OprF is a general outer membrane porin of Pseudomonas aeruginosa, a well-known human opportunistic pathogen associated with severe hospital-acquired sepsis and chronic lung infections of cystic fibrosis patients. A multiphenotypic approach, based on the comparative study of a wild-type strain of P. aeruginosa, its isogenic oprF mutant, and an oprF-complemented strain, showed that OprF is required for P. aeruginosa virulence. The absence of OprF results in impaired adhesion to animal cells, secretion of ExoT and ExoS toxins through the type III secretion system (T3SS), and production of the quorum-sensing-dependent virulence factors pyocyanin, elastase, lectin PA-1L, and exotoxin A. Accordingly, in the oprF mutant, production of the signal molecules N-(3-oxododecanoyl)-L-homoserine lactone and N-butanoyl-L-homoserine lactone was found to be reduced and delayed, respectively. Pseudomonas quinolone signal (PQS) production was decreased, while its precursor, 4-hydroxy-2-heptylquinoline (HHQ), accumulated in the cells. Taken together, these results show the involvement of OprF in P. aeruginosa virulence, at least partly through modulation of the quorum-sensing network. This is the first study showing a link between OprF, PQS synthesis, T3SS, and virulence factor production, providing novel insights into virulence expression.

SummaryThe Burkholderia cepacia complex (Bcc) is composed of 17 closely related species. These bacteria are widely but heterogeneously distributed in the natural environment, such as soil, water and rhizosphere. Bcc strains are able to colonize various ecological niches by adopting versatile lifestyles, including saprophytism and (positive or deleterious) association with eukaryotic cells. Bcc strains have proven to be very efficient in biocontrol, plant growth promotion and bioremediation. However, they also are important opportunistic pathogens that can cause severe respiratory infections among individuals suffering from cystic fibrosis or chronic granulomatous disease. Therefore, considering that the distinction between plant beneficial and clinical strains is not obvious, biotechnological applications of Bcc strains are currently not allowed. This minireview provides an overview of the wide range of lifestyles that Bcc bacteria can adopt, leading to glimpses into their tremendous adaptation potential and highlighting remaining questions concerning potential implicated mechanisms.

There is some debate about the potential survival of Pseudomonas fluorescens at temperatures above 37 degrees C and its consequences for infectious potential, owing to the heterogeneity of clinical strains. Seven clinical strains growing at 37 degrees C or more were submitted for polyphasic identification; 2 were identified as Pseudomonas mosselii and 4 were precisely characterized as P. fluorescens bv. I or II. The binding indexes on glial cells of the strains identified as P. fluorescens bv. I and P. mosselii were compared with that of a reference psychrotrophic strain, P. fluorescens MF37 (bv. V). Clinical P. fluorescens had a similar adherence potential range than strain MF37. Conversely, the binding indexes for P. mosselii strains were 3 times greater than that for strain MF37. These data, and those obtained by comparing the cytotoxic activities of P. fluorescens clinical strains, suggest the existence of different virulence mechanisms, leading either to a low infectious form or to a microorganism with cytotoxic activity in the same range as that of P. mosselii or even Pseudomonas aeruginosa.