aureus frequently occur in animal and human microbiomes ( Holden et al., 2004 Martin-Vivaldi et al., 2010 Lawley et al., 2012 Cruz et al., 2013 Kommineni et al., 2015) wherein they can be pathogenic or commensal. Here, we use experimental evolution to test whether a mildly pathogenic, resident microbe ( Enterococcus faecalis) can evolve to defend its host ( Caenorhabditis elegans) against infection by a more virulent pathogen ( Staphylococcus aureus), and thus cross the parasitism–mutualism continuum. Despite this, evolutionary responses by resident microbes against pathogen infection have not before been considered. Can microbes evolve to protect their host in response to virulent pathogen challenge, and, in doing so make an evolutionary transition to mutualism? It is well established that infecting pathogens can undergo rapid adaptation ( Brockhurst and Koskella, 2013) in response to transmission opportunity and mode ( Messenger et al., 1999), prior immune exposure ( Mackinnon and Read, 2004) and multi-strain coinfection ( Garbutt et al., 2011) with host defences known to reciprocally evolve to pathogen adaptation ( Schulte et al., 2010 Morran et al., 2011). The large population sizes and short generation times of microbes also create the potential for the rapid evolution of such defences. Evolution of this nature would represent microbes evolving along the parasitism–mutualism continuum ( Chamberlain et al., 2014). Resident microbes can therefore provide strong protection against virulent pathogens, and corresponding microbial traits might be evolutionarily advantageous. As pathogens invade the host, they can not only be targeted by the host immune system, but also interact with pathogenic and commensal microbial species already present within the host ( McFall-Ngai et al., 2013). These protective microbes provide an important complement to the host's defence systems ( Abt and Artis, 2013 Hooper et al., 2012 McFall-Ngai et al., 2013). Microbes can cause infectious disease, but they can also act to protect hosts from pathogens, a phenomenon observed in a range of animals ( Dillon et al., 2005 Dong et al., 2009 Jaenike et al., 2010 Koch and Schmid-Hempel, 2011), including humans ( Kamada et al., 2013), and in plants at the root–soil interface ( Mendes et al., 2011 May and Nelson, 2014). Microbes can have effects on host biology far beyond their core impacts on digestion ( Dillon et al., 2000 Cerf-Bensussan and Gaboriau-Routhiau, 2010 Brucker and Bordenstein, 2013 Lize et al., 2014).
KILL 2 OR MORE ENEMIES RAPIDLY DRIVER
Our results indicate that microbes living within a host may make the evolutionary transition to mutualism in response to pathogen attack, and that microbiome evolution warrants consideration as a driver of infection outcome. faecalis-mediated protection was through increased production of antimicrobial superoxide, which was confirmed by biochemical assays. Genomic analysis implied that the mechanistic basis for E. Microbe-mediated protection was also effective against a broad spectrum of pathogenic S.
Host protection evolved in all six, independently selected populations in response to within-host bacterial interactions and without direct selection for host health. Using experimental evolution of a novel, tripartite interaction, we demonstrate that mildly pathogenic bacteria ( Enterococcus faecalis) living in worms ( Caenorhabditis elegans) rapidly evolved to defend their animal hosts against infection by a more virulent pathogen ( Staphylococcus aureus), crossing the parasitism–mutualism continuum.
Microbes can defend their host against virulent infections, but direct evidence for the adaptive origin of microbe-mediated protection is lacking.
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