Fitness costs and benefits vary for two facultative symbionts of the social amoeba, .

Unlabelled: Hosts and their associated microbes can enter into different relationships, which can range from mutualism, where both partners benefit, to exploitation, where one partner benefits at the expense of the other. Many host-microbe relationships have been presumed to be mutualistic, but frequently only benefits to the host, and not the microbial symbiont, have been considered. Here, we address this issue by looking at the effect of host association on the fitness of two facultative members of the microbiome ( and ). Using two indicators of bacterial fitness, growth rate and abundance, we determined the effect of on fitness. In liquid culture, we found that amoebas lowered the growth rate of both species. In soil microcosms, we tracked the abundance of grown with and without over a month and found that had larger populations when associating with while was not significantly affected. Overall, we find that both and pay a cost to associate with , but can also benefit under some conditions. Understanding how fitness varies in facultative symbionts will help us understand the persistence of host-symbiont relationships.

Open Research Badges: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://openscholarship.wustl.edu/data/15/.