Impact of the defined Oligo-MM12 microbiota on intestinal colonisation and dissemination of Listeria monocytogenes

Abstract Listeria monocytogenes is a foodborne pathogen of global concern, particularly for immunocompromised individuals at risk of severe disease. In mice, infection outcomes are strongly influenced by host immunity and gut microbiome composition. The Oligo-MM 12 defined microbiota mouse model, containing a simplified community of 12 bacterial strains, offers a controlled system to study L. monocytogenes pathogenesis and microbiome interactions. Defined or reduced-complexity microbiota models are increasingly used to investigate colonisation resistance and identify protective taxa. In this study, we compared Oligo-MM 12 mice with conventionally raised Specific Pathogen Free (SPF) mice to assess how microbiome complexity shapes infection. This allowed us to explore how microbiome complexity affects resistance to L. monocytogenes . We performed an in vivo infection study to assess host responses and pathogen-related outcomes, alongside an ex vivo fermentation assay that simulated the murine distal colon, to monitor microbial dynamics. Building on our earlier work, we now demonstrate that in vivo, Oligo-MM 12 mice showed significantly higher L. monocytogenes shedding in faeces during infection, whereas SPF mice progressively reduced L. monocytogenes levels. Despite this, L. monocytogenes dissemination to internal organs after three days of infection was similar in both models. Alterations to gut Prevotella , Akkermansia and Blautia species following L. monocytogenes infection were noteworthy. Ex vivo fermentation mirrored in vivo patterns, validating the Oligo-MM 12 system for mechanistic studies. Together, these results highlight the importance of microbiome complexity in modulating infection outcomes and establish a foundation for identifying protective taxa and mechanisms of colonization resistance.