Fig. 4.

Captured bacteria are incorporated into a backpack upon reaching the rear of the A. castellanii trophozoite as the amoeba moves forward. (A) Time series monitoring the capture and relocation of fluorescent L. monocytogenes on the surface of a moving and a nonmoving A. castellanii trophozoite. Motile L. monocytogenes from all directions (green rods, marked with circles of color according to their time of arrival, in the order yellow, red, blue, purple, white, orange) were trapped on the surface of an A. castellanii trophozoite. A backpack, marked with a green arrowhead, was observed 8 min after the first capture by the moving Acanthamoeba trophozoite. In contrast, L. monocytogenes cells trapped on nonmoving Acanthamoeba trophozoites did not assemble into a backpack. The white cross indicates the position of the geometric center of the A. castellanii trophozoite, and the white line and dots indicate its trajectory as the trophozoite moves (Movies S2 and S3). (B) Schematic representation of our proposed model for backpack formation. The initial location of the anteriormost bacterium is marked with a blue bar. As the A. castellanii trophozoite moves on a surface, the bacteria (in green) remain stationary relative to the substratum until they reach the posterior of the trophozoite, where the bacteria are accumulated into a backpack. (C) Hypothetic model for backpack formation associated with the sol–gel–sol theory for the movement of amoebae on surfaces. Actin filaments are polymerized at the front of the trophozoite to form plasmagel and are subsequently depolymerized at the rear to form plasmasol that contains a pool of unpolymerized monomers. Bacteria (in green) captured on the surface of the trophozoite are deposited in a backpack at the region where plasmagel becomes plasmasol.