cells.*

We use Genetically Encoded Multimeric nanoparticles (GEMs) to probe the microrheology of the Escherichia coli cytoplasm. We reconstruct three-dimensional trajectories from optical microscopy images obtained with a custom-built biplane microscope. The use of different sized GEM particles enables us to explore diffusion of objects ranging in size from 20 to 50 nm, similar in scale to ribosomes and other macromolecular complexes in the cell. We also vary the total charge of the fluorescent proteins from -17 e to +48 e and investigate the effect on diffusion. Using specific small molecule drug treatment, we show progress towards understanding the effects of the nucleoid and cell metabolic state on the 3D diffusion of particles inside bacterial cells.