The causative agent of the human infection Q fever, Coxiella burnetii, was first described in Australia in 1935. This pathogen is a unique obligate intracellular bacterium that replicates to high numbers within a pathogen-derived lysosome-like vacuole. Unlike other bacteria, Coxiella thrives within this low pH, hydrolytic and hostile environment. In 2009, researchers developed means to axenically culture Coxiella in the laboratory, paving the way for the development of tools to genetically manipulate the organism. These advances have allowed the field to finally identify and understand the bacterial factors that mediate the intracellular success of Coxiella.
We conducted a visual screen on an arrayed library of C. burnetii NMII transposon insertion mutants to identify genes required for biogenesis of a mature Coxiella-containing vacuole. Mutants defective in Dot/Icm type IV secretion system function or the PmrAB regulatory system were incapable of intracellular replication. In addition, several mutants with intracellular growth defects were found to have insertions in genes encoding effector proteins translocated into host cells by the Dot/Icm system. This research has demonstrated that the Dot/Icm secretion system, and the cohort of effector proteins introduced into host cells via this apparatus, is central to Coxiella pathogenesis. Investigating the functional roles of the arsenal of Dot/Icm effectors has already implicated eukaryotic vesicular trafficking pathways as a key target for manipulation by Coxiella and will continue to reveal important virulence strategies.