Dynamics of Escherichia coli’s passive response to a sudden decrease in external osmolarity Renata Budaa,1,2, Yunxiao Liu (刘云啸)b,1, Jin Yang (杨津)b,1, Smitha Hegdea,1, Keiran Stevensona, Fan Baib,3, and Teuta Pilizotaa,3 aCentre for Synthetic and Systems Biology, Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3FF, United Kingdom; and bBiodynamic Optical Imaging Centre (BIOPIC), School of Life Sciences, Peking University, Beijing 100871, China Edited by Janet M. Wood, University of Guelph, Guelph, ON, Canada, and accepted by Editorial Board Member Herbert Levine July 28, 2016 (received for review November 10, 2015) For most cells, a sudden decrease in external osmolarity results in vate the nonspecific export of solutes through mechanosensitive fast water influx that can burst the cell. To survive, cells rely on the channels (MSCs), such as MscS and MscL (Fig. 1A) (8). As the passive response of mechanosensitive channels, which open under solutes leave the cell, so does the cytoplasmic water, enabling the increased membrane tension and allow the release of cytoplasmic cell to recover original volume and pressure (Fig. 1A). solutes and water. Although the gating and the molecular Mechanosensitive channels are found in a wide range of cells Escherichia coli structure of mechanosensitive channels found in (9–11), displaying great diversity. The precise gating mechanism have been extensively studied, the overall dynamics of the whole of these pressure-controlled channels has attracted a lot of at- cellular response remain poorly understood. Here, we characterize tention from scientific community. Despite the efforts, it remains a E. coli’s passive response to a sudden hypoosmotic shock (down- challenge (12). To our current knowledge, E. coli possesses seven shock) on a single-cell level. We show that initial fast volume ex- pansion is followed by a slow volume recovery that can end below different mechanosensitive channels (13). Of those seven, four the initial value. Similar response patterns were observed at play the dominant role: the mechanosensitive channel of small downshocks of a wide range of magnitudes. Although wild-type conductance (MscS), the large mechanosensitive channel (MscL) cells adapted to osmotic downshocks and resumed growing, cells (9, 14, 15), the mechanosensitive channel of miniconductance of a double-mutant (ΔmscL, ΔmscS) strain expanded, but failed to (MscM) (16), and the potassium-dependent mechanosensitive fully recover, often lysing or not resuming growth at high osmotic channel (MscK) (17). Since their discovery in giant spheroplasts of downshocks. We propose a theoretical model to explain our ob- E. coli (13, 18), crystal structures of some of the channels have servations by simulating mechanosensitive channels opening, and been obtained (19–21), and channel function has been extensively subsequent solute efflux and water flux. The model illustrates studied in vitro (13, 18, 19, 22–25). The most widely used in vitro how solute efflux, driven by mechanical pressure and solute chem- technique, electrophysiology, enabled measurements of channels’ ical potential, competes with water influx to reduce cellular os- pressure sensitivity, open dwell time, conductance, as well as ion motic pressure and allow volume recovery. Our work highlights selectivity (18, 26). For example, in vitro-measured opening time the vital role of mechanosensation in bacterial survival. of MscS or MscL is on the order of 20–30 ms (27, 28), and the channels close immediately upon the decrease in tension (13). osmotic downshock | bacterial mechanosensing | single-cell imaging Significance iology offers an array of intriguing mechanical solutions, both Bactive and passive, often exceeding what is currently possible with man-made methods. Understanding how biological systems Mechanosensation is central to life. Bacteria, like the majority of achieve different functionalities under mechanical stimuli can in- walled cells, live and grow under significant osmotic pressure. By form new, thus-far-unexplored design principles. One such passive relying on mechanosensitive regulation, bacteria can adapt to control system is the bacterial response to sudden decreases in dramatic changes in osmotic pressure. Studying such mechanical external osmolarities. sensing and control is critical for understanding bacterial survival A Gram-negative cell’s fluid cytoplasm is separated from the in a complex host and natural environment. Here, we investigate Escherichia coli’ external environment by the inner membrane, the periplasmic the fundamental design principles of s passive space, and the outer membrane. Ordinarily, the total solute con- mechanosensitive response to osmotic downshocks by imple- menting single-cell high-resolution imaging. We explain the centration within the cytoplasm is higher than that of the envi- observed cell volume changes by modeling flux of water and ronment, resulting in a positive osmotic