Figure 1.

Summary of the intrinsic and extrinsic factors that modulate QS signalling in microbial cells. The intracellular modulators include the dynamics of signal detection and the activity of the positive feedback loop that characterises signal production, and this can vary between species or strains within a species. Metabolic state, and the effects of metabolism of autoinducer (AI) production, can also impact AI production. AI mimics that inhibit AI detection have been described. In bacteria, other environment-responsive regulatory elements, such as transcription factors and small RNAs, can positively or negatively tune the QS circuit. The extracellular factors that modulate QS signalling include physical and chemical elements that impact the stability of secreted AI molecules and the action of enzymes that can degrade AIs. In addition, the population size and factors that influence the diffusion and availability of the AI will influence the QS circuit.