Maintenance and loss of microbe-mediated protection in the absence of pathogens

Protective microbes are known for their service to hosts. While they allow hosts to survive infection, microbes, too, benefit from successful inhibition of incoming pathogens. Under constant pathogen exposure, protective symbionts should thus be selected for. Yet, it is less clear if, and how, microbe-mediated protection is maintained in symbionts in the absence of pathogen pressure. Addressing the stability of protective symbiosis during bacterial adaptation to healthy hosts, we studied microbe-mediated protection of Pseudomonas lurida MYb11 against pathogenic Bacillus thuringiensis Bt247 in its natural host Caenorhabditis elegans MY316. Specifically, we assessed host protection and in vitro inhibition of the pathogen for a collection of derived MYb11 isolates, which were previously evolved during serial passaging in healthy C. elegans hosts. We found that all evolved MYb11 isolates continued to inhibit the pathogen in vitro, while most, albeit not all, continued to protect hosts. We focused on two of these isolates, MT5 and MT11, one with and one without protection, and found that intact protection is associated with high symbiont colonization and resulting lower pathogen proportions. In sum, our study dissects the stability of a natural protective symbiosis and suggests that high colonization ability ensures microbe-mediated protection, even if microbes adapt to host association in the absence of pathogen selection.