Modeling Quorum Sensing in Sinorhizobium meliloti

Background: For bacteria, it is important for survival to sense their population density or quorum. We use a well-known quorum sensing system of the nitrogen-fixing bacterium Sinorhizobium meliloti in order to study and model its molecular basis. The quorum sensing system depends upon the players ExpR, SinR, SinI and AHL, the signaling molecule within the system. Results: We present a minimal mathematical model for quorum sensing based on a system of differential equations. The quorum sensing system undergoes three stages. At low cell density, all produced AHL molecules quickly leave the cells. In the medium, the concentration of AHLs increases only slowly. If the number of cells grows, the concentration of AHLs in the medium increases and AHLs can be transported back into the cell where they regulate their own production, i.e. they are responsible for a positive feedback loop. Finally, if cells are even more abundant, production of AHLs is switched off. Conclusions: Although our mathematical model is only qualitative it can correctly predict several empirical observations from wild-type and mutant strains.