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Background Pseudomonas aeruginosa (PA) is a critically important healthcare-associated pathogen responsible for a variety of infections including bloodstream infection (bacteremia), pneumonia, and urinary tract infection. PA bacteremia is a significant cause of morbidity and mortality, especially in immunocompromised patients; However, little is known about the in-host infection dynamics of PA bacteremia and the impact of individually infected patients on transmission in the healthcare environment. Methods We utilized animal modeling in conjunction with sequencing technology to dissect the infection dynamics of PA bloodstream infections. BALB/c mice were challenged intravenously with a human bacteremia isolate, PABL012. At various time points post infection, organs were harvested and the surviving PA enumerated. In parallel, PABL012 engineered to express the luciferase cassette was used to track PA in live mice over time using the IVIS imaging system. STAMP (sequence tag-based analysis of microbial populations) analysis was then applied to define the population dynamics of PA bloodstream infection. Results Bacterial enumeration and IVIS imaging revealed that systemically infected mice have a focus of bacterial expansion in their gallbladders (GB). Surprisingly, the same mice also shed PA in their gastrointestinal tract (GI), a phenomenon not previously appreciated following bloodstream infection. Finally, STAMP analysis revealed that (1) PA experiences a severe in vivo bottleneck when trafficking to the GB, (2) the population in the GB expands tremendously during infection and (3) this population is ultimately the source of excreted bacteria in the GI tract. Conclusion Our research, using murine models, provides the first evidence that the GB acts as a sanctuary site for PA replication following systemic infection and links replication with fecal excretion. Fecal excretion of PA from hospitalized patients is observed, but the direct link between acute infection, GI shedding, and transmission remains unclear. Our observations have significant implications on understanding how PA evades initial host clearance, the identity of protected expansion niches, and how PA might exit the human host in the healthcare environment facilitating a transmission event. Disclosures All authors: No reported disclosures.

2566. infection Dynamics of Pseudomonas aeruginosa Bloodstream Infections