THE BIOPHYSICS OF THE GRAM-NEGATIVE PERIPLASMIC SPACE

When subject to an osmotic 'up-shock', water flows outward from bacter ial cytoplasm of the bacterium. Lipid bilayers can shrink very Little in area and therefore must wrinkle to accommodate the smaller volume. The usual consequence is that all the layers of the cell envelope must become wrinkled together because they adhere to each other and must n ow cover a smaller surface. Plasmolysis spaces are formed if the cytop lasmic membrane (CM) separates from the other components of the wall. However, because the CM bilayer is essentially an incompressible two-d imensional Liquid, this constraint restricts the location and shape of plasmolysis spaces. With mild up-shocks they form at the pole and aro und constricting regions in the cell. Elsewhere their creation require s the formation of endocytotic or exocytotic vesicles. The formation o f endocytotic vesicles occurs in animal and plant cells as well as in bacterial cells. With stronger up-shocks tubular structures (Bayer adh esion sites), or other special geometric shapes (e.g., Scheie structur es) allow the bilayer to surround an irregular shaped cytoplast. Perio smotic agents, that is, those that extract water from the periplasm as well as the cytoplasm, are molecules such as poly-vinyl-pyrrolidone a nd alpha-cyclodextrin that are too large to pass through the porins in the outer membrane. They were found to significantly inhibit the form ation of plasmolysis spaces. Presumably, they inhibit the plasmolysis process, which requires that extracellular fluid enter between the CM and the outer membrane (OM). In the extreme case, with the dehydrating action of both osmotic agents and periosmotic agents, periplasmic spa ce formation tends to be prevented and a new kind of space develops wi thin the cytoplasm. We have designated these as 'cytoplasmic voids'. T hese novel structures are not bounded by lipid bilayers, in contrast t o the endocytotic vesicles. These new spaces appear to result from the negative turgor pressure generated by the application of the combinat ion of osmotic and periosmotic agents causing bubble formation. Severa l ideas in the literature about the wall biology (periseptal annuli, l eading edge, osmotic pressure in the periplasm) are presented and crit iqued. The basic criticism of these is that much of the phenomena can be explained because of the physics of the phospholipid bilayers and o smotic forces and thus does not imply the existence of a special contr ol mechanism to regulate growth and division.