Stochastic Assembly Produces Heterogeneous Communities in the C. elegans Intestine

Author SummaryHost-associated bacterial communities vary extensively between individuals, but it can be very difficult to determine the sources of this variation. In this manuscript, we demonstrate experimentally how randomness in colonization can result in large differences in the composition of host-associated bacterial communities, using the nematode worm C. elegans as a tractable host model. Here, the amount of variation between individual communities is a function of two rates relevant to how bacteria colonize the host intestine, the colonization rate and the birth rate. We can therefore control the degree of variation between communities by controlling the colonization rate, using the amount of bacteria presented to the worms to control the rate at which migrants enter the intestine. When worms are fed with two neutrally-competing fluorescently labeled bacterial strains at low colonization rates, we are able to produce noise-induced bistability in this system, where each community is dominated by bacteria of only one color. These results demonstrate the potential importance of noise as a driver of variation between communities and highlight the utility of the simple model organism C. elegans for studying questions relevant to host-associated microbial communities.\n\nAbstractHost-associated bacterial communities vary extensively between individuals, but it can be very difficult to determine the sources of this heterogeneity. Here we demonstrate that stochastic bacterial community assembly in the C. elegans intestine is sufficient to produce strong inter-worm heterogeneity in community composition. When worms are fed with two neutrally-competing fluorescently labeled bacterial strains, we observe stochastically-driven bimodality in community composition, where approximately half of the worms are dominated by each bacterial strain. A simple model incorporating stochastic colonization suggests that heterogeneity between worms is driven by the low rate at which bacteria successfully establish new intestinal colonies. We can increase this rate experimentally by feeding worms at high bacterial density; in these conditions the bimodality disappears. These results demonstrate that demographic noise is a potentially important driver of diversity in bacterial community formation and suggest a role for C. elegans as a model system for ecology of host-associated communities.