Direct imaging of the circular chromosome in a live bacterium

New assays for quantitative imaging 1–6 and sequencing 7–11 have yielded great progress towards understanding the organizational principles of chromosomes. Yet, even for the well-studied model bacterium Escherichia coli , many basic questions remain unresolved regarding chromosomal (sub-)structure 2,11 , its mechanics 1,2,12 and dynamics 13,14 , and the link between structure and function 1,15,16 . Here we resolve the spatial organization of the circular chromosome of bacteria by directly imaging the chromosome in live E. coli cells with a broadened cell shape. The chromosome was observed to exhibit a torus topology with a 4.2 μm toroidal length and 0.4 μm bundle thickness. On average, the DNA density along the chromosome shows dense right and left arms that branch from a lower-density origin of replication, and are connected at the terminus of replication by an ultrathin flexible string of DNA. At the single-cell level, the DNA density along the torus is found to be strikingly heterogeneous, with blob-like Mbp-size domains that undergo major dynamic rearrangements, splitting and merging at a minute timescale. We show that prominent domain boundaries at the terminus and origin of replication are induced by MatP proteins, while weaker transient domain boundaries are facilitated by the global transcription regulators HU and Fis. These findings provide an architectural basis for the understanding of the spatial organization of bacterial genomes.