Bioluminescent imaging to investigate <i>Coxiella burnetii</i> pathogenesis identifies adipose tissue as a host niche for infection

<i>Coxiella burnetii</i> is a gram-negative, obligate intracellular pathogen that causes Q fever in humans. <i>In vivo</i> research on <i>C. burnetii</i> is limited due to the classification of the Nine Mile phase I (NMI) strain as a select agent that requires biosafety level 3 containment. The isogenic Nine Mile phase II (NMII) strain can be cultured safely at biosafety level 2 and has been shown to infect immunocompromised mice, which suggests this strain could be used to investigate virulence phenotypes <i>in vivo</i>. This study developed a bioluminescent imaging (BLI) model using NMII to non-invasively monitor <i>C. burnetii</i> infections in mice. Here, we show that BLI enables tracking of bacteria in an animal host, identification of bacterial virulence differences, and investigation of host determinants of immunity. Using BLI, we show that NMII resides primarily in visceral adipose tissue following intraperitoneal infection of mice. Intracellular replication of <i>C. burnetii</i> in adipocytes was confirmed using cultured cells <i>ex vivo</i>. These data indicate that adipose tissue can serve as a niche for <i>C. burnetii</i> replication. This study underscores the utility of BLI in advancing <i>C. burnetii</i> research and highlights the need for further exploration into the role of adipocytes in the disease Q fever and bacterial persistence <i>in vivo</i>.