Brucella is a facultative intracellular pathogen that replicates within a specialised compartment called the replicative Brucella-containing vacuole (rBCV). rBCVs have been reported to carry ER markers, but little is known about how the rBCV is established and how it relates to the genuine ER and other cellular organelles. In this Research Article, Christoph Dehio and colleagues (Sedzicki et al., 2018) reconstruct the 3D ultrastructure of rBCVs and associated ER using correlative structured illumination microscopy (SIM) and focused ion beam/scanning electron microscopic (FIB/SEM) tomography. The authors analyse the compartmentalisation of Brucella during growth within HeLa cells or in tropoblasts during mouse infections. They demonstrate that there are extensive interactions between rBCVs and other host cell membranes, and further, that rBCVs are continuous with genuine ER. The authors next try to quantify the level of networking between rBCV membranes and the ER, and find that most bacteria appear to reside inside a complex 3D ER meshwork that has many connections to other host membrane compartments. Finally, the authors show, for post-division bacteria, that the continuity between their separating rBCVs is limited to small surface areas. The beautiful series of images in this paper provide strong evidence for a model in which the lumen of rBCVs directly interfaces with the lumen of the ER, suggesting that rBCVs are extensions of genuine ER.