ABSTRACT Successful colonization of a multitude of ecological niches by the bacterium Pseudomonas aeruginosa relies on its ability to respond to concentrations of self-produced signal molecules. This intercellular communication system known as quorum sensing (QS) tightly regulates the expression of virulence determinants and a diversity of survival functions, including those required for social behaviours. In planktonic cultures of P. aeruginosa , the transcriptional regulator LasR is generally considered on top of the QS circuitry hierarchy; its activation relies on binding to 3-oxo-C 12 -homoserine lactone (3-oxo-C 12 -HSL), a product of the LasI synthase. Transcription of lasI is activated by LasR, resulting in a positive feedback loop. Few studies have looked at the function of QS during surface growth even though P. aeruginosa typically lives in biofilm-like communities under natural conditions. Here, we show that surface-grown P. aeruginosa readily produces 3-oxo-C 12 -HSL in absence of LasR, and that this phenotype is frequent upon surface association in naturally occurring environmental and clinical LasR- defective isolates, suggesting a conserved alternative function for the signal. Indeed, even in the absence of the cognate regulator LasR, 3-oxo-C 12 -HSL upregulates the autologous expression of pyocyanin and of LasR-controlled virulence determinants in neighboring cells. This highlights a possible role for 3-oxo-C 12 -HSL in shaping community responses and provides a possible evolutive benefit for mixed populations to carry LasR-defective cells, a common feature of natural of P. aeruginosa . IMPORTANCE The bacterium Pseudomonas aeruginosa colonizes and thrives in many environments, in which it is typically found in surface-associated polymicrobial communities known as biofilms. Adaptation to this social behavior is aided by quorum sensing (QS), an intercellular communication system pivotal in the expression of social traits. Regardless of its importance in QS regulation, the loss of function of the master regulator LasR is now considered a conserved adaptation of P. aeruginosa , irrespective of the origin of strains. By investigating the QS circuitry in surface-grown cells, we found accumulation of QS signal 3-oxo-C 12 -HSL in absence of its cognate receptor and activator, LasR. The current understanding of the QS circuit, mostly based on planktonic growing cells, is challenged by investigating the QS circuitry of surface-grown cells. This provides a new perspective on the beneficial aspects that underline the frequency of LasR-deficient isolates.

Surface growth of<i>Pseudomonas aeruginosa</i>reveals a regulatory effect of 3- oxo-C<sub>12</sub>-homoserine lactone in absence of its cognate receptor, LasR

Surface growth ofPseudomonas aeruginosareveals a regulatory effect of 3- oxo-C12-homoserine lactone in absence of its cognate receptor, LasR