Virulence of the Pseudomonas fluorescens clinical strain MFN1032 towards Dictyostelium discoideum and macrophages in relation with type III secretion system

International audienceBackground: Pseudomonas fluorescens biovar I MFN1032 is a clinical isolate able to grow at 37°C. This straindisplays secretion-mediated hemolytic activity involving phospholipase C and cyclolipopeptides, and acell-associated hemolytic activity distinct from the secreted hemolytic activity. Cell-associated hemolysis isindependent of biosurfactant production and remains in a gacA mutant. Disruption of the hrpU-like operon (thebasal part of type III secretion system from rhizospheric strains) suppresses this activity. We hypothesized that thisphenotype could reflect evolution of an ancestral mechanism involved in the survival of this species in its naturalniche. In this study, we evaluated the hrpU-like operon’s contribution to other virulence mechanisms using a panelof Pseudomonas strains from various sources.Results: We found that MFN1032 inhibited the growth of the amoebae Dictyostelium discoideum and that thisinhibition involved the hrpU-like operon and was absent in a gacA mutant. MFN1032 was capable of causingmacrophage lysis, if the hrpU-like operon was intact, and this cytotoxicity remained in a gacA mutant.Cell-associated hemolytic activity and macrophage necrosis were found in other P. fluorescens clinical isolates, butnot in biocontrol P. fluorescens strains harbouring hrpU-like operon. The growth of Dictyostelium discoideum wasinhibited to a different extent by P. fluorescens strains without correlation between this inhibition and hrpU-likeoperon sequences.Conclusions: In P. fluorescens MFN1032, the basal part of type III secretion system plays a role in D. discoideumgrowth inhibition and macrophage necrosis. The inhibition of D. discoideum growth is dependent on the GacS/GacA system, while cell-associated hemolytic activity and macrophage lysis are not. Virulence against eukaryoticcells based on the hrpU-like operon may be more than just a stochastic evolution of a conserved system dedicatedto survival in competition with natural predators such as amoebae. It may also mean that there are some importantmodifications of other type III secretion system components, which remain unknown. Cell-associated hemolysismight be a good indicator of the virulence of Pseudomonas fluorescens strain