Counting Heads or Measuring Space? – A Close Look at Bacterial Communication Strategies

What exactly do bacteria learn from the signal substances? There have been two theories: the release of signal substances is understood to be either a cooperative strategy to determine the cell density (quorum sensing) or – alternatively – a non-cooperative strategy in which the signal substance is only used to determine the dimensions of the space surrounding the cell (diffusion sensing). However, both theories have not been shown to work under natural conditions, which usually are much more complex than those in laboratory.

Scientists from the GSF – National Research Center for Environment and Health (member of the Helmholtz-Gemeinschaft) have been able to show that both approaches are merely theoretical extremes of an overall strategy by which bacteria determine whether the amount of energy required to produce substances, such as antibiotics or exoenzymes, is worth while in a particular environmental situation. “This overall strategy – called efficiency sensing – combines existing theories and first allows an understanding of how bacterial communication works and which purpose it serves”, explains Dr. Burkhard Hense from the GSF Institute of Biomathematics and Biometry (IBB), who analysed the various strategies using mathematical models.

Microbial communication was first discovered in mixed liquid laboratory cultures, e.g. of the luminescent bacterium Vibrio fischeri, which only shows bioluminescence from a certain cell density. Therefore, the release of signal molecules was first understood as a strategy to determine the cell density (quorum sensing). With its cooperative approach, however, quorum sensing does not provide a stable survival strategy from an evolutionary point of view, because “cheaters” can also benefit from the released substances without having to pay for their production. The approach of diffusion sensing is slightly simpler: it is assumed that the bacterium uses the signal substances to measure whether the cell sourrounding space is adequate to achieve the concentration of active substances required for efficient action. This is in contrast to the quorum sensing concept, when other bacteria do not necessarily have to be involved.

In a more complex and heterogeneous environment, such as the root compartment of plants, however, both communication strategies have their weaknesses: the root surface is a highly complex matrix in which solids, gels, liquids and gases are found within a small space and where numerous other organisms interfere with the communication on top of that. Therefore, within the framework of the interdisciplinary project “Molecular Interactions in the Rhizosphere” Hense and his colleagues of the GSF-Institute of Biomathematics and Biometry (IBB) investigated this habitat in cooperation with Professor Dr. Anton Hartmann and Dr. Michael Rothballer from the GSF Department Microbe-Plant-Interaction ( AMP).

Based on experimental observations, it could be shown by mathematical modelling that the spatial distribution of the bacteria in the rhizosphere often has a stronger influence on the communication than the cell density or the dimensions of the space surrounding them. Therefore, the scientists developed a synthesis of the two models, which they named “efficiency sensing”: the microbes always perceive a mixture of cell density, cell distribution and diffusion limitation due to spatial conditions, because these aspects cannot be strictly separated – it depends on the circumstances and habitat quality which aspect is predominant. The problem of the “cheaters“ is also avoided, if the spatial distribution of the cells is taken into consideration: on root surfaces or in biofilms related organisms often form clonal micro-colonies. Since in this case all relatives are in the immediate proximity, they are also most likely to encounter the signal substances and the reactions triggered by the signal substances – strangers are largely excluded. Thus, such aggregations of closely related cells allow stable cooperation in terms of evolution and offer effective protection from external interference.

“Efficiency sensing was developed based on observations and models of the conditions on root surfaces, but it can be transferred to other bacterial habitats”, Hense emphasizes. Therefore, manipulations of the bacterial signal system are a highly promising approach in various spheres of life, e.g. in agriculture (support of plant-growth-promoting bacteria, inhibition of noxious organisms) or in medicine (fighting pathogens). A better understanding of the ecological mechanisms of bacterial signaling under natural conditions, as is possible with the “efficiency sensing” concept, is a prerequisite for this.