How bacteria avoid bursting when outside concentrations drop

Bacteria live and grow under significant osmotic pressure - the difference between osmolarity inside the cell and that of the environment. When placed in environments with low osmolarity, bacteria manage to stay in control of their pressure and cell volume by opening a hole in the membrane and responding to competition arising from a propensity of the water and intracellular molecules to enter or exit the cell.

HFSP Program Grant holders Fan Bai and Teuta Pilizota and colleagues
authored on Tue, 10 January 2017

Bacteria rely heavily on mechanosensitive channels to help them cope with shifts in osmotic pressure.  Dr. Teuta Pilizota and Dr. Fan Bai and their teams used single-cell, high resolution imaging to monitor what happens when the common gut bacteria, Escherichia coli, is subject to a sudden decrease in external osmolarity (called a ‘downshock’). Water floods into the cell, which would burst the cell if mechanosensitive channels were not forced open by the increase in tension in the membrane of the swollen cell. The channels then allow solutes and water to flow out, which stabilises the cell volume and prevents the cell from bursting. The process, while allowing control of cell volume and pressure, is passive. Once the channels open, the control happens as a consequence of competition between solutes flowing inwards and water flowing both outwards and inwards. For the first time, single-cell analysis of live bacteria showed that after a ‘downshock’, a bacterial cell will first swell rapidly and then shrink back slowly to its original (and often smaller) size. This is a consequence of the passive nature of the process, and the mechanosensitive channels serve as an emergency pressure release valve. Indebted to this passive control, the bacterial cells could continue to grow normally, apparently unaffected by the shock.

Figure: Change in volume of an individual bacterial cell during an osmotic downshock

The team built and tested a model of this sequence of events using parameters associated with the flow of solutes and water across the cell membrane through the mechanosensitive channels. The model provided an accurate prediction of cell size changes.

The research was part of an HFSP Program Grant aimed at looking at cellular energy flows under different stresses, osmotic stress being one of them. Having understood the osmotic pressure and volume regulation, the team is moving on to look at the energy profiles during this, as well as other stresses. The final goal is to understand free energy balance within bacterial cells.

Reference

Dynamics of Escherichia coli's passive response to a sudden decreases in external osmolarity. Buda R*, Liu Y*, Yang J*, Hegde S*, Stevenson K, Bai F** and Pilizota T**. PNAS September 2016, doi:10.1073/pnas.1522185113.

Link to article

Pubmed link