ABSTRACT Clinical outcomes in invasive gram-negative bacterial infections are determined by the interplay of patient, treatment, and bacterial variables. Significant progress has been made in understanding patient and treatment factors; however, little is known about how bacterial genetic variation influences patient outcomes. This is an important gap in understanding microbial pathogenesis and limits the ability to identify drug targets critical in human pathogenesis. To address this problem, the long-term goal is to interrogate bacterial pathogenesis in humans by defining the impact of bacterial genetic variation on the outcome of patients with gram-negative bacteremia. This knowledge can be leveraged to identify novel treatments. The career development plan reflects this long-term goal as the emphasis is to develop quantitative and lab expertise to independently identify and explore associations between bacterial genetics and patient outcomes. The objective in this proposal is to characterize two putative pathogenicity islands (PPI), variably present in E. coli, that are promising for their roles in pathogenesis and potential as drug targets. The two PPI were identified by the applicant and encode genes homologous to a type III secretion system (T3SS) structural apparatus (PPI-1) and translocases/adhesin (PPI-2). PPI-1/PPI-2 are not present in typical laboratory strains of E. coli and so have not been significantly studied. Presence of PPI-1/PPI-2 associated with increased mortality in patients with E. coli bacteremia, and deletions in PPI-1/PPI-2 allowed for complement-mediated killing of E. coli in serum and impaired E. coli-host cell interactions (decreased invasion). Further, PPI-1/PPI-2 functioned as a prognostic biomarker that improved ability to identify patients at high risk of mortality. The central hypothesis is that PPI-1 and PPI-2 function together as a T3SS that mediates complement-mediated serum resistance and tissue invasiveness to influence patient outcomes, and presence of PPI-1/PPI-2 provides prognostic value in patients with E. coli BSI. This will be tested through two specific aims (SA): 1) Determine how PPI-1 and PPI-2 affect virulence to influence patient outcome, and 2) Define value of PPI-1 and PPI-2 genotype as prognostic biomarkers for E. coli bacteremia. SA1 will identify how PPI-1/PPI-2 mediates resistance to complement activation, verify that PPI-1/PPI-2 is a functional T3SS, and determine the relative contributions of serum killing versus T3SS function to virulence. SA2 will use an existing external validation cohort to show that incorporating bacterial genetics with clinical variables improves prognostic models of clinical outcomes in E. coli bacteremia. Characterization of PPI-1/PPI-2 will increase our understanding of E. coli pathogenesis and pave the way for novel therapeutics. For example, a protein in PPI-2 is homologous to a drug target that the applicant and others previously exploited in Pseudomonas aerug...