Date Published: June 23, 2017
Publisher: Public Library of Science
Author(s): Mikkel Lindstrøm Sørensen, Peter Dahl, Thomas Sams, Szabolcs Semsey.
Bacteria in a biofilm colony have the capacity to monitor the size and growth conditions for the colony and modify their phenotypical behaviour to optimise attacks, defence, migration, etc. The quorum sensing systems controlling this involve production and sensing of diffusive signal molecules. Frequently, quorum sensing systems carry a positive feedback loop which produces a switch at a threshold size of the colony. This all-or-none switch can be beneficial to create a sudden attack, leaving a host little time to establish a defence. The reaction-diffusion system describing a basal quorum sensing loop involves production of signal molecules, diffusion of signal molecules, and detection of signal molecules. We study the ignition process in a numerical solution for a basal quorum sensor and demonstrate that even in a large colony the ignition travels through the whole colony in a less than a minute. The ignition of the positive feedback loop was examined in different approximations. As expected, in the exact calculation the ignition was found to be delayed compared to a calculation where the binding of signal molecules was quasistatic. The buffering of signal molecules is found to have little effect on the ignition process. Contrary to expectation, we find that the ignition does not start when the threshold is reached at the center—instead it allows for the threshold to be approached in the whole colony followed by an almost simultaneous ignition of the whole biofilm aggregate.
Quorum sensing (QS) is a biological regulation process utilised by bacteria to control behaviour in accordance with size, density, and growth-rate of a bacterial population . The process is based on diffusible signal molecules, produced by the bacteria at a background level. The signal molecules are able to bind to regulator molecules within the bacteria, thereby activating the regulator .
The quorum sensing system considered in this article, is based on the reaction-diffusion model proposed by Ferkinghoff-Borg and coworkers [15, 16, 25]. This model examines a dimer based regulator system with concentrations dependent on both spatial coordinates and time in a spherically symmetric geometry. These reaction-diffusion equations are obtained by considering the reactions that occur, when the unactivated regulator molecules dimerize and are subsequently activated by ligand binding as illustrated in Fig 1.
In Fig 2, the activated regulator concentration at the center of the colony, ra, is displayed as a function of the size, Σ. The curves represent the full model with and without buffering (broken lines) compared with the static and quasi-static solutions (full lines).
We have modelled a generic single-loop quorum sensing system with positive feedback. The primary goal of the study has been to study of the space-time structure of the ignition of the switch produced by the positive feedback in the quorum sensor.