Bacterial predation transforms the landscape and community assembly of biofilms.

The bacterium Bdellovibrio bacteriovorus attaches to the exterior of a Gram-negative prey cell, enters the periplasm, and harvests resources to replicate before lysing the host to find new prey.(1-7) Predatory bacteria such as this are common in many natural environments,(8-13) as are groups of matrix-bound prey cell clusters, termed biofilms.(14-16) Despite the ubiquity of both predatory bacteria and biofilm-dwelling prey, the interaction between B. bacteriovorus and prey inside biofilms has received little attention and has not yet been studied at the micrometer scale. Filling this knowledge gap is critical to understanding bacterial predator-prey interaction in nature. Here we show that B. bacteriovorus is able to attack biofilms of the pathogen Vibrio cholerae, but only up until a critical maturation threshold past which the prey biofilms are protected from their predators. Using high-resolution microscopy and detailed spatial analysis, we determine the relative contributions of matrix secretion and cell-cell packing of the prey biofilm toward this protection mechanism. Our results demonstrate that B. bacteriovorus predation in the context of this protection threshold fundamentally transforms the sub-millimeter-scale landscape of biofilm growth, as well as the process of community assembly as new potential biofilm residents enter the system. We conclude that bacterial predation can be a key factor influencing the spatial community ecology of microbial biofilms.