Evolution and maintenance of microbe-mediated protection under occasional pathogen infection.

Anke Kloock,Michael B Bonsall,Kayla C King

doi:10.1002/ece3.6555

Abstract

Every host is colonized by a variety of microbes, some of which can protect their hosts from pathogen infection. However, pathogen presence naturally varies over time in nature, such as in the case of seasonal epidemics. We experimentally coevolved populations of Caenorhabditis elegans worm hosts with bacteria possessing protective traits (Enterococcus faecalis), in treatments varying the infection frequency with pathogenic Staphylococcus aureus every host generation, alternating host generations, every fifth host generation, or never. We additionally investigated the effect of initial pathogen presence at the formation of the defensive symbiosis. Our results show that enhanced microbe‐mediated protection evolved during host‐protective microbe coevolution when faced with rare infections by a pathogen. Initial pathogen presence had no effect on the evolutionary outcome of microbe‐mediated protection. We also found that protection was only effective at preventing mortality during the time of pathogen infection. Overall, our results suggest that resident microbes can be a form of transgenerational immunity against rare pathogen infection.

Highlights

In nature, all plants and animals are colonized by microbes (Barrière, & Feĺix, 2005; Ley, Peterson, & Gordon, 2006; Vántus, Kovács, & Zsolnai, 2014)
Basic procedures were adopted from the experimental evolution, but with the following alterations to keep the assays feasible with higher accuracy when scoring dead and alive worms: 400 L1 worms were exposed to 200 μl of food and E. faecalis on 6-cm Nematode Growth Medium (NGM) plates, while 60μl of S. aureus overnight culture was used to inoculate 6-cm TSB plates
It has been shown that hosts receive the greatest benefits from protective microbes under constant pathogen infection

Summary

| INTRODUCTION

All plants and animals are colonized by microbes (Barrière, & Feĺix, 2005; Ley, Peterson, & Gordon, 2006; Vántus, Kovács, & Zsolnai, 2014) The composition of these microbial communities is highly diverse and includes harmful, neutral, and beneficial microbial species (Ley et al, 2006), including those that can be important players in host defense against parasites, a phenomenon referred to as “defensive mutualism” (King, 2019; May & Nelson, 2014). While hosts can benefit from microbe-mediated protection, defensive symbionts can be less beneficial to the host in the absence of enemies, due to metabolic and physiological costs (King, 2019). These results suggest that even occasional pathogen infection can select for defensive mutualism, revealing the potential for this phenomenon to be widespread in nature

| MATERIALS AND METHODS

Findings

| DISCUSSION