Four-Dimensional Imaging of E. coli Nucleoid Organization and Dynamics in Living Cells

• The E. coli nucleoid exhibits longitudinal density bundling and radial confinement
• Longitudinal density waves flux through the E. coli nucleoid over seconds-to-minutes
• A mechanical model explains segregation of bacterial chromosomes without a spindle
• Sisters segregate in pulses via cyclic accumulation and relief of mechanical stress

SummaryVisualization of living E. coli nucleoids, defined by HupA-mCherry, reveals a discrete, dynamic helical ellipsoid. Three basic features emerge. (1) Nucleoid density coalesces into longitudinal bundles, giving a stiff, low-DNA-density ellipsoid. (2) This ellipsoid is radially confined within the cell cylinder. Radial confinement gives helical shape and directs global nucleoid dynamics, including sister segregation. (3) Longitudinal density waves flux back and forth along the nucleoid, with 5%–10% of density shifting within 5 s, enhancing internal nucleoid mobility. Furthermore, sisters separate end-to-end in sequential discontinuous pulses, each elongating the nucleoid by 5%–15%. Pulses occur at 20 min intervals, at defined cell-cycle times. This progression includes sequential installation and release of programmed tethers, implying cyclic accumulation and relief of intranucleoid mechanical stress. These effects could comprise a chromosome-based cell-cycle engine. Overall, the presented results suggest a general conceptual framework for bacterial nucleoid morphogenesis and dynamics.

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