Counting heads or measuring space?

Apr 02, 2007

Bacteria can “talk” to each other: by using signal substances they inform their neighbours as to whether or not it is worth switching certain genes on or off. This communication between bacterial cells is essential for the adaptation to changing environments and for the survival. 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.

Citation: "Opinion: Does efficiency sensing unify diffusion and quorum sensing?" Burkhard A. Hense, Christina Kuttler, Johannes Müller, Michael Rothballer, Anton Hartmann and Jan-Ulrich Kreft; Nature Reviews Microbiology 5, 230-239 (March 2007), doi:10.1038/nrmicro1600

Source: National Research Center for Environment and Health

Explore further: 'Office life' of bacteria may be their weak spot

add to favorites email to friend print save as pdf

Related Stories

Smart paint signals when equipment is too hot to handle

Jul 04, 2014

(Phys.org) —NJIT researchers have developed a paint for use in coatings and packaging that changes color when exposed to high temperatures, delivering a visual warning to people handling material or equipment ...

Managing the data jungle

Jul 01, 2014

Many biology labs fight with a glut of measurement data. New software aims to make this a thing of the past: it simplifies laboratory experiment evaluation and unifies how data is saved. It even identifies ...

Odor code for food is based on a few volatile substances

Jun 25, 2014

(Phys.org) —The actual flavor of a food is experienced through our sense of smell rather than with our tongue. However, of the large number of volatile compounds in foods, only about 230 are involved in ...

Recommended for you

Transparent larvae hide opaque eyes behind reflections

3 hours ago

Becoming invisible is probably the ultimate form of camouflage: you don't just blend in, the background shows through you. And this strategy is not as uncommon as you might think. Kathryn Feller, from the University of Maryland ...

Peacock's train is not such a drag

4 hours ago

The magnificent plumage of the peacock may not be quite the sacrifice to love that it appears to be, University of Leeds researchers have discovered.

Iberian pig genome remains unchanged after five centuries

10 hours ago

A team of Spanish researchers have obtained the first partial genome sequence of an ancient pig. Extracted from a sixteenth century pig found at the site of the Montsoriu Castle in Girona, the data obtained indicates that ...

User comments : 0