Natural release of virulence factors in membrane vesicles by Pseudomonas aeruginosa and the effect of aminoglycoside antibiotics on their release.

Pseudomonas aeruginosa (and various other gram-negative pathogens) liberate membrane vesicles during normal growth. These bilayered vesicles consist of endotoxin (lipopolysaccharide), outer membrane proteins and several potent hydrolytic enzymes including protease, alkaline phosphatase, phospholipase C and peptidoglycan hydrolase. The vesicles contain pro-elastase and alkaline phosphatase (which are periplasmic constituents) and so are important for packaging periplasmic components as they are liberated to the outside of the cell. Once liberated, the vesicles are capable of fusing with the membranes of epithelial cells and liberating their virulence factors into host cells where they degrade cellular components, thereby aiding infection by the pathogen. The aminoglycoside antibiotic, gentamicin, is thought to kill bacteria by inhibiting protein synthesis, yet this cationic antibiotic can also perturb the packing order of lipids, thereby destabilizing bilayered membranes. For pathogens with highly anionic lipopolysaccharide on their surface, such as P. aeruginosa, this membrane destabilization can be so serious that it can cause cell lysis; these cells are therefore killed by a combination of protein synthesis inhibition and surface perturbation. By destabilizing the membranes of P. aeruginosa, gentamicin increases the release of membrane vesicles three- to five-fold. This may help account for some of the bacterium-mediated toxicity encountered during patient treatment with aminoglycoside antibiotics.