Figure 2: Quorum sensing in V. harveyi. There are three parallel quorum sensing systems, including AI-1/LuxM, AI-2/ LuxS, and CAI-1/CqsA. At low cell populations, no signal molecules are detected. While at a high cell population, these three AIs are produced and accumulated to reach a critical concentration to be sensed by their receptors. All the three signal transfer pathways are achieved by a series of hierarchical phosphorylation and dephosphorylation by the same components.