COS 87-3 - Propagule size has context-dependent effects on colonization success in mixtures of gut microbial communities

Abstract: Colonization plays an important role during assembly of the gut microbiome and influences microbiome composition throughout the life of the host, but determinants of colonization success are not yet fully understood. The size of introduced populations and frequency of their introduction can be important drivers of colonization success for introduced species. However, the effect of these factors, which are known together as “propagule pressure,” varies across environments and can be modulated by the strength of competition between introduced and resident species. In vitro microbial communities provide a tractable, high-throughput experimental system to interrogate the influence of propagule pressure on colonization. To quantify the importance of propagule size during colonization in gut microbial communities, we derived in vitro communities from human stool samples and systematically mixed eight pairs of communities at seven ratios ranging from 1:1000 to 1000:1. We compared our results to the predictions of a neutral model in which the relative abundance of each species remains proportional to its inoculation dose, a measure of propagule size. We observed that community co-cultures maintained similar levels of species diversity to the parent communities regardless of mixture ratio, suggesting that community diversity may be limited by resource competition even when many species are initially present. Likewise, community composition was generally maintained across mixture ratios, in contrast to neutral predictions. However, certain bacterial taxa showed varying levels of dose-dependence across mixtures, suggesting that the effect of propagule size depends on community composition and may be affected by the strength of competitive interactions in each mixture. Our results highlight the importance of both propagule pressure and competition in determining colonization success in gut microbial communities.