One of the coolest studies I’ve come across so far this year is the fascinating story about microbe-mediated protection in worms by Kayla King et al.
The bacterium Enterococcus faecalis normally causes mild disease in worms (Caenorhabditis elegans). After a week with this bacterial infection, fewer than one in a hundred worms dies. In contrast, Staphylococcus aureus is a highly dangerous bacteria, killing over half of the worms within a day. Interestingly, when you mix the two pathogenic bacteria together, E. faecalis protects the worms from the more virulent competitor, reducing the worms’ mortality rate from 52% to only 18%.
King et al. wanted to investigate if it was possible to select for this mutualistic defensive trait exhibited by E. faecalis, so they continually harvested the bacterium from the worms for 15 worm-generations. The worms and the virulent S. aureus, however, were derived every time from the same genetically identical stock to make sure only E. faecalis could evolve.
After the end of the experiment, E. faecalis had evolved exceptional protective abilities for its worm host against S. aureus; only one percent of the worms now died after a mixed infection. What’s more fascinating is how fast this happened. After only five worm generations, E. faecalis was already a highly efficient defensive microbe. The authors challenged the worms with seven different S. aureus strains, but E. faecalis could handle them all.
Whole-genome resequencing was then used to investigate the evolved protective abilities of E. faecalis. Mutations that were associated with the production of superoxide were found, a substance that is harmful to many microbes, including S. aureus.
To conclude, competition between two microbes may quickly result in efficient host-protective traits. My guess is that we are going to see much more of these microbe-mediated host protective effects in the near future.
Reference: King et al. 2016. Rapid evolution of microbe-mediated protection against pathogens in a worm host. The ISME Journal. doi:10.1038/ismej.2015.259