Cancer-chemotherapy associated Clostridium difficile infection (CDI) has an adverse effect on treatment outcomes in cancer patients. It leads to treatment interruptions and/or dose intensity reduction with calamitous effects on tumor regression. Unfortunately, efforts at reducing CDI incidence in cancer patients have been hampered by limited understanding of CDI pathophysiology and poor performance of established predictors particularly in cytoreductive cancer chemotherapy (CCC) treated patients. Until more reliable predictors of CDI in CCC patients are identified and the basic science of CDI development is understood, efforts to curtail incidence and improve treatment strategies will remain inadequate. The research aims of this proposal are focused on preventing CDI in CCC-treated patients while providing the training scaffold for Dr. Apewokin's transition to becoming an independent physician-scientist with a focus on cancer supportive care. Protection against gastrointestinal infections in the normal host is mediated by many processes. Immunoglobulins provide humoral protection while the endogenous gut microbial community structure favors resistance to pathogenic organism colonization. We have demonstrated in pilot studies that cytoreductive cancer chemotherapy modifies these factors and leads to reduction of CDI-specific immunoglobulins and gut microbial diversity. We propose a conceptual model of CCC-associated CDI that captures these processes and accordingly hypothesize that CCC-induced loss of CD-specific humoral protection and microbial diversity transforms CD colonization to infection. To test this conceptual model we will establish that AIM #1, CCC patients who develop CDI (cases) have lower CD-specific humoral protection. In AIM #2, CCC patients who develop CDI (cases) have lower gut microbial diversity, and AIM#3, CCC patients who develop CDI undergo clonal expansion of existing CD organisms during CCC. We will conduct a case-control study of 16 CDI-positive cases and 16 CDI-negative controls. We will use metagenomics and induced human intestinal organoid models (iHIOs) as a model system to interrogate CDI development in CCC patients. iHIOs recapitulate human physiology and disease pathology, and incorporate components critical to disease and human host response. We will also employ shotgun sequencing to evaluate microbial factors contributing to CDI during CCC. Integrating these state-of-the-art methodologies with collaborative synergistic research approaches will allow us to characterize CDI pathophysiology and elucidate mechanisms that could not be performed by traditional methods and models.Training in 1) protein toxin biology and quantification techniques, 2) molecular biology and metagenomics, 3) biostatistics and immunogenetics, and 4) clinical trial design will serve as a vehicle for Dr.Apewokin to transition to become an independent physician-scientist competitive for external R type awards. A mentoring team consisting of complementary renowned experts -Drs. A. Weiss, P. Scaglioni, D. Haslam and T. Mersha - has been assembled to achieve this goal.