The effect of nanoparticle charge and size on 3D diffusion in live Escherichia coli cells.

We study the rheology of the bacterial cytoplasm using a combination of biplane microscopy and single particle tracking. We reconstruct the 3D motion of Genetically Encoded Multimeric nanoparticles (GEMs) inside living Escherichia coli cells. GEM sizes range from 20 to 50 nm, similar in scale to ribosomes and other complexes in the cell. GEMs spatially segregate by size, with 20nm GEMs enriched in the nucleoid, 50nm GEMs excluded to the periphery, and 40nm GEMs localized in both regions. Fixing particle size to 40 nm, we explore the effects of surface charge. Positive GEMs (+1800e) are confined to the cell periphery, while highly negative GEMs (-2160e) localize in the nucleoid preferentially. Motion is sub-diffusive for all GEMs with the anomalous exponent determined primarily by the spatial localization of the particles, while charge and size determine the apparent diffusion coefficient. We compare our experiments to whole-cell colloidal simulations to explore the role of geometry, confinement, particle size and charge in anomalous diffusion in bacteria.