Many bacteria use extracellular signals to communicate and coordinate social activities, a process referred to as quorum sensing. Many quorum signals have significant hydrophobic character, and how these signals are trafficked between bacteria within a population is not understood. Here we show that the opportunistic human pathogen Pseudomonas aeruginosa packages the signalling molecule 2-heptyl-3-hydroxy-4-quinolone (pseudomonas quinolone signal; PQS) into membrane vesicles that serve to traffic this molecule within a population. Removal of these vesicles from the bacterial population halts cell-cell communication and inhibits PQS-controlled group behaviour. We also show that PQS actively mediates its own packaging and the packaging of other antimicrobial quinolines produced by P. aeruginosa into vesicles. These findings illustrate that a prokaryote possesses a signal trafficking system with features common to those used by higher organisms and outlines a novel mechanism for delivery of a signal critical for coordinating group behaviour in P. aeruginosa.

The opportunistic human pathogen Pseudomonas aeruginosa causes persistent airway infections in patients with cystic fibrosis (CF). To establish these chronic infections, P. aeruginosa must grow and proliferate within the highly viscous sputum in the lungs of CF patients. In this study, we used Affymetrix GeneChip microarrays to investigate the physiology of P. aeruginosa grown using CF sputum as the sole source of carbon and energy. Our results indicate that CF sputum readily supports high-density P. aeruginosa growth. Furthermore, multiple signals, which reduce swimming motility and prematurely activate the Pseudomonas quinolone signal cell-to-cell signaling cascade in P. aeruginosa, are present in CF sputum. P. aeruginosa factors critical for lysis of the common CF lung inhabitant Staphylococcus aureus were also induced in CF sputum and increased the competitiveness of P. aeruginosa during polymicrobial growth in CF sputum.

Pseudomonas aeruginosa is a gram-negative opportunistic human pathogen often infecting the lungs of individuals with the heritable disease cystic fibrosis and the peritoneum of individuals undergoing continuous ambulatory peritoneal dialysis. Often these infections are not caused by colonization with P. aeruginosa alone but instead by a consortium of pathogenic bacteria. Little is known about growth and persistence of P. aeruginosa in vivo, and less is known about the impact of coinfecting bacteria on P. aeruginosa pathogenesis and physiology. In this study, a rat dialysis membrane peritoneal model was used to evaluate the in vivo transcriptome of P. aeruginosa in monoculture and in coculture with Staphylococcus aureus. Monoculture results indicate that approximately 5% of all P. aeruginosa genes are differentially regulated during growth in vivo compared to in vitro controls. Included in this analysis are genes important for iron acquisition and growth in low-oxygen environments. The presence of S. aureus caused decreased transcription of P. aeruginosa ironregulated genes during in vivo coculture, indicating that the presence of S. aureus increases usable iron for P. aeruginosa in this environment. We propose a model where P. aeruginosa lyses S. aureus and uses released iron for growth in low-iron environments.Pseudomonas aeruginosa is a gram-negative opportunistic human pathogen commonly found in water and soil. P. aeruginosa causes a number of chronic and acute infections and is noted for its inherent resistance to many clinically relevant antibiotics. Two of the most common infections caused by P. aeruginosa are chronic colonization of the lungs of individuals with the genetic disease cystic fibrosis (CF) (19) and peritonitis in individuals undergoing continuous ambulatory peritoneal dialysis (CAPD) (25). The lungs of CF patients are commonly colonized before the age of 8, and most individuals maintain these infections throughout their lifetimes. High infection rates are also associated with CAPD, which is often used to treat end-stage renal disease.P. aeruginosa physiology and gene expression during in vivo growth is largely unknown. Using in vivo expression technology (IVET), Wang et al. identified 19 P. aeruginosa genes inducible during growth in a neutropenic mouse (40). Although that study identified several new genes important for virulence in P. aeruginosa, it did not provide a comprehensive analysis of in vivo gene expression. Two recent studies using Pasteurella multocida and Borrelia burgdorferi have provided a more comprehensive view of in vivo bacterial gene expression by using DNA microarrays (3, 4, 33). These studies illustrate that a significant number of genes (2 to 8% of all the genes in the genomes) are differentially regulated in vivo, suggesting that the in vivo environment is distinct from normal in vitro culture conditions.Although evaluation of the transcriptomes of in vivo-grown bacteria provides a snapshot of transcription under monoculture growth conditions, it is clear that many in...