The mechanical properties of gram-negative bacteria are governed by a rigid peptidoglycan (PG) cell wall and the turgor pressure generated by the large concentration of solutes in the cytoplasm. The elasticity of the PG has been measured in bulk and in isolated sacculi and shown to be compliant compared to the overall stiffness of the cell itself. However, the stiffness of the cell wall in live cells has not been measured. In particular, the effects that pressure-induced stress might have on the stiffness of the mesh-like PG network have not been addressed even though polymeric materials often exhibit large amounts of stress-stiffening. We study bulging Escherichia coli cells using atomic force microscopy to separate the contributions of the cell wall and turgor pressure to the overall cell stiffness. We find strong evidence of power-law stress-stiffening in the E. coli cell wall, with an exponent of 1.07 \pm 0.25 , such that the wall is significantly stiffer in live cells (E %DIFDELCMD < \sim32\pm10 %%% MPa\pm \pm  ) than in unpressurized saculli . These measurements also indicate that the turgor pressure in  E. coli is 26 \pm 4 kPa.
We study intact and bulging Escherichia coli cells using atomic force microscopy to separate the contributions of the cell wall and turgor pressure to the overall cell stiffness. We find strong evidence of power-law stress-stiffening in the E. coli cell wall, with an exponent of 1.22 \pm 0.12 , such that the wall is significantly stiffer in intact cells (E %DIFDELCMD < \sim32\pm10 %%% = 23\pm 8 MPa and 49\pm 20 MPa in the axial and circumferential directions ) than in unpressurized sacculi . These measurements also indicate that the turgor pressure in living cells E. coli is 29 \pm 3 kPa.
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The mechanical properties of gram-negative bacteria are governed by a rigid peptidoglycan (PG) cell wall and the turgor pressure generated by the large concentration of solutes in the cytoplasm. The elasticity of the PG has been measured in bulk and in isolated sacculi and shown to be compliant compared to the overall stiffness of the cell itself. However, the stiffness of the cell wall in live cells has not been measured. In particular, the effects that pressure-induced stress might have on the stiffness of the mesh-like PG network have not been addressed even though polymeric materials often exhibit large amounts of stress-stiffening. We study
We study intact and
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1.07
1.22
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0.25
0.12
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live
intact
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MPa
= 23
A
8 MPa and 49
A
20 MPa in the axial and circumferential directions
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saculli
sacculi
A
living cells
R
26
29
R
4
3