Modulation of prey size reveals adaptability and robustness in the cell cycle of an intracellular predator

Abstract

Despite the remarkable diversity of bacterial lifestyles, the sophisticated regulatory networks underlying bacterial replication have only been investigated in a limited number of model species so far. In bacteria that do not rely on canonical binary division for proliferation, the coordination of major cellular processes is still mysterious. Moreover, bacterial growth and division remain largely unexplored within spatially confined niches where nutrients are limited. This includes the lifecycle of the model endobiotic predatory bacterium Bdellovibrio bacteriovorus, which grows by filamentation within its host or prey cells and produces a variable number of daughter cells. Here, we examined how the size of the micro-compartment in which predators replicate (i.e., the prey bacterium) impacts their cell cycle progression at the single-cell level. Using Escherichia coli with genetically encoded size differences, we show that the duration of the predator cell cycle scales with prey size. As a result, the size of the prey determines the number of predator offspring, through a relationship that is maintained across prey species. Strikingly, the nutritional quality of the prey cell determined the specific growth rate of predators regardless of prey size, reminiscent of the effect of medium composition on the growth of other bacteria. Tuning the predatory cell cycle by modulating prey dimensions also allowed us to reveal invariable temporal connections between key cellular processes. Altogether, our data uncover adaptability and robustness shaping the enclosed replication of B. bacteriovorus. Consequently, predators optimally exploit the finite prey resources and space while ensuring a strict cell cycle progression. This study opens the way to explore diverse cell cycle control strategies by extending their characterization to intracellular lifestyles, beyond canonical models and growth conditions.

Distinct dynamics and proximity networks of hub proteins at the prey-invading cell pole in a predatory bacterium

“…As shown by immunoblotting ( Supp Fig 2C ), the limited amount of DivIVA proteins during growth likely prevents the nucleation of DivIVA foci in the elongated predator cell. The disappearance of DivIVA foci during the growth phase was not observed in a previous study (13), possibly because smaller prey was employed, resulting in a reduced predator growth phase (9), and the imaging of different bdelloplasts at each timepoint separated by longer time intervals (reproduced in Supp Fig 2D ). However, in both previously used and our conditions, new DivIVA-msfGFP foci gradually emerged at visible constriction sites ( Fig 3A , white asterisks) and at cell poles ( Fig 3A , blue asterisks), towards the end of the growth phase ( Fig 3A, Supp Fig 2B, Movie S1 ).…”

“…During this growth phase (GP), B. bacteriovorus replicates its chromosome asynchronously, resulting in odd or even copy numbers corresponding to the number of daughter cells (8). The duration of the GP, the timing of non-binary cell division, and therefore, the number of predator offspring, are determined by prey cell size and composition (9). When exiting the prey, newborn predators display the same polarity as before prey invasion (5, 8).…”

Lifecycle of a predatory bacterium vampirizing its prey through the cell envelope and S-layer

“…Furthermore, the quantification of offspring number showed that B. exovorus produces “fixed” numbers of progenies, typically three and less frequently two, irrespective of the prey size. This contrasts with the behaviour of the closely related endobiotic predatory bacterium B. bacteriovorus , which adjusts its growth phase to prey variability, resulting in the scaling of progeny number with prey cell size 32 . It is conceivable that growing inside a prey provides a protective nest enabling the exploitation of all available prey resources, if the predator can adjust its cell cycle to the variability of the prey – which is the case with B. bacteriovorus .…”

“…Our data suggest that the type of predation could also impact the feeding profile of the prey. Unlike B. bacteriovorus , which does not seem to require prey IM disruption for feeding (a shrinking but clearly defined prey cytoplasm can be seen throughout the predator growth phase; e.g., 31,32,37 ), predation by B. exovorus quickly results in the loss of prey IM integrity, possibly facilitating digestion and/or import of nutrients from the cytoplasm. Our results also suggest that the prey cellular content is digested in situ by B. exovorus .…”

Moving toward a better understanding of the model bacterial predator Bdellovibrio bacteriovorus

“…The long-held requirement for filamentous growth and division has been challenged by observations with smaller Proteus mirabilis prey (< 2 µm), that reveal a remarkable shift to binary fission -this is accompanied by a requirement to remodel the old flagellar pole into a new invasion pole [61]. Additionally, progeny number can be conclusively linked to prey cell size [62], and the origin of replication is always located at the non-flagellated invasive pole [63].…”