Signal processing in bacterial quorum sensing

Cell-to-cell communication in bacteria is a process known as quorum sensing (QS) that relies on the production, detection, and response to the extracellular accumulation of signaling molecules called autoinducers (AIs). The signal processing circuit in the model quorum-sensing bacterium Vibrio harveyi is exquisitely designed which involves integration of the information from multiple autoinducers and complicated feedback regulations of the signaling components. Applying quantitative methods and physical reasoning, we investigated how cells integrate and interpret the information contained within multiple autoinducers. Using single-cell fluorescence microscopy, we quantified the signaling responses to and analyzed the integration of multiple autoinducers in the V. harveyi cells. Our results revealed that signals from two distinct autoinducers, AI-1 and AI-2, are combined strictly additively in a shared phosphorelay pathway, with each autoinducer contributing very nearly equally to the total response. We found a coherent response across the population with little cell-to-cell variation, indicating that the entire population of cells can reliably distinguish several distinct conditions of external autoinducer concentration. We speculate that the use of multiple autoinducers allows a growing population of cells to synchronize gene expression during a series of distinct developmental stages. We also identified and characterized two negative feedback loops that act to facilitate precise quorum-sensing signaling. The quorum-sensing central response regulator LuxO autorepresses its own transcription and the Qrr small regulatory RNAs (sRNAs) posttranscriptionally repress luxO by base-pairing with the luxO mRNA transcript. We discovered that the two cooperative negative feedback loops determine the point at which V. harveyi has reached a quorum and control the range of autoinducers over which the quorum-sensing responses occur. Our findings suggest that sRNA-mediated feedback regulation is a common design feature that permits fine-tuning of gene regulation and maintenance of homeostasis.