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: Insect mating behavior has lessons for drones

Related Stories

Charter nears deal for Time Warner Cable

54 minutes ago

Charter Communications Inc. is close to buying Time Warner Cable for about $55 billion, according to two people familiar with the negotiations.

The monopoly of aluminium is broken

2 hours ago

Discovering Majorana's was only the first step, but utilizing it as a quantum bit (qubit) still remains a major challenge. An important step towards this goal has just been taken, as shown by researchers ...

Yik Yak's frat-bro founders shrug off growing pains

2 hours ago

The most popular post of all time on Yik Yak is a dirty joke. Less than 2 years old, the Atlanta-based social network is geared mostly toward college students who access and post unsigned announcements through an app on their ...

Fears for pink iguanas as Galapagos volcano erupts

3 hours ago

A volcano in the Galapagos islands erupted for the first time in more than 30 years Monday, sending streams of lava flowing down its slopes and potentially threatening the world's only colony of pink iguanas.

Recommended for you

Insect mating behavior has lessons for drones

11 hours ago

Male moths locate females by navigating along the latter's pheromone (odor) plume, often flying hundreds of meters to do so. Two strategies are involved to accomplish this: males must find the outer envelope ...

Bacterial tenants in fungal quarters

21 hours ago

Ludwig Maximilian University of Munich researchers have sequenced the genome of a bacterial symbiont hosted by a mycorrhizal fungus. Analysis of the symbiont's genetic endowment reveals previously unknown ...

Natural enzyme examined as antibiotics alternative

May 29, 2015

In 1921, Alexander Fleming discovered the antimicrobial powers of the enzyme lysozyme after observing diminished bacterial growth in a Petri dish where a drop from his runny nose had fallen. The famed Scottish ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.