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: Manatees could lose their endangered species status

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

( —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

( —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 ...

New optical sensors swell when exposed to target gas

Jun 17, 2014

Using microscopic polymer light resonators that expand in the presence of specific gases, researchers at MIT's Quantum Photonics Laboratory have developed new optical sensors with predicted detection levels ...

Recommended for you

France fights back Asian hornet invader

41 minutes ago

They slipped into southwest France 10 years ago in a pottery shipment from China and have since invaded more than half the country, which is fighting back with drones, poisoned rods and even chickens.

Tide turns for shark fin in China

51 minutes ago

A sprawling market floor in Guangzhou was once a prime location for shark fin, one of China's most expensive delicacies. But now it lies deserted, thanks to a ban from official banquet tables and a celebrity-driven ...

New research reveals clock ticking for fruit flies

1 hour ago

The army of pesky Queensland fruit flies that annually inflict many millions of dollars-worth of damage on the nation's horticultural industry may be about to see their numbers take a significant dive thanks ...

The ABC's of animal speech: Not so random after all

3 hours ago

The calls of many animals, from whales to wolves, might contain more language-like structure than previously thought, according to study that raises new questions about the evolutionary origins of human language.

Manatees could lose their endangered species status

13 hours ago

About 2,500 manatees have perished in Florida over the last four years, heightening tension between conservationists and property owners as federal officials prepare to decide whether to down-list the creature to threatened ...

User comments : 0