Abstract : Pseudomonas fluorescens is a ubiquitous Gram negative bacterium with a high adaptation potential. This bacterium is frequently encountered in hospitals as a contaminant of injectables, and cases of nosocomial infection have been reported. Although P. fluorescens shows most of the feature of an opportunistic pathogen, the scientific background concerning the real potential of this species is very limited.
In the present study we took advantage of the recent identification of a clinical strain of P. fluorescens biovar I, MFN1032, considered as the causative germ of human lung infection, to investigate the secreted factors involved in the virulence of the bacterium.
First, the hemolytic power of MFN1032 was studied in vitro, with the aim to identify the factors responsible for this activity. We show that a phospholipase C, PlcC, is involved in the hemolytic activity of this bacterium, and sequences comparison reveals a phospholipase C belonging to a new group of PLC. Analysis of the results strongly suggests that PlcC is closely related to the production of biosurfactants, by a transcriptional regulator belonging to the GntR family. Moreover, biosurfactants also appear to be involved in the hemolytic activity.
Furthermore, the adaptation potential and the virulence of the strain are enhanced by phenotypic variation. Analysis of the phenotype of several variants suggests that the expression of hemolysis by strain MFN1032 is a phase variable trait.
On a molecular point of view, the complementation by the gacA or gacS genes restored wild phenotype of some variants. Surprisingly, an overexpression of gacA/gacS genes in trans decreased the total number of variants, although the gac complementation did not restore wild phenotype of hyperadherents strains.
The GacA/GacS system plays a key role in phenotypic variation in MFN1032, but the results suggested that PlcC and the transcriptional regulator GntR could also participate to this process.