Pseudomonas aeruginosa is a gram-negative bacterium that causes serious infections in immunocompromised individuals and cystic fibrosis patients. This opportunistic pathogen controls many of its virulence factors and cellular functions through the activity of three cell-to-cell signals, N-(3-oxododecanoyl)-L-homoserine lactone, N-butyryl-L-homoserine lactone, and the Pseudomonas quinolone signal (PQS). The activity of these signals is dependent upon their ability to dissolve in and freely diffuse through the aqueous solution in which P. aeruginosa happens to reside. Despite this, our data indicated that PQS was relatively insoluble in aqueous solutions, which led us to postulate that P. aeruginosa could be producing a PQS-solubilizing factor. In this report, we show that the P. aeruginosa-produced biosurfactant rhamnolipid greatly enhances the solubility of PQS in aqueous solutions. The enhanced solubility of PQS led to an increase in PQS bioactivity, as measured by both a gene induction assay and an apoptosis assay. This is the first demonstration of the importance of a bacterial surfactant in the solubilization and bioactivity of a cell-to-cell signal.Pseudomonas aeruginosa is an environmental microbe that can cause serious infections when introduced to immunocompromised individuals or those suffering from cystic fibrosis. This opportunistic pathogen uses multiple cell-to-cell signals to communicate with its siblings and thereby control a host of bacterial functions, including virulence (33). One of these signals, the Pseudomonas quinolone signal (PQS), is a unique cell-to-cell signal that was identified as 2-heptyl-3-hydroxy-4-quinolone (34). Although PQS is different from the acyl-homoserine lactone signals produced by P. aeruginosa, it is still a part of the quorum-sensing signaling cascade (23).The production of PQS is positively controlled by N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C 12 -HSL) and the las quorum-sensing system, while its bioactivity depends on and can activate the rhl quorum-sensing system (23, 34). Synthesis of PQS requires at least seven genes (6, 12), some of which are regulated in a complex manner. LasR and PqsR positively control the induction of the pqsABCDE operon, which is part of the PQS synthetic gene cluster, while RhlR appears to repress this operon (21). PQS production is also unusual in that it begins during the logarithmic phase of growth (9, 19), but unlike 3-oxo-C 12 -HSL and N-butyryl-Lhomoserine lactone (C 4 -HSL), it is not produced maximally until late in the stationary phase of growth (23).Why P. aeruginosa produces PQS is still unknown, but this signal has been shown to control multiple virulence factors (4,9,12,23,34) and is required for virulence in nematodes, plants, and mice (11,12,35). Most interestingly, PQS is produced in the lungs of cystic fibrosis patients infected with P. aeruginosa (5), which implies that the signal is important for adaptation to the lung environment.The ability of PQS to function as an intercellular signal in culture media and pr...
