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.

€

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

€

P = RT(Cin − Cout )

€

˙ C in = −α P − P0( )

€

˙ ε ~dPdt

+ 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!