Biology Reference
In-Depth Information
of Aliivibrio salmonicida has been sequenced and the quorum sensing system predicted [Hjerde et al. ,
2008].
An abstraction of the quorum sensing model would be comparable to an 'on' and 'off' system under
the influence of molecular noise [Goryachev et al. , 2006]. Bacteria that use quorum sensing constantly
produce small, freely diffusible signaling molecules (also called autoinducers or pheromones). The
detection of the autoinducer works on the basis of specific receptors which recognize external signaling
molecules. The likelihood of a bacterium detecting its own signaling molecules is very low and can be
dismissed. When the inducer binds to the receptor, the transcription of certain genes will be activated.
Among those genes, the synthesis of the autoinducer will be stimulated. At low density, when the
bacteria exist only as a few individuals, diffusion of the autoinducer into the environment reduces the
concentration to almost zero. Until a certain threshold concentration of the autoinducer is achieved in
the environment, the intracellular network remains in the 'off' state. This is a kind of a stand-by level
of autoinducer production. When the population grows and cell density increases, the concentration
of the inducer increases and will pass a certain threshold level. When this threshold level is achieved
the incoming signal will activate the quorum sensing network through intracellular and extracellular
receptors which turns on the expression of the phenotype specific genes. In many cases the activation
often strongly increases the production of autoinducer. The intracellular network then switches into the
'on' state.
This phenomenon is a so-called positive feedback loop. The network becomes fully activated and
activates phenotype specific genes. The intracellular circuitry of the cell will change in order to initiate
processes such as motility, biofilm formation, virulence factor production and secretion or other processes
to achieve advantages for adaptation to the environment and survival.
In general, vibrios have the ability to perform quorum sensing in a similar fashion [M uller et al. , 2006].
Communication via LuxI/LuxR-type signaling circuits appears to be the standard mechanism by which
many gram-negative bacteria communicate. It was in V. fischeri that the quorum sensing transcriptional
regulators LuxR and LuxI (the autoinducer synthase) were first discovered [Antunes et al. , 2007]. The
LuxI-produced signaling molecule, an acyl homoserine lactone (acyl-HSL), diffuses out of the cell, and
under conditions of high cell density, returns back into the cell. Within the cell it activates LuxR and
thus the lux bioluminescence operon, which are under LuxR control [Visick et al. , 2005].
Studies of quorum sensing in Aliivibrio salmonicida subsequently revealed a much more complex
system for the LuxI/LuxR-type pathway, including two LuxR transcriptional regulators. Figure 1
demonstrates a proposed quorum sensing network of Aliivibrio salmonicida under low and high popula-
tion density which is based on experimental data [Hjerde et al. , 2008]. It is fascinating to the scientific
community how the proposed network appears and how the intracellular circuitry of signal transduction
and gene expression adapts to environmental perturbations. There are still many unanswered questions
and unexplored directions in this circuitry. The aim is to answer those questions and to expand the
relevant paths in the network by predicted information, quantitative experimental data and literature to
construct a sophisticated cell-to-cell-communication model.
Petri nets
A Petri net is a mathematical modeling language with an exact mathematical definition of its execution
semantics. Petri nets offer a graphical notation for stepwise processes that include choice, iteration, and
concurrent execution. The advantage of Petri nets is that they provide a balance between modeling power
and analyzability. Using Petri nets, complex systems can be modeled and simulated in a sophisticated
Search WWH ::




Custom Search