Random encounters and amoeba locomotion drive the predation of Listeria monocytogenes by Acanthamoeba castellanii

Abstract

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.

Flagellin O-linked glycans are required for the interactions between Campylobacter jejuni and Acanthamoebae castellanii

“…Interestingly, this aggregation phenomenon was also observed during Listeria monocytogenes interactions with Acanthamoeba and was termed ‘backpacking’ [18]. This phenotype was concluded to be driven by amoebae locomotion [18]. By contrast, C.…”

“…Given that C. jejuni ∆ flaA mutants are immobile, and motility was speculated to be important for bacterial encounters with Acanthamoebae [ 18 ], we tested whether this defect hindered interactions with A. castellanii . Heat- (70 °C for 45 min) or gentamycin- (100 µgml –1 for 2 h) killed bacteria were used to ablate the motility properties of the flagella.…”

“… C. jejuni aggregate on the surface of Acanthamoebae prior to phagocytosis [ 10, 11 ], although it is now apparent that this aggregation is not essential to induce a productive internalization given that phagocytosis of the ∆ flaB mutant was dramatically reduced. Interestingly, this aggregation phenomenon was also observed during Listeria monocytogenes interactions with Acanthamoeba and was termed ‘backpacking’ [ 18 ]. This phenotype was concluded to be driven by amoebae locomotion [ 18 ].…”

“…Interestingly, this aggregation phenomenon was also observed during Listeria monocytogenes interactions with Acanthamoeba and was termed ‘backpacking’ [ 18 ]. This phenotype was concluded to be driven by amoebae locomotion [ 18 ]. By contrast, C. jejuni aggregation was not a consequence of amoebae movement, and considering many other bacterial species that interact with Acanthamoebae are flagellated and their interactions are intentional [ 19, 20 ], it is likely, at least for C. jejuni , that an intact flagella facilitates adhesion on the surface of the amoebae, which in turn triggers a productive internalization.…”

Zooming in on the intracellular microbiome composition of bacterivorous Acanthamoeba isolates

“…Notably, Acanthamoeba can package and discharge undigested bacteria such as Vibrio cholerae in the form of expelled food vacuoles (EFVs), which can protect the bacteria from multiple external stresses and make them more infectious both in vitro and in vivo [ 14 ]. Due to the random feeding feature of Acanthamoeba [ 22 ], the intracellular multi-microbial communities in the same food vacuole could serve as a “genetic melting pot” and enhance the emergence of microbes with increased abilities to endure intracellularly in amoeba as well as in cells of higher eukaryotes [ 16 ]. Such patho-adaptations in Acanthamoeba hosts are now broadly accepted as an environmental training ground for the evolution and transmission of potential bacterial pathogens [ 23 ].…”