Morphological remodeling of Coxiella burnetii during its biphasic developmental cycle revealed by cryo-electron tomography

Coxiella burnetii is an obligate zoonotic bacterium that targets macrophages causing a disease called Q fever. It has a biphasic developmental life cycle where the extracellular and metabolically inactive small cell variant (SCV) transforms inside the host into the vegetative large cell variant (LCV). However, details about the morphological and structural changes of this transition are still lacking. Here, we used cryo-electron tomography to image both SCV and LCV variants grown either under axenic conditions or purified directly from host cells. We show that SCVs are characterized by equidistant stacks of inner membrane that presumably facilitate the transition to LCV, a transition coupled with the expression of the Dot/Icm type IVB secretion system (T4BSS). A class of T4BSS particles were associated with extracellular densities possibly involved in host infection. Also, SCVs contained spherical multilayered membrane structures of different sizes and locations suggesting no connection to sporulation as once assumed.