Microbial evolution along the parasitism-mutualism continuum

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Animals and plants host diverse microbial communities consisting of both harmful (parasitic) and beneficial (mutualistic) species. However, these roles are not fixed and may depend on which other species are present. Microbes exist along a continuum between parasitism and mutualism, but we do not yet know what drives some to be parasitic and others to be mutualistic. Crucially, we do not know how microbial interactions within a host combine with transmission between hosts to drive selection along the parasitism-mutualism continuum. The overarching long-term goal of my research program is to develop and test novel theory to understand how and why microbes evolve to be parasitic or mutualistic. Over the next five years, my team will address three short-term objectives: (1) theoretically determine how within- and between-host processes drive the evolution of parasitic and mutualistic traits in a microbe; (2) develop theoretical predictions for how microbial coevolution shifts constituent- and community-level parasitism and mutualism; and (3) predict evolution in specific microbial communities inside a host and test these predictions in collaboration with lab scientists using established evolution experiments. We will develop mathematical models that capture microbial interactions across scales (within- and between-host) and analyse how microbial traits such as virulence, transmissibility, and host protection (co-)evolve. This approach will allow us to delineate the impact of the within- and between-host scales on selection for parasitic and mutualistic traits. Then, we will test our modelling predictions by working with lab scientists to experimentally evolve bacteria in microscopic worms. Our research is key to understanding host-microbe evolutionary ecology, providing evolutionary biologists with novel theoretical frameworks and testable predictions. Understanding microbial evolution along the parasitism-mutualism continuum also has important implications for future therapeutics to treat infectious diseases. For example, transplantation of microbial communities from healthy human donors is known to be an effective treatment for certain antimicrobial resistant infections. With crucial insights into the fundamental processes of how microbes (co)evolve in communities, we will be better placed to manipulate these communities inside our bodies safely and purposefully to create new therapeutic options for a variety of infectious diseases.