The broad goal of the research in the Young Lab is to understand the role of bacteria in acute and chronic gastrointestinal (GI) illness. We study the role of what would traditionally be considered "pathogenic bacteria" in gastrointestinal illness. A multifaceted approach is used to examine various aspects of the host/microbe interaction.
Helicobacter hepaticus and murine inflammatory bowel disease
Inflammatory bowel disease (IBD) encompasses the clinical entities ulcerative colitis and Crohn's disease. These conditions are characterized by an abnormal, dysregulated inflammatory response in the gastrointestinal tract. Intestinal bacteria have been implicated in the initiation and progression of this inflammatory response.
We study a mouse model of IBD in IL-10 knockout (IL-10-/-) mice that is triggered by the presence of the bacterium H. hepaticus. Experimental infection of IL-10-/- mice with H. hepaticus results in severe typhlocolitis, whereas uninfected mice remain diseases free.
Cecal histopathology in IL-10-/- mouse (left) infected with H. hepaticus. Uninfected mice (right) is without inflammation and hyperplasia
Cytolethal distending toxin in H. hepaticus-associated disease
Cytolethal distending toxin (CDT) is a virulence factor that is elaborated by a heterogeneous group of pathogenic bacteria including C. jejuni and other Campylobacter species, certain E. coli strains, S. dysenteriae, H. ducreyi, and A. actinomycetemcomitans. We have identified CDT in H. hepaticus and have developed genetic tools to construct isogenic CDT mutants. We have shown that infection of IL-10-/- mice with an isogenic H. hepaticus CDT-negative mutant is associated with development of less severe IBD than that encountered in mice infected with wild type H. hepaticus. Additionally, mice are eventually able to clear infection with a CDT-negative H. hepaticus strain. We are investigating the possibility that this is secondary to the ability of CDT-producing strains to blunt the host immune response to the organism.
CDT has shown to be structurally homologous to mammalian type I DNases. We have expressed and purified recombinant H. hepaticus CDT and are using this for in vitro studies on the mechanism of action of the toxin.
Microbial ecology of gastrointestinal bacterial infections
Over the past decade, molecular-based approaches have revealed enormous phylogenetic diversity in the microbial world that is not yet represented in culture. This information has come almost entirely by retrieval of small subunit (SSU) rRNA sequence information, which provides a phylogenetic context in which to quantify such diversity, and typically involves cloning and sequencing the SSU rRNA-encoding gene (i.e. the SSU rDNA). By comparing 16S rDNA sequences retrieved from clone libraries, the requirement for cultivation has been circumvented and we have obtained an illuminated view of the remarkable microbial diversity that is actually present in many habitats.
Fluorescent in situ hybridation (FISH) revealing the gastrointestinal microbiota in close contact with the intestinal mucosa.
We are applying culture-independent bacterial community analysis to examine alterations in the microbial ecology of the murine gut using the H. hepaticus/IL-10-/- model of inflammatory bowel disease (IBD) described above. The endogenous GI microbiota is felt to play a key role in the pathogenesis of IBD. Therefore we are using qualitative and quantitative methods to study the diversity of the bacterial intestinal microbiota of mice before and after infection with H. hepaticus, and following further changes that may occur during progression of disease. This work should provide insight into the role that the intestinal microbiota may play in the development and progression of IBD.
