<i>Escherichia coli tol-pal</i> Mutants Form Outer Membrane Vesicles

<jats:title>ABSTRACT</jats:title> <jats:p> Mutations in the <jats:italic>tol-pal</jats:italic> genes induce pleiotropic effects such as release of periplasmic proteins into the extracellular medium and hypersensitivity to drugs and detergents. Other outer membrane defective strains such as <jats:italic>tolC</jats:italic> , <jats:italic>lpp</jats:italic> , and <jats:italic>rfa</jats:italic> mutations are also altered in their outer membrane permeability. In this study, electron microscopy and Western blot analyses were used to show that strains with mutations in each of the <jats:italic>tol-pal</jats:italic> genes formed outer membrane vesicles after growth in standard liquid or solid media. This phenotype was not observed in <jats:italic>tolC</jats:italic> and <jats:italic>rfaD</jats:italic> cells in the same conditions. A <jats:italic>tolA</jats:italic> deletion in three different <jats:italic>Escherichia coli</jats:italic> strains was shown to lead to elevated amounts of vesicles. These results, together with plasmid complementation experiments, indicated that the formation of vesicles resulted from the defect of any of the Tol-Pal proteins. The vesicles contained outer membrane trimeric porins correctly exposed at the cell surface. Pal outer membrane lipoprotein was also immunodetected in the vesicle fraction of <jats:italic>tol</jats:italic> strains. The results are discussed in view of the role of the Tol-Pal transenvelope proteins in maintaining outer membrane integrity by contributing to target or integrate newly synthesized components of this structure. </jats:p>