Coming together: Chemotactic attraction leads to aggregate formation in bacteria

October 12, 2016

Chemical communication, also called quorum sensing, is well known to regulate several different behaviors in bacteria. Scientists at the Max Planck Institute for Terrestrial Microbiology have now added yet another behavior to this list. They have shown that the gut bacterium Escherichia coli can use communication via the universal signaling molecule autoinducer 2 to attract individual bacteria into aggregates. This work demonstrates that the ability to actively self-aggregate using long-range signal exchange is not limited to eukaryotic organisms but can also be exhibited by motile bacteria, thus highlighting commonalities between mechanisms of collective behavior across domains of life. Given the fact that autoaggregation is widespread among bacteria, the described mechanism is likely to be common in nature.

Many bacteria can alternate between distinctly different lifestyles. Most prominent example of such lifestyle transition is formation of multicellular aggregates, which can be either associated with the surface (so-called biofilms) or free-floating in liquid as clumps. These multicellular structures provide bacteria with shelter, making them more resistant to rapidly changing environmental conditions.

As another facet of their collective behavior, most bacteria can also communicate with each other by means of producing and sensing of small molecules. This ability to sense and respond to their neighbors, called „quorum sensing“, is likely to be critical for establishing and maintaining a multicellular lifestyle. Surprisingly, however, there are only few established links between quorum sensing and formation of structured aggregates, indicating that many aspects of the community behavior of bacteria need to be discovered.

Autoaggregation of E. coli: a “dance suite” in three parts. Adhesin (Ag43) mediated autoaggregation of E. coli can be schematically divided into three phases. Phase I corresponds to formation of initial ‘seeding’ aggregates by random collisions of Ag43-expressing motile cells. These initial aggregates subsequently grow during phase II. Here gradients of AI-2 that are produced by the aggregates serve to attract additional cells. Finally, phase III reflects dispersal of the aggregates. During this phase, increased levels of AI-2 in the culture lead to the loss of chemotaxis due to high background. This results in the subsequent gradual disaggregation, because more cells detach from aggregates than join them. 

The Sourjik lab at the MPI for Terrestrial Microbiology has established a new connection between the formation of multicellular aggregates and quorum sensing. The authors studied the process of autoaggregation (formation of suspended cell aggregates in liquid), a common form of collective behavior that exists in many bacteria but is significantly less studied than the biofilm formation on the surface. In contrast to the preexisting view of autoaggregation as a purely passive adherence of multiple cells, this behavior in a model organism E. coli turned out to be much more sophisticated. Besides requiring adhesion molecules on the cell surface, it was shown to rely on chemotaxis, the ability to follow chemical gradients in the environment.

“Chemotaxis is typically seen as a hallmark of planktonic lifestyle of individual bacterial cells, enabling them to locate sources of nutrients and to avoid harmful chemicals” says Prof. Victor Sourjik. “Our work has shown that chemotaxis is equally important for the collective behavior of bacteria, being utilized by forming aggregates to actively attract individual planktonic cells.” Interestingly, this self-attraction was shown to be mediated by autoinducer 2 (AI-2), a universal quorum-sensing signal shared by a number of Gram-positive and Gram-negative bacteria. Its observed role as a self-attraction signal might thus by physiologically important not only in E. coli but also in many other bacteria. 


References:

Laganenka, L. et al. (2016) Chemotaxis towards autoinducer 2 mediates autoaggregation in Escherichia coli. Nat. Commun 7: 12984. doi: 10.1038/ncomms12984.
http://www.nature.com/ncomms/2016/160930/ncomms12984/full/ncomms12984.html

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