how they evolve over time has many important applications such as the control of microbial infections in humans and the engineering of fertilisers to increase crop efficiency.

An individual-based computer simulation of bacterial communities has been developed in the Foster lab to explore how spatial patterns within these communities influence the evolution of social phenotypes. In agreement with social evolution theory, the amount of mixing between different bacterial strains has been found to strongly predict whether social phenotypes will spread in a population [1]. My current work involves (1) testing these predictions in the laboratory and (2) extending the computational model - and eventually the experimental system - to include multiple interacting species.

To test how spatial organisation within bacterial groups affects the evolution of social phenotypes, we are conducting experiments using fluorescently-labelled strains of a pathogenic bacterium, Pseudomonas aeruginosa. As well as expressing a number of social phenotypes, including the secretion of signalling molecules, colonies of P. aeruginosa display interesting spatial patterns (see image above).

In parallel to this work, we have been exploring the evolution of multispecies interactions using computer simulations. A first step in extending the existing model is to understand how the social phenotypes of one species can evolve in the presence of a second clonal species under varying ecological conditions. Consistent with previously published experimental data [2], the model predicts that under high competition for nutrients, the addition of new species can reduce selection for cooperative phenotypes within the focal species. However, the model also identifies conditions where the opposite effect is predicted. Under high nutrient conditions, additional species can “protect” secretors from non-secretors and allow them to thrive. Finally, we have used the model to study the evolution of cooperation among species (mutualism) whereby each species secretes something that benefits the other. This revealed a number of important constraints on the evolution of such mutualism among species. In particular, among-species cooperation is only likely when secretions are cost-free or whenever the two species do not compete for the same nutrients [3].