Acanthamoeba feature a unique backpacking strategy to trap and feed on Listeria monocytogenes and other motile bacteria.

Despite its prominent role as an intracellular human pathogen, Listeria monocytogenes normally features a saprophytic lifestyle, and shares many environmental habitats with predatory protozoa. Earlier studies claimed that Acanthamoeba may act as environmental reservoirs for L. monocytogenes, whereas others failed to confirm this hypothesis. Our findings support the latter and provide clear evidence that L. monocytogenes is unable to persist in Acanthamoeba castellanii and A. polyphaga. Instead, external Listeria cells are rapidly immobilized on the surface of Acanthamoeba trophozoites, forming large aggregates of densely packed bacteria that we termed backpacks. While the assembly of backpacks is dependent on bacterial motility, flagellation alone is not sufficient. Electron micrographs showed that the aggregates are held together by filaments of likely amoebal origin. Time-lapse microscopy revealed that shortly after the bacteria are collected, the amoeba can change direction of movement, phagocytose the backpack and continue to repeat the process. The phenomenon was also observed with avirulent L. monocytogenes mutants, non-pathogenic Listeria, and other motile bacteria, indicating that formation of backpacks is not specific for L. monocytogenes, and independent of bacterial pathogenicity or virulence. Hence, backpacking appears to represent a unique and highly effective strategy of Acanthamoeba to trap and feed on motile bacteria.