A small diffusion pore in the outer membrane of Pseudomonas aeruginosa

The permeability properties of the outer membrane of Pseudomonas aeruginosa were re‐examined, since the reported conclusions are conflicting [Decad, M. G. and Nikaido, H. (1976) J. Bacteriol. 128, 325–336; Caulcott, C. A., Brown, M. R. W. and Gonda, I. (1984) FEMS Microbiol. Lett. 21, 119–123]. On the basis of the experimental evidence to be described below we conclude that the exclusion limit of the outer membrane of P. aeruginosa is smaller than the size of uncharged disaccharides but larger than the size of hexose. This conclusion is based on the following evidence. (a) Penetration of monosaccharides into the expanded periplasm was large and that of disaccharides was small, after the cells were plasmolyzed with 600 mosM NaCl. (b) A significant amount of protein was released after osmotic down‐shock of cells treated with the hypertonic monosaccharides but not of cells treated with the hypertonic saccharides larger than disaccharides. (c) Centrifuged pellets of cells treated with hypertonic di, tri and tetrasaccharides weighed about 15–20% less than that of cells treated with the isotonic monosaccharide, suggesting that the osmotic pressure was exerted on the outer membrane causing dehydration and shrinking of the cells. By contrast, cells treated with the hypertonic pentose and hexoses weighed about 0.1% and 6% less, respectively, than cells treated with the isotonic saccharide, suggesting that pentose diffused through the outer membrane freely.