Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Microbiology

First Advisor

Roy, Craig

Abstract

Coxiella burnetii is a Gram-negative, obligate intracellular bacterium that infects livestock and causes Q fever in humans, which can range from acute flu-like symptoms to severe, persistent infections. C. burnetii relies on a Dot/Icm type IVB secretion system to translocate effector proteins, manipulating host cell processes to support intracellular replication. While axenic media have facilitated in vitro pathogenesis research, in vivo studies remain limited due to the classification of the virulent Nine Mile phase I strain as a category B select agent. Although the avirulent phase II strain RSA493 is exempt from this classification, it is rapidly cleared in immunocompetent mouse models, restricting its use in in vivo studies. However, SCID mice have shown susceptibility to phase II infection, offering a viable model. To expand accessible in vivo C. burnetii research, I developed a bioluminescent imaging (BLI) model, enabling non-invasive tracking of infection dynamics and effector phenotypes.Here, I performed a series of studies demonstrating that phase II C. burnetii can infect A/J and C57BL/6 mouse models and that BLI enables precise tracking of bacterial burden. Furthermore, BLI allows for sensitive detection of virulence differences, including both severe Dot/Icm-dependent defects and subtle effects of individual effector protein mutants such as emcA::Tn. This technique can also be used to investigate host determinants of disease, as demonstrated using the complement C3 knockout mouse model to better understand the role of this protein in phase II C. burnetii clearance. Beyond in vivo imaging, BLI has also proven valuable ex vivo, revealing that visceral adipose tissue serves as a primary replication site for C. burnetii following intraperitoneal infection. Follow-up experiments confirmed that C. burnetii replicates in adipocytes both in vitro and in vivo. These findings suggest that adipose tissue represents a previously unrecognized niche for C. burnetii replication, offering potential insights into persistent infections in Q fever patients. Overall, this dissertation introduces a widely accessible and versatile model for studying phase II C. burnetii pathogenesis in vivo and highlights the utility of BLI for accurately tracking bacterial burden, assessing virulence differences, and analyzing host immune responses, further advancing C. burnetii pathogenesis research.

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