AbstractInflammatory bowel disease (IBD) is caused by an inappropriate immune response, typically against intestinalmicrobes, resulting in inflammation of the intestines. In the last five years alone, 70,000 new IBD cases havebeen diagnosed in the US, representing a serious public health challenge of increasing medical and socio?economic importance. IBD is triggered by environmental factors, which within the intestine, is rich in microbes -beneficial, commensal as well as pathogenic. Here we will study the interactions between a beneficial bacteriaBacillus subtilis and an opportunistic fungal pathogen Candida albicans. Overgrowth of C. albicans in the guthas been linked to inflammation of the digestive tract. Some associated genetic and environmental factorshave been characterized but the underlying molecular and immunological mechanisms remain unknown.This proposal will test the role of microbial interactions and associated immune responses. Microbes residingin the host niche of the gut must cope with alterations during long-term persistence in the host. They must alsoadapt rapidly to changes to ensure growth and survival during infection. Our goal is to determine the molecularmechanisms of cross-kingdom microbial interaction and host-microbe interactions with in a live host to gain abetter understanding of healthy interactions versus disease. Our working hypothesis is that B. subtilis hasprobiotic properties because pre-exposure to B. subtilis protects the host from future C. albicansinfections. We are just beginning to understand the genes involved in the interaction between C. albicans andB. subtilis as well as C. albicans and the host. Here, we will identify the molecular mechanisms that governdisease progression, and determine how specific mutations directly affect microbial interactions, hostrecognition, host response, and virulence. One of the major virulence factors in C. albicans is its morphogenicplasticity. We will use multiple morphogenic forms of C. albicans ? pathogenic forms as well as GUT-adaptedcommensals forms that are able to evade the host innate immune system thereby affecting disease outcomes.We will also use B. subtilis mutants that are unable to form protective biofilms and strains where key proteinsare fluorescently tagged for microscopy. These investigations will be conducted in the intestine of a live animalhost ? C. elegans. We will address the following questions: (1) what bacterial genes dictate the outcome of afungal infection? (2) how fit are these mutants under physiologically relevant infection conditions within a livehost? (3) what host innate immunity genes are involved in maintaining a healthy microbiota? (4) how does thehost recognize and respond to the repertoire of morphologies it encounters? These studies will advance ourunderstanding of microbial interactions from the pathogen perspective and will help reveal the role of the hostin maintaining balanced relationship in healthy individuals as well as how disruption of this interaction causesdevastating inflammation in the host.