Organization of bacterial cell division ring, and bacterial morphology and growth under mechanical compression
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This dissertation studies fundamental questions in biomechanics and biophysics of bacterial cell division. Specifically, we examine morphology and growth of prototypical bacterial cells by employing both experimental and theoretical tools. We use novel techniques in fluorescence microscopy and microfluidics to reveal the organization of the bacterial division ring (Z-ring). We also examine bacterial morphology and growth under mechanical compression, and explain the experimental data using existing mechanochemical models. The bacterial division ring (Z-ring) is essential for cytokinesis in bacteria. The Z-ring is a ring-shaped cell division complex and whose primary component is FtsZ, a filamentous tubulin homologue that serves as a scaffold for the recruitment of other cell division related proteins. FtsZ forms filaments and bundles. In the cell, it has been suggested that FtsZ filaments form the arcs of the ring, and are aligned in the cell circumferential direction. Using polarized fluorescence microscopy in live Escherichia coli cells, we measure the structural organization of FtsZ filaments in the Z-ring. The data suggests a disordered organization: a substantial portion of FtsZ filaments is aligned in the cell axis direction. FtsZ organization in the Z-ring also appears to depend on the bacterial species. Taken together, the unique arrangement of FtsZ suggests novel unexplored mechanisms in bacterial cell division. Related to cytokinesis, bacterial cell morphology and growth are controlled by a combination of physical and chemical processes. In standard medium, Escherichia coli cells are rod-shaped, and maintain a constant diameter during exponential growth. We demonstrate that by applying compressive forces to growing rod-shaped E. coli, cells no longer retain their rod-like shapes but grow and divide with a flat pancake-like geometry. The deformation is reversible: deformed cells can recover back to rod-like shapes in several generations after compressive forces are removed. During compression, the cell elongation rate, proliferation rate, DNA replication rate, and protein synthesis are not significantly different from those of the normal rod-shaped cells. Quantifying the rate of cell wall growth under compression reveals that the cell wall growth rate depends on the local cell curvature. MreB not only influences the rate of cell wall growth, but also influences how the growth rate scales with cell geometry. The result is consistent with predictions of a mechanochemical model, and suggests an active mechanical role for MreB during cell wall growth. The developed compressive device is also useful for studying bacterial cells in unique geometries.
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