Roles of bacterial growth competition systems in colonization of the murine gut

The gut microbiome is essential for human health. Although the gut microbiota is largely stable at the species level in healthy individuals, strain-level variation remains less understood. Many bacterial strains encode toxin delivery systems that may shape competition within the gut. Here, we investigate how contact-dependent growth inhibition (CDI) and colicins influence intestinal colonization by a competitive murine Escherichia coli isolate, R12. We show that R12 can colonize an intact mouse gut microbiota by displacing resident Enterobacteriaceae , but success depends on multiple interacting factors. CDI systems and colicins provide a competitive advantage against resident E. coli , particularly during early colonization, while metabolic flexibility and access to alternative carbon sources support long-term persistence. Colonization outcomes vary between hosts and are shaped by resident microbiota composition, strain-level competition, and the initial invader-to-resident ratio. Overall, successful gut invasion is determined by the combined effects of bacterial antagonistic systems, metabolic capacity, and ecological context.