Social evolution in P. aeruginosa

Many microbes live in highly dense communities, such as bacterial colonies, in which individual cells can strongly affect the growth and survival of neighbouring cells. These traits, which are “social” in an evolutionary sense include secreted 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. During my postdoc, I used Pseudomonas aeruginosa, a model organism and human pathogen to study factors influencing social phenotypes in microbes. I combined lab experiments with an individual-based computer simulation of bacterial communities, which had been developed in the Foster lab to explore microbial life in these dense communities.

Much of this work has focused on how spatial patterns in microbial communities influence the evolution of social phenotypes. Theory has shown that spatial patterns will play an important role in the likelihood of social phenotypes spreading within a population. Furthermore, computer simulations predict that spatial patterns will depend on environmental factors, such as nutrient abundance [1]. To explore the effect of nutrients on spatial patterns, I have studied mixtures of fluorescently-labeled strains of P. aeruginosa as they grow in colonies on agar surfaces and develop intriguing spatial patterns (see right). This system has allowed us to measure the effect of nutrient concentrations on spatial patterns: the more nutrients are available to a growing colony, the more slowly it loses its diversity [2].

I have also investigated how these spatial patterns affect the spreading of social phenotypes. Using computer simulations, we have established that the amount of mixing between different bacterial strains strongly predicts whether social phenotypes will spread in a population [1,3]. To test these predictions, together with members of Kevin Foster’s lab, we are studying how mixing affects selection for antibiotic resistance (manuscript in preparation). Quorum sensing, a social phenotype whereby signalling molecules are used to organize collective behaviour in microbes, has also been found to depend on spatial organization: our computer simulations have revealed that competition in a spatially limited environment can provide a novel explanation for the evolution of quorum sensing in microbes [4].

 

References and related publications

1. Nadell CD, Xavier JB, Foster KR (2008) Emergence of spatial structure in cell groups and the evolution of cooperation. PLoS Comp Biol 6:e1000716.

2. Mitri S, Clarke E, Foster KR (2015) Resource limitation drive spatial organization in microbial groups. ISMEJ. [html] [pdf]

3. Mitri S, Xavier JB, Foster KR (2011) Social evolution in multispecies biofilms. PNAS 108:10839-46. [html] [pdf]

4. Schluter J, Schoech AP, Foster KR, Mitri S (2016) The evolution of quorum sensing as a mechanism to infer kinship. PLoS Comp Biol 12: e1004848.

5. Mitri S, Foster KR (2016) Pleiotropy and the cost of individual traits promote cooperation. Evolution 70:488-94. [html] [pdf]