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Physics Colloquium: Kerwyn Huang (Stanford)

December 12, 2014 - 6:45pm to December 13, 2014 - 7:45am

Every Friday 10:45-11:45 a.m., COB 267 (except as noted). Tea and cookies will be served from 10:30 - 10:45 a.m. Questions regarding the seminar series should be directed to Prof. Chih-Chun Chien



It Came As A Shock: Regulation Of Bacterial 

Growth By Osmotic Pressure 

By K.C. Huang 

Associate Professor,

Department of Bioengineering and Microbiology & Immunology, and Biochemistry 

Stanford University 


It has long been proposed that turgor pressure plays an essential role during bacterial growth by driving mechanical expansion of the cell wall. This hypoth-esis is based on analogy to plant cells, for which this mechanism has been es-tablished, and on experiments in which the growth rate of bacterial cultures was observed to decrease as the osmolarity of the growth medium was in-creased. To distinguish the effect of turgor pressure from pressure-independent effects that osmolarity might have on cell growth, we monitored the elongation of single Escherichia coli cells while rapidly changing the osmo-larity of their media. By plasmolyzing cells, we found that cell-wall elastic strain did not scale with growth rate, suggesting that pressure does not drive cell-wall expansion. Furthermore, in response to hyper- and hypoosmotic shock, E. coli cells resumed their pre-shock growth rate and relaxed to their steady-state rate after several minutes, demonstrating that osmolarity modulates growth rate slowly, independently of pressure. Oscillatory hyperosmotic shock re-vealed that while plasmolysis slowed cell elongation, the cells nevertheless “stored” growth such that once turgor was re-established the cells elongated to the length that they would have attained had they never been plasmolyzed. In contrast, Bacillus subtilis cells exhibit highly regular growth oscillations in re-sponse to hypoosmotic shock that are dependent on peptidoglycan synthesis. The period of these oscillations scales linearly with the magnitude of the shock. By applying a simple mathematical theory to these data, we show that growth oscillations are initiated by mechanical-strain-induced growth arrest. This demonstrates that B. subtilis has developed an elegant system by which turgor pressure both up- and down-regulates the final steps of cell growth.