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Cell RheologyThe mechanical properties of the bacterium and how they
regulate cell growth
Rico RojasHuang and Theriot Labs
Goal: To measure and understand how cell growth depends on the osmotic pressure within the cell.
Vibrio
The osmotic pressure within bacteria is much higher than atmospheric pressure.
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P ~ (Cin − Cout )
Morse Equation
Gram negatives: P ~ 1 atmGram positives: P ~ 10 atm
The bacterial cell wall is a cross-linked polymeric gel that encloses the cell.
Polysaccharides Polypeptides
Gan et al., 2008
Bacillus
Mechanical stress the in cell wall balances the turgor pressure and stretches the wall.
Does stress also determine strain rate of the cell wall, i.e., growth rate of the cell?
Bacillus
ε = strain =Δl/le
ξ Mesh Size
χ Cross-Link Conc.
Spring Constant
Rate of Cross-Link Dissociation
Ball-and-Spring Model of the Cell Wall
Strain Rate
Bacteria have a number of mechanisms for regulating their turgor.
Wood, 2006
Biological materials have complex mechanical properties.
Koenderink et al., 2006
Actin Rheology
A Rheometer
Characterizing the response of cells to changes in osmolarity – the “Cell Rheometer.”
Single cell measurements
Raw Data: length vs. time
T=30 s
Strain rate vs. time
n=32
Turgor pressure modulates growth rate
T=30 s
The phase is constant across a range of driving frequencies
“Gram-negative” bacteria (e.g. E. coli) have two membranes
The outer membrane may bear significant stress
Plasmolysis Dissolution of outer membrane
Pressurized
Plasmolyzed
Lysed
B. subtilis
Comparative study
B. subtilis E. coli
Highly non-linear osmoregulation in Gram-positive species
Too simple a model
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P = RT(Cin − Cout )
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˙ C in = −α P − P0( )
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˙ ε ~dP
dt+ kd P
⎛
⎝ ⎜
⎞
⎠ ⎟I. Constitutive Equation
II. Morse Equation
III. Osmoregulation
{ {
GrowthElasticity
Jen Hsin
For this year:
-Finish characterizing the mechanics of the cell wall and osmoregulatory mechanisms.-Write cell-scale model that integrates these with growth/wall synthesis.
Thanks!