The study by Pesci et al. (1999) reports the discovery of a novel cell-to-cell signaling molecule in *Pseudomonas aeruginosa*, designated as the Pseudomonas quinolone signal (PQS). This molecule, 2-heptyl-3-hydroxy-4-quinolone, is distinct from the previously known acyl-homoserine lactones (AHLs) used by *P. aeruginosa* for quorum sensing. PQS was found to control the expression of the major virulence factor, LasB elastase, by activating the transcriptional activator protein LasR. The synthesis and bioactivity of PQS were shown to be mediated by the las and rhl quorum sensing systems, respectively. The presence of PQS in wild-type *P. aeruginosa* culture media was demonstrated to induce the expression of *lasB* in a *P. aeruginosa* lasR mutant, indicating that PQS is an additional intercellular signal involved in virulence gene regulation. The study also highlights the complexity of the quorum sensing hierarchy in *P. aeruginosa*, where PQS acts as a secondary signal that may interact with other signals like 3-oxo-C12-HSL and C4-HSL. The purification and identification of PQS, along with its synthetic preparation, confirmed its role in cell-to-cell communication. The findings suggest that 4-quinolones, known for their antibiotic activity, may also play a role in bacterial signaling and could be potential targets for antibacterial therapies.The study by Pesci et al. (1999) reports the discovery of a novel cell-to-cell signaling molecule in *Pseudomonas aeruginosa*, designated as the Pseudomonas quinolone signal (PQS). This molecule, 2-heptyl-3-hydroxy-4-quinolone, is distinct from the previously known acyl-homoserine lactones (AHLs) used by *P. aeruginosa* for quorum sensing. PQS was found to control the expression of the major virulence factor, LasB elastase, by activating the transcriptional activator protein LasR. The synthesis and bioactivity of PQS were shown to be mediated by the las and rhl quorum sensing systems, respectively. The presence of PQS in wild-type *P. aeruginosa* culture media was demonstrated to induce the expression of *lasB* in a *P. aeruginosa* lasR mutant, indicating that PQS is an additional intercellular signal involved in virulence gene regulation. The study also highlights the complexity of the quorum sensing hierarchy in *P. aeruginosa*, where PQS acts as a secondary signal that may interact with other signals like 3-oxo-C12-HSL and C4-HSL. The purification and identification of PQS, along with its synthetic preparation, confirmed its role in cell-to-cell communication. The findings suggest that 4-quinolones, known for their antibiotic activity, may also play a role in bacterial signaling and could be potential targets for antibacterial therapies.