ABSTRACT
The cytoplasmic membrane is freely permeable to water but forms an effective barrier for solutes of the environment or the cytoplasm. In general, the total concentration of osmotically active solutes within a cell is higher than in the environment, causing water to flow down its chemical potential into the cell. As a result, a hydrostatic pressure, the so-called turgor, is exerted by the cytoplasmic membrane toward the cell wall. Consequently, turgor balances the difference in osmotic pressure between the cell interior and its surroundings. Turgor is maintained throughout the growth cycle as the cell elongates, and it is considered necessary for enlargement of the cell envelope and thus for growth and division [27]. Although turgor is very difficult to quantify in bacteria [60], values of 3 to 5 bar for Gram-negative bacteria
and approximately 20 bar for Gram-positive bacteria have been estimated [7,19,56]. The much higher value in Gram-positive bacteria is thought to reflect the large cytoplasmic solvent pool needed for expansion of the multilayer peptidoglycan. Because the environment of a bacterial cell is frequently subjected to fluctuations in osmolality, bacteria were forced to develop efficient adaptation mechanisms to cope with both, a decrease or an increase in the external osmolality, or, in other words, with a hypoosmotic shift (osmotic downshift) or a hyperosmotic shift (osmotic upshift).
