Abstract
Microbes can defend their host against virulent infections, but direct evidence for the adaptive origin of microbe-mediated protection is lacking. 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. Host protection evolved in all six, independently selected populations in response to within-host bacterial interactions and without direct selection for host health. Microbe-mediated protection was also effective against a broad spectrum of pathogenic S. aureus isolates. Genomic analysis implied that the mechanistic basis for E. faecalis-mediated protection was through increased production of antimicrobial superoxide, which was confirmed by biochemical assays. 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.
Highlights
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)
We performed an analysis of variance (ANOVA) on untransformed data to test for the difference in nematode mortality caused by E. faecalis and S. aureus independent colonisation, as well as their co-colonisation
Separate ANOVAs were performed on transformed cfu values for each of E. faecalis and S. aureus within a dead, co-colonised nematode to test the effects of treatment on bacterial fitness
Summary
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). To examine the fitness differences of E. faecalis tode carcasses were picked from a single replicate (ancestor vs SE at G15 vs CCE at G15) in co-colonised population and placed in a 1.5 ml centrifuge tube nematode hosts with S. aureus, we measured the with 1 ml M9 buffer.
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