The Complex Quorum Sensing Circuitry of Is Both Hierarchically and Homeostatically Organized.

The genome of the bacterium encodes three complete LuxI/LuxR-type quorum sensing (QS) systems: BtaI1/BtaR1 (QS-1), BtaI2/BtaR2 (QS-2), and BtaI3/BtaR3 (QS-3). The LuxR-type transcriptional regulators BtaR1, BtaR2, and BtaR3 modulate the expression of target genes in association with various -acyl-l-homoserine lactones (AHLs) as signaling molecules produced by the LuxI-type synthases BtaI1, BtaI2, and BtaI3. We have systematically dissected the complex QS circuitry of strain E264. Direct quantification of -octanoyl-homoserine lactone (C-HSL), -3-hydroxy-decanoyl-homoserine lactone (3OHC-HSL), and -3-hydroxy-octanoyl-homoserine lactone (3OHC-HSL), the primary AHLs produced by this bacterium, was performed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in the wild-type strain and in QS deletion mutants. This was compared to the transcription of , , and using chromosomal mini-CTX- transcriptional reporters. Furthermore, the levels of expression of , , and were monitored by quantitative reverse transcription-PCR (qRT-PCR). We observed that C-HSL, 3OHC-HSL, and 3OHC-HSL are differentially produced over time during bacterial growth and correlate with the , , and gene expression profiles, revealing a successive activation of the corresponding QS systems. Moreover, the transcription of the , , and genes is modulated by cognate and noncognate AHLs, showing that their regulation depends on themselves and on other QS systems. We conclude that the three QS systems in are interdependent, suggesting that they cooperate dynamically and function in a concerted manner in modulating the expression of QS target genes through a successive regulatory network. Quorum sensing (QS) is a widespread bacterial communication system coordinating the expression of specific genes in a cell density-dependent manner and allowing bacteria to synchronize their activities and to function as multicellular communities. QS plays a crucial role in bacterial pathogenicity by regulating the expression of a wide spectrum of virulence/survival factors and is essential to environmental adaptation. The results presented here demonstrate that the multiple QS systems coexisting in the bacterium , which is considered the avirulent version of the human pathogen and thus commonly used as an alternative study model, are hierarchically and homeostatically organized. We found these QS systems to be finely integrated into a complex regulatory network, including transcriptional and posttranscriptional interactions, and further incorporating growth stages and temporal expression. These results provide a unique, comprehensive illustration of a sophisticated QS network and will contribute to a better comprehension of the regulatory mechanisms that can be involved in the expression of QS-controlled genes, in particular those associated with the establishment of host-pathogen interactions and acclimatization to the environment.