Probing passive diffusion of flagellated and deflagellated Escherichia coli.

Using particle-tracking techniques, the translational and rotational diffusion of paralyzed E. coli with and without flagella are studied experimentally. The position and orientation of the bacteria are tracked in the lab frame and their corresponding mean-square displacements are analyzed in the lab frame and in the body frame to extract the intrinsic anisotropic translational diffusion coefficients as well as the rotational diffusion coefficient for both strains. The deflagellated strain is found to show an anisotropic translational diffusion, with diffusion coefficients that are compatible with theoretical estimates based on its measured geometrical features. The corresponding translational diffusion coefficients of the flagellated strain have been found to be reduced as compared to those of the deflagellated counterpart. Similar results have also been found for the rotational diffusion coefficients of the two strains. Our results suggest that the presence of flagella --even as a passive component-- has a significant role in the dynamics of E. coli, and should be taken into account in theoretical studies of its motion.