Probing the role of commensal intestinal fungi during Salmonella pathogenesis

PROJECT SUMMARYDespite modern sanitation and hygiene, gastrointestinal infections continue to be a significant health problem inthe United States and represent a tremendous economic burden in terms of loss of work. One of the crucialsteps of Salmonella pathogenicity is the establishment of colonization in the gut. While the host eventually clearsthe infection with Salmonella, initial host responses benefit the growth of Salmonella. Similarly, also somecommensal bacteria promote Salmonella intestinal colonization. These resident bacteria liberate for exampleotherwise inaccessible carbon sources. However, previous studies focused exclusively on the role of commensalbacteria during Salmonella pathogenesis and did not address the role of another important group ofmicroorganisms that resides in the gut: commensal intestinal fungi. In recent years, this under-studiedcomponent of the microbiota has been implicated in a variety of diseases including Inflammatory Bowel Disease.However, the role of commensal fungi during pathogenesis of gut pathogens like Salmonella is unknown.This is despite the fact that Salmonella encodes genes that make an interaction with commensal gut fungi highlylikely. In this regard, Salmonella produces three receptors for fungal and bacterial siderophores (so-calledxenosiderophores). Siderophores are small molecules that are secreted by fungi and bacteria for iron acquisitionin low iron environments like the inflamed gut. Our preliminary data show that expression of two fungalsiderophore receptors confers a growth advantage to Salmonella in conventionally raised but not in germ-freemice. Furthermore, chitinases produced by Salmonella could weaken the chitin-containing fungal cell wall toenable Salmonella to acquire fungal nutrients including intracellularly stored siderophores. We thereforehypothesize that Salmonella acquires xenosiderophores and accesses fungal nutrients with the help ofchitinases to increase colonization in the gut. We plan to investigate in Aim 1 how fungal xenosiderophorespromote Salmonella colonization in the gastrointestinal tract by depletion or addition of fungi. We will sequencethe fungal microbiota to identify siderophore-producing fungi. We will also determine the growth promoting effectof the third xenosiderophore receptor, which binds a bacterial siderophore. In Aim 2, we propose to study howmembers of the microbiota compete with Salmonella for xenosiderophores. For this, we will reduce thecomplexity of the microbiota. Germ-free mice allow for specific colonization with producers and competitors forfungal siderophores. We will test if common fungal and bacterial commensals that do not produce siderophoreslike Candida, Saccharomyces, and Bacteroides compete with Salmonella for xenosiderophores. In Aim 3, wewill elucidate if Salmonella chitinases enhance Salmonella pathogenicity. We will test two different functions ofchitinases: weakening the fungal cell wall to access fungal nutrients including intracellularly stored siderophoresand degradation of mucin or glycans on epithelial cells. We expect that our approaches will enhance ourunderstanding of the role of commensal fungi during infection with Salmonella. In a broader context, we expectour work will help to design treatments, i.e. effective competitors to intestinal pathogens.