pressure on the cell wall solutes across the cell membrane. A better characterization of (termed turgor pressure) (1). Escherichia coli is able to respond to bacterial mechanosensitive response can help us map their re- both increases and decreases in external concentrations. An in- action to environmental threats. crease in external osmolarity (hyperosmotic shock or upshock) results in water efflux from the cell interior, causing cellular vol- Author contributions: T.P. designed research; R.B., S.H., and T.P. performed research; K.S. ume to shrink and osmotic pressure to drop to zero (2). E. coli contributed new reagents/analytic tools; J.Y., F.B., and T.P. developed the model; J.Y. and responds by actively accumulating specific solutes (osmolytes), K.S. fitted the data to the model; R.B., Y.L., S.H., and T.P. analyzed data; and R.B., Y.L., such as potassium, proline, and glycine-betaine (2). Accumulation J.Y., S.H., K.S., F.B., and T.P. wrote the paper. of osmolytes in the cell’s cytoplasm causes reentry of water, cell The authors declare no conflict of interest. volume increase, and recovery of osmotic pressure (3, 4). A This article is a PNAS Direct Submission. J.M.W. is a Guest Editor invited by the Editorial Board. downward shift in external osmolarity (termed hypoosmotic 1 shock or downshock) causes fast water influx into the cell’s cy- R.B., Y.L., J.Y., and S.H. contributed equally to this work. 2Present address: Laboratory of Cell Biophysics, Division of Molecular Biology, Ruder toplasm. As a result, the osmotic pressure increases and the cell Boskovic Institute, 10000 Zagreb, Croatia. expands in a nonlinear fashion (5, 6). Turgor pressure in E. coli 3To whom correspondence may be addressed. Email: [email protected] or fbai@ has been estimated to lie between 0.3 and 3 atm (5, 7), rising up pku.edu.cn. to 20 atm upon a large downshock (6). An increase in the inner This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. membrane tension, caused by the expansion, is thought to acti- 1073/pnas.1522185113/-/DCSupplemental. E5838–E5846 | PNAS | Published online September 19, 2016 www.pnas.org/cgi/doi/10.1073/pnas.1522185113 Downloaded by guest on September 25, 2021 At this rate, full transition to the lower osmolarity media is completed PNAS PLUS within 0.8 s (Materials and Methods). Cytoplasmic volume was mon- itored via cytoplasmically expressed eGFP, sampled at a frame every 0.2 s for initial 15 min, and at a frame every 5 s for the rest of the 75-min recording. Characteristic phases were identified and indicated with different background colors as follows: (I) expansion phase, observed immediately after downshock; (II) decrease phase of vol- ume recovery, observed postexpansion, lasting several minutes; as the volume decreases in this phase, a characteristic “overshoot” below the initial volume is often observed; (III) increase phase of volume recovery, observed after minimum volume (Vmin) has been reached and lasting until initial volume is reestablished, that is, ∼30 min; (IV) cell growth phase, observed post-volume recovery. Fig. 1C gives raw images corresponding to different phases shown in Fig. 1B. We analyzed volume changes in 609 wild-type cells before, dur- ing, and after downshock for the following shock magnitudes: 103, 190, 460, 790, 960, 1,130, and 1,337 mOsmol. Fig. 2, Left, shows Fig. 1. Characteristic cell volume response to a sudden downshock. (A)Upona average traces with SDs of 103- to 1,130-mOsmol shocks. All cells sudden decrease in external concentration, cell volume expands, which leads to quickly expand in phase I and show characteristic slow volume re- opening of mechanosensitive channels. Consequently, solutes exit the cell, covery in phase II. As the shock increases, the length of phase II and allowing recovery of cell volume through loss of cytoplasmic water. (B)A the overshoot increase. SI Appendix,Fig.S1,Left, shows average characteristic single-cell volume response for a 1,130-mOsmol downshock. The traces with SDs over longer time periods; phase IV, that is, growth, trace was normalized by the initial volume, that is, the volume before the is visible for all shock magnitudes. SI Appendix,Fig.S3,Left, shows downshock. Different phases of the recovery response are indicated with dif- average trace with SDs of our largest shock, 1,337 mOsmol. We ferent colors. In gray is the expansion phase (phase I), followed by two volume observe expansion in phase I; however, only small recovery in phase recovery phases. Phase II (in orange) is characterized by volume decrease, and phase III (in green), by volume increase upon reaching the minimum volume. II is visible, with no characteristic overshoot and no phase IV. In Phase IV (in purple) indicates recommenced growth. Initial 15 min are sampled fact, a large number of cells in 1,337-mOsmol condition lyse at 5 Hz and an additional 1 h at a frame every 5 s. (C) Still images from dif- during our recording (SI Appendix,Fig.S10).