Killers and protectors: rapid microbial evolution across the mutualist-parasite continuum

Many animal and plant species, and even humans, harbour microbes that provide them with protection against parasites. These ‘protective microbes’ can be a significant component of host defence. Using experimental evolution of a novel, tripartite interaction, we demonstrate that mildly pathogenic bacteria (Enterococcus faecalis) living in worms (Caenorhabditis elegans) can rapidly evolve to defend their animal hosts against infection by a more virulent pathogen (Staphylococcus aureus), crossing the parasitism-mutualism continuum. We also show the consequences for host and pathogen evolution. Our results indicate that evolution of beneficial bacteria in the host microbiome are important in shaping infection outcomes.