Sunday, August 14, 2016

Scientists Discover That Bacteria Have a Collective Memory

Scientists Discover That Bacteria Have a Collective Memory

 Collective motion can be observed in biological systems over a wide
range of length scales, from large animals to bacteria because
collective systems always work better for adaptation than those which
are singular. 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 in a study by Eawag and ETH Zurich scientists published in PNAS.

A central question in the study of biological
collective motion is how the traits of individuals give rise to the
emergent behavior at population level. This question is relevant to
the dynamics of general self-propelled particle systems, biological
self-organization, and active fluids. Bacteria provide a tractable
system to address this question, because bacteria are simple and their
behavior is relatively easy to control.

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

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.

Martin Ackermann comments: "If we understand
this collective effect, it may improve our ability to control
bacterial populations." The findings are relevant, for example, to our
understanding of how pathogens can resist antibiotics, or how the
performance of bacterial cultures in industrial processes or
wastewater treatment plants can be maintained under dynamic
conditions. After all, bacteria play a crucial role in almost all bio-
and geochemical processes. From a human perspective, depending on the
particular process, they are either beneficial -- e.g. if they break
down pollutants or convert nutrients into energy -- or harmful,
especially if they cause diseases. For the researchers, says Mathis,
another important conclusion can be drawn: "If you want to understand
the behaviour and fate of microbial populations, it's sometimes
necessary to analyse every single cell."

Bacteria also have the collective capacity to
generate many neurotransmitters and neuromodulators. For example,
certain Lactobacillus and Bifidobacterium species produce gamma-aminobutyric acid (GABA); Escherichia, Bacillus and Saccharomyces produce norepinephrine (NE).

Future studies of how microbes contribute to the function of their
host on all levels will play an important role in advancing
understanding of health disorders as well as disorders of social


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