Bacteriolytic effect of membrane vesicles from Pseudomonas aeruginosa on other bacteria including pathogens: conceptually new antibiotics.

Pseudomonas aeruginosa releases membrane vesicles (MVs) filled with periplasmic components during normal growth, and the quantity of these vesicles can be increased by brief exposure to gentamicin. Natural and gentamicin-induced membrane vesicles (n-MVs and g-MVs, respectively) are subtly different from one another, but both contain several important virulence factors, including hydrolytic enzyme factors (J. L. Kadurugamuwa and T. J. Beveridge, J. Bacteriol. 177:3998-4008, 1995). Peptidoglycan hydrolases (autolysins) were detected in both MV types, especially a periplasmic 26-kDa autolysin whose expression has been related to growth phase (Z. Li, A. J. Clarke, and T. J. Beveridge, J. Bacteriol. 178:2479-2488, 1996). g-MVs possessed slightly higher autolysin activity and, at the same time, small quantities of gentamicin. Both MV types hydrolyzed isolated gram-positive and gram-negative murein sacculi and were also capable of hydrolyzing several glycyl peptides. Because the MVs were bilayered, they readily fused with the outer membrane of gram-negative bacteria. They also adhered to the cell wall of gram-positive bacteria. g-MVs were more effective in lysing other bacteria because, in addition to the autolysins, they also contained small amounts of gentamicin. The bactericidal activity was 2.5 times the MIC of gentamicin, which demonstrates the synergistic effect of the antibiotic with the autolysins. n-MVs were capable of killing cultures of P. aeruginosa with permeability resistance against gentamicin, indicating that the fusion of n-MV to the outer membrane liberated autolysins into the periplasm, where they degraded the peptidoglycan and lysed the cells. g-MVs had even greater killing power since they liberated both gentamicin and autolysins into these resistant cells. These findings may help develop a conceptually new group of antibiotics designed to be effective against hard-to-kill bacteria.