Figure 1.

The generation of turgor and resistance to the force. In E. coli cells growing in a medium of ∼240 mOsm (a standard minimal medium or LB containing 5 g/L NaCl) one may confidently expect to find ∼200 mm cytoplasmic anions and ∼300 mm K⁺. Approximately 100 mm of the K⁺ matches fixed anions and is thus not considered for the calculation of the outward turgor of ∼10 atm. Given the medium contributes ∼6 atm the net turgor pressure is ∼4 atm. MS channels will gate if there is a net outward pressure of ∼0.1 atm and thus the cell wall and outer membrane, between them, contribute a resistance of ≥4 atm to maintain MS channels closed. There are at least two contributions to the strength of the cell wall — the first, already described, is the crosslinking of the peptidoglycan and the second is the outer membrane that can provide some resistive force through the binding together of the lipopolysaccharide chains by divalent cations [46].