Surface growth of Pseudomonas aeruginosa reveals a regulatory effect of 3-oxo-C12-homoserine lactone in the absence of its cognate receptor, LasR

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 behaviors. 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-C12-homoserine lactone (3-oxo-C12-HSL), a product of 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, including prototypical strain PA14, produces 3-oxo-C12-HSL in the absence of LasR. This phenotype is commonly observed upon surface association in naturally occurring environmental and clinical LasR-defective isolates, suggesting a conserved alternative function for the signal. Notably, in surface-grown cultures, 3-oxo-C12-HSL reaches higher levels than planktonic cells, and there is a delayed timing of its production. Accordingly, 3-oxo-C12-HSL upregulates the autologous expression of pyocyanin and LasR-controlled virulence determinants in neighboring cells even in the absence of the cognate regulator LasR. This highlights a possible role for 3-oxo-C12-HSL in shaping community responses and provides a possible evolutive benefit for mixed populations to carry LasR-defective cells, a common feature of natural populations 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 the strains. By investigating the QS circuitry in surface-grown cells, we found an accumulation of QS signal 3-oxo-C12-HSL in the 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