The Rhl Quorum-Sensing System Is at the Top of the Regulatory Hierarchy under Phosphate-Limiting Conditions in Pseudomonas aeruginosa PAO1.

is a major nosocomial pathogen that presents high-level resistance to antibiotics. Its ability to cause infections relies on the production of multiple virulence factors. Quorum sensing (QS) regulates the expression of many of these virulence factors through three QS systems: Las, Rhl, and PQS. The Las system positively regulates the other two systems, so it is at the top of a hierarchized regulation. Nevertheless, clinical and environmental strains that lack a functional Las system have been isolated, and, surprisingly, some of them still have the ability to produce virulence factors and infect animal models, so it has been suggested that the hierarchy is flexible under some conditions or with atypical strains. Here, we analyze the PAO1 type strain and its Δ-derived mutant and report, for the first time, a growth condition (phosphate limitation) where LasR absence has no effect either on virulence factor production or on the gene expression profile, in contrast to a condition of phosphate repletion where the LasR hierarchy is maintained. This work provides evidence on how the QS hierarchy can change from being a strictly LasR-dependent to a LasR-independent RhlR-based hierarchy under phosphate limitation even in the PAO1 type strain. is an important pathogen, considered a priority for the development of new therapeutic strategies. An important approach to fight its infections relies on blocking quorum sensing. The Las system is the main regulator of the quorum-sensing response, so many research efforts aim to block this system to suppress the entire response. In this work, we show that LasR is dispensable in a phosphate-limited environment in the PAO1 type strain, which has been used to define the quorum-sensing response hierarchy, and that under this condition RhlR is at the top of the regulation hierarchy. These results are highly significant, since phosphate limitation represents a similar environment to the one that faces when establishing infections.