Aug. 31 2009 - 1:09 pm | 0 views | 0 recommendations | 0 comments

Tiny little subterfuge

By ALEXANDER HONKALA

: Image via Wikipedia

You are never alone, even when you might want to be. 100 trillion of our closest friends, our intestinal microbiota, colonize and inhabit our intestines, helping us to digest our food, regulate our inflammatory immune system response, and even shape the functional morphology of our gut epithelia. Luckily, evolution has selected for the survival of bacteria that help us out, whether by synthesizing the Vitamin K we need, breaking down complex carbohydrates for us, or secreting the butyrate that helps gastrointestinal epithelia to mature correctly. Even so, the competition of bacteria to survive and inhabit that space is fierce and cutthroat and oddly enough, that competition has led to the evolution of both cooperative behaviors and subterfuge in bacterial populations.

Recent research in PLoS Computational Biology [Czárán T, Hoekstra RF (2009) Microbial Communication, Cooperation and Cheating: Quorum Sensing Drives the Evolution of Cooperation in Bacteria. PLoS ONE 4(8): e6655. doi:10.1371/journal.pone.0006655] has helped to reconcile the evolutionary contradiction of one organism helping another survive when it could use that energy to reproduce with observed cooperative behavior through computational modeling. Dr. Czárán et al used a cellular automaton model in which a point was assigned cell-like functions. In this case, Dr. Czárán assigned functions that mimicked a metabolic cooperation system in bacteria: an emitter for a common good product, a detector for other bacteria, and an emitter of an “I’m here!” signal. When some bacteria give off the “I’m here!” signal, other bacteria that produce the detector can tell whether or not potentially beneficial neighbors are around. When there are enough good neighbors, the bacteria know that they will get something in return if they secrete a common metabolic good product and they switch that machinery on. Dr. Czárán explored the conditions under which this kind of cooperative behavior emerges.

By placing these cellular models into a 3D lattice where each can interact with its neighbors in an automatic manner dependent upon whether or not they are able to make a product, see the signal to make it, or produce the signal, it was found that cooperation only emerges under conditions in which the dispersal of related cells in a liquid medium is limited, and this aligns well with evolutionary theory in that it helps one cell spread its own genes by helping closely related cells. If the medium in which cells are living is turbulent, then the probability that cells would be helping related cells is decreased and cooperative behavior doesn’t make much metabolic sense. It was also found that cooperation can exist without a molecular system to detect other bacteria, but the presence of cooperation in a population pushes for the selection of quorum sensing so that cooperating cells can guard against parasites. Parasites will still emerge even with quorum sensing systems in that some cells will realize the evolutionary benefit of secreting signals that get other cells to make communal metabolic products, but the quorum sensing helps cooperating cells discriminate whether or not to produce a common good. Overall, this modeling has revealed the evolutionary logic and mechanisms behind cooperative behavior in bacterial populations with diverse emergent behaviors.

This research sheds considerable light into the dynamics of large bacterial populations under a variety of conditions. This could help scientists decipher the drama being played out in our microbiomes that can acutely impact our health.