|Abstract:Pseudomonas aeruginosa is a ubiquitous environmental bacterium responsible for a variety of infections in humans, as well as in animal hosts. While the evolution of virulence in P. aeruginosa strains isolated from chronic lung infection in cystic fibrosis (CF) patients has been extensively studied, the virulence phenotype of P. aeruginosa isolated from other infection types or from the environment is currently not well characterized. Here we report an extensive analysis of the virulence of P. aeruginosa strains isolated from acute infections and compared it to population structure. Virulence profiles of individual strains were also compared to the expression levels of the rhlR gene, the transcriptional regulator of the rhl quorum-sensing system, and the gene encoding Crc, a global regulator controlling catabolite repression and carbon metabolism. Additionally, the presence/absence of the two mutually exclusive genes exoU and exoS encoding effectors of the type III secretion system was also assessed. In order to capture the widest range of genetic variability, a collection of 120 clinical strains was initially characterized by repetitive-element-based PCR genotyping, and a selection of 27 strains belonging to different clonal lineages was subsequently tested using three different virulence assays, including two D. discoideum assays on different growth media, and a C. elegans fast-killing assay. We show that the parallel application of virulence assays can be used to quantitatively assess this complex, multifactorial phenotypic trait. We observed a wide spectrum of virulence phenotypes ranging from weakly- to highly-aggressive, indicating that clinical strains isolated from acute infections can present a reduced or altered virulence phenotype. Genotypic associations only partially correlated with virulence profiles and virulence gene expression, whereas the presence of either exoU or exoS was not significantly correlated with virulence. Interestingly, the expression of rhlR showed a significant and positive correlation with the virulence profiles obtained with the three assays, while the expression of crc was either negatively or not correlated with virulence, depending on the assay.