In vivo microbial coevolution favours host protection and plastic downregulation of immunity

Microbiota can protect their hosts from infection. The short timescales in
which microbes can evolve presents the possibility that ‘protective
microbes’ can take-over from the immune system of longer-lived hosts in
the coevolutionary race against pathogens. Here, we found that coevolution
between a protective bacterium (Enterococcus faecalis) and a virulent
pathogen (Staphylococcus aureus) within an animal population
(Caenorhabditis elegans) resulted in more disease suppression than when
the protective bacterium adapted to uninfected hosts. At the same time,
more protective E. faecalis populations became costlier to harbour and
altered the expression of 134 host genes. Many of these genes appear to be
related to the mechanism of protection, reactive oxygen species
production. Crucially, more protective E. faecalis populations
downregulated a key immune gene, sodh-1, known to be effective against S.
aureus infection. These results suggest that a microbial line of defence
is favoured by microbial coevolution and may cause hosts to plastically
divest of their own immunity.