Bdellovibrio bacteriovorus cells are small deltaproteobacterial cells that feed on other gram-negative bacteria, including human pathogens. Using cryo-electron tomography, we demonstrated that B. bacteriovorus cells are capable of substantial flexibility and local deformation of the outer and inner membranes without loss of cell integrity. These shape changes can occur in less than 2 min, and analysis of the internal architecture of highly bent cells showed that the overall distribution of molecular machines and the nucleoid is similar to that in moderately bent cells. B. bacteriovorus cells appear to contain an extensive internal network of short and long filamentous structures. We propose that rearrangements of these structures, in combination with the unique properties of the cell envelope, may underlie the remarkable ability of B. bacteriovorus cells to find and enter bacterial prey.Bacteria of the genus Bdellovibrio are highly motile predators that reside in diverse aquatic and terrestrial environments, as well as in the mammalian digestive tract (28; for a review, see reference 33). They prey on a variety of gram-negative bacteria, including several human pathogens, and could become an important tool in the probiotic treatment of disease (32). Within this genus, Bdellovibrio bacteriovorus is the species that has been best characterized biochemically and genetically (27). Investigations over the last four decades (20, 27, 33) have helped elucidate several aspects of the complex life cycle of this organism, which includes a free-swimming or attack phase and a prey-bound phase. Attack-phase B. bacteriovorus is propelled by a single long sheathed flagellum and is capable of attaining speeds up to 160 m/s. The predator swims and turns in apparently random directions until it finds and attaches to a prey cell. It then digests a region of the host cell outer membrane to make a small entry pore, penetrates into the host periplasm, and forms a growth chamber by reshaping and resealing the host outer membrane. The established parasite depletes the host cytoplasm and undergoes growth to form a single elongated spiral cell and segmented division to generate multiple progeny, and it develops a flagellum to gain motility. Ultimately, the chamber ruptures, releasing the nascent bacteria to reinitiate the life cycle.Conventional electron microscopic (EM) studies have provided valuable insights into morphological changes that occur during the life cycle of B. bacteriovorus (1, 2, 6, 7). However, room temperature EM imaging typically requires specimen preparation steps that include chemical fixation, removal of the aqueous phase by treatment with organic solvents, and embedding in plastic resins. These procedures can introduce artifacts, such as distortion of the cell shape, deterioration of membrane structures, and aggregation of soluble multiprotein complexes. Moreover, the contrast in the images originates primarily from the stain and not from the intrinsic density of target structures, limiting the resolution attainable...