As well as demonstrating enormous environmental versatility, Pseudomonas aeruginosa is an important source of Gram-negative nosocomial infections and a major cause of death and morbidity in cystic fibrosis patients. Key to the success of this microorganism is the complexity of regulation that controls crucial processes that contribute to its physiological adaptability. A recent report in Microbiology adds another level of understanding to the regulation of one such process — quorum sensing.

Evidence has amassed that quorum sensing, which allows bacteria to detect the density of their own species, is crucially important to the pathogenicity of P. aeruginosa. Two quorum-sensing systems, which rely on the N-acylhomoserine lactone (AHL) signal, have been identified in this microorganism: the las system and the rhl system. These two systems do not operate independently and are hierarchically arranged with the las system at the top of the signalling cascade. The quorum-sensing circuitry is subject to regulation by a number of different regulators, including Vfr, RsaL, GacA and DksA. Juhas and co-workers have now identified a new regulator, VqsR (virulence and quorum-sensing regulator), that has an important role in the quorum-sensing cascade of P. aeruginosa. When the vqsR gene was inactivated, the production and secretion of AHLs was abolished and the subsequent production of virulence factors was also severely compromised. To investigate the importance of VqsR to P. aeruginosa virulence, the mutant strain was tested in a nematode infection model system and was shown to be significantly attenuated. Oligonucleotide microarrays were then used to investigate the effect of VqsR on global changes in the gene-expression profile of the microorganism, both in the presence of human serum and under oxidative stress. Disruption of this regulator activated the expression of genes that are known to be repressed by quorum sensing and repressed the expression of genes previously shown to be promoted by this process. Furthermore, this analysis also demonstrated that VqsR is involved in the regulation of siderophores and membrane-bound elements of antibiotic resistance.

In conclusion, Juhas et al. have identified a new regulator of P. aeruginosa cell-to-cell communication and virulence, thereby adding a new layer to the regulatory complexity that underpins the extensive environmental adaptability of this unique organism.