Sunday, 17 April 2016

Collective memory discovered in bacteria

Individual bacterial cells have short memories. But groups of bacteria can develop a collective memory that can increase their tolerance to stress. This has been demonstrated experimentally for the first time.

Bacteria exposed to a moderate concentration of salt survive subsequent exposure to a higher concentration better than if there is no warning event. But in individual cells this effect is short-lived: after just 30 minutes, the survival rate no longer depends on the exposure history. Now two Eawag/ETH Zurich microbiologists, Roland Mathis and Martin Ackermann, have reported a new discovery made under the microscope with Caulobacter crescentus, a bacterium ubiquitous in freshwater and seawater.

When an entire population is observed, rather than individual cells, the bacteria appear to develop a kind of collective memory. In populations exposed to a warning event, survival rates upon a second exposure two hours after the warning are higher than in populations not previously exposed. Using computational modelling, the scientists explained this phenomenon in terms of a combination of two factors. Firstly, salt stress causes a delay in cell division, leading to synchronization of cell cycles; secondly, survival probability depends on the individual bacterial cell's position in the cell cycle at the time of the second exposure. As a result of the cell cycle synchronization, the sensitivity of the population changes over time. Previously exposed populations may be more tolerant to future stress events, but they may sometimes even be more sensitive than populations with no previous exposure.

For more on this, see:

Roland Mathis, Martin Ackermann. Response of single bacterial cells to stress gives rise to complex history dependence at the population level. PNAS, March 7, 2016 DOI: 10.1073/pnas.1511509113

 Posted by Dr. Tim Sandle