Predatory protozoa play an essential role in shaping microbial populations. Among these protozoa, Acanthamoeba are ubiquitous in the soil and aqueous environments inhabited by Listeria monocytogenes. Observations of predator-prey interactions between these two microorganisms revealed a predation strategy in which Acanthamoeba castellanii assemble L. monocytogenes in aggregates, termed backpacks, on their posterior. The rapid formation and specific location of backpacks led to the assumption that A. castellanii may recruit L. monocytogenes by releasing an attractant. However, this hypothesis has not been validated, and the mechanisms driving this process remained unknown. Here, we combined video microscopy, microfluidics, single-cell image analyses, and theoretical modeling to characterize predator-prey interactions of A. castellanii and L. monocytogenes and determined whether bacterial chemotaxis contributes to the backpack formation. Our results indicate that L. monocytogenes captures are not driven by chemotaxis. Instead, random encounters of bacteria with amoebae initialize bacterial capture and aggregation. This is supported by the strong correlation between experimentally derived capture rates and theoretical encounter models at the single-cell level. Observations of the spatial rearrangement of L. monocytogenes trapped by A. castellanii revealed that bacterial aggregation into backpacks is mainly driven by amoeboid locomotion. Overall, we show that two nonspecific, independent mechanisms, namely random encounters enhanced by bacterial motility and predator surface-bound locomotion, drive backpack formation, resulting in a bacterial aggregate on the amoeba ready for phagocytosis. Due to the prevalence of these two processes in the environment, we expect this strategy to be widespread among amoebae, contributing to their effectiveness as predators.
Keywords: Acanthamoeba; Listeria; capture dynamics; predation; random encounter.