Abstract
The cytoplasm of bacteria is maintained at a higher osmolality than the growth medium, which generates a turgor pressure. The cell membrane (CM) cannot support a large turgor, so there are two possibilities for transferring the pressure to the peptidoglycan cell wall (PGW): (1) the CM could be pressed directly against the PGW, or (2) the CM could be separated from the PGW by a periplasmic space that is isoosmotic with the cytoplasm. There is strong evidence for gram-negative bacteria that a periplasm exists and is isoosmotic with the cytoplasm. No comparable studies have been done for gram-positive bacteria. Here I suggest that a periplasmic space is probably essential in order for the periplasmic proteins to function, including especially the PBPs that remodel the peptidoglycan wall. I then present a semi-quantitative analysis of how teichoic acids could support a periplasm that is isoosmotic with the cytoplasm. The fixed anionic charge density of teichoic acids in the periplasm is ∼0.5 M, which would bring in ∼0.5 M Na+ neutralizing ions. This approximately balances the excess osmolality of the cytoplasm that would produce a turgor pressure of 19 atm. The 0.5 M fixed charge density is similar to that of proteoglycans in articular cartilage, suggesting a comparability ability to support pressure. An isoosmotic periplasm would be especially important for cell division, since it would allow CM constriction and PGW synthesis to avoid turgor pressure.
Highlights
Bacterial cytoplasm has a high concentration of proteins and nucleic acids, plus their neutralizing counterions and various small molecule osmolytes
I will conclude that the periplasm of both gram-negative and gram-positive bacteria are likely isoosmotic with the cytoplasm, which means that cytokinesis does not need to generate a force to overcome turgor pressure
I have presented arguments that a periplasmic space is needed for the peptidoglycan wall (PGW) synthesis proteins to function, and I developed a quantitative estimate that Teichoic acids (TAs) could generate a concentration of counterions that approximately balances the osmolality of the cytoplasm
Summary
Bacterial cytoplasm has a high concentration of proteins and nucleic acids, plus their neutralizing counterions and various small molecule osmolytes. The envelope of gram-negative bacteria comprises an inner cytoplasmic membrane (CM), a peptidoglycan wall (PGW) and an outer membrane (OM). Some authors have defined the periplasm as the space between the inner and outer membranes for gram-negative bacteria, but our definition provides consistency for gram-positive bacteria. In case 1 the CM is pressed against the PGW by cytoplasmic turgor pressure, and there is essentially no periplasmic space. I will first present a general argument that a periplasmic space is necessary for periplasmic proteins, in particular PGW remodeling enzymes, to function. I will conclude that the periplasm of both gram-negative and gram-positive bacteria are likely isoosmotic with the cytoplasm, which means that cytokinesis does not need to generate a force to overcome turgor pressure
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