The trillions of commensal (non-harmful) microbes that live in and on our bodies form a protective barrier against invasive species and opportunistic pathogens (pathobionts), like a second skin or an extension of the innate immune system. This barrier is largely maintained by metabolic competition between commensals and pathobionts and by interactions between commensals, pathobionts, and the host immune system. Some microbiota (commensal microbial communities) are more permissive to invasion than others. Once a pathobiont has colonized, commensal microbes can both facilitate and thwart virulence, depending on ecological context. Both symptomatic and asymptomatic carriers of a pathobiont can spread it to others in the population. In order to reduce the burden of infectious disease, a better understanding of how our commensal microbiota can be bolstered to protect us from invasive, opportunistic pathogens is needed. This year’s course and symposium are dedicated to exploring the mechanisms that underlie the barrier function of our microbiota against infectious diseases.

ISB hosted a series of events in October of 2023 that leverage data sets from recurrent Clostridioides difficile patients before and after fecal transplant to understand the determinants of pathobiont colonization and highlight leading microbiome researchers who are working to better understand pathobiont-commensal-host interactions in health and disease.