Mice colonized with the defined microbial community OMM19.1 are susceptible to 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 () 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 mouse gut microbiota. Here, we examined the susceptibility of OMM19.1 colonized mice to infection (CDI) at a range of infectious doses (10, 10, and 10 spores) without prior antibiotic treatment. We found that mice colonized with OMM19.1 were susceptible to CDI regardless of the dose. The clinical scores increased with increasing dosage. Infection with was correlated with a significant increase in and , while the relative abundance of was significantly decreased following CDI. Our results demonstrate that the OMM19.1 community requires additional bacteria to enable colonization resistance.IMPORTANCEThe human gut microbiota consists of a wide range of microorganisms whose composition and function vary according to their location and have a significant impact on health and disease. The ability to generate and test the defined microbiota within gnotobiotic animal models is essential for determining the mechanisms responsible for colonization resistance. The exact mechanism(s) by which healthy microbiota prevents infection is unknown, although competition for nutrients, active antagonism, production of inhibitory metabolites (such as secondary bile acids), and microbial manipulation of the immune system are all thought to play a role. Here, we colonized germ-free C57BL/6 mice with a synthetic bacterial community (OMM19.1) that mimics the specific pathogen-free mouse microbiota. Following breeding, to enable immune system development, F1 mice were infected with three different doses of . Our research suggests that there are additional essential microbial functions that are absent from the current OMM19.1 model.