Mice colonized with the defined microbial community OMM19.1 are susceptible to C. difficile infection without prior antibiotic treatment.

Diverse gut microorganisms present in humans and mice are essential for the prevention of microbial pathogen colonization. However, antibiotic-induced dysbiosis of the gut microbiome reduces microbial diversity and allows C. difficile to colonize the intestine. The Oligo Mouse Microbiota 19.1 (OMM19.1) is a synthetic community that consists of bacteria that are taxonomically and functionally designed to mimic the specific pathogen-free (SPF) mouse gut microbiota. Here, we examined the susceptibility of OMM19.1 colonized mice to C. difficile infection at a range of infectious doses (10 ³ , 10 ⁵ , and 10 ⁷ spores) without prior antibiotic treatment. We found that mice colonized with OMM19.1 were susceptible to C. difficile infection regardless of the dose. The clinical scores increased with increasing C. difficile dosage. Infection with C. difficile was correlated with a significant increase in Ligilactobacillus murinus and Escherichia coli , while the abundance of Bacteroides caecimuris, Akkermansia muciniphila, Extibacter muris, and Turicimonas muris significantly decreased following C. difficile infection. Our results demonstrate that the OMM19.1 community requires additional bacteria to enable colonization resistance.