The Dependence of Bacterial Cell Growth on Turgor Pressure

Rico Rojas

Goal: To measure and understand how expansion of the bacterial cell wall depends on

mechanical force.

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

This growth in size of the cells appears to be the result of the progressive effect of endosmosis. They distend under the influence of the liquid, and gradually expand like soap bubbles expand under the influence of air which distends them. Cell walls themselves are composed of molecules, and also experience development, particularly the trend of expansion.

ξ Mesh Size

χ Cross-Link Conc.

Spring Constant

Rate of Cross-Link Dissociation

Ball-and-Spring Model of the Cell Wall

Rojas, et al. 2011

Strain Rate

Furchtgott et al., 2011

Jen Hsin

Ball-and-Spring Simulation Platform

B. mycoides

Growth rate depends on the osmolarity of the medium.

Scott, 1953; Christian and Scott 1955

Conc. of Sucrose or Salt

Growth rate vs. medium osmolarityof Salmonella in different media

Christian, 1955

Measures, 1975

Green 1971

Turgor Determines Growth Rate in Plants

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

B. subtilis

Turgor Pressure Modulates Growth Rate.

φT=1/ω

E. coli

Things to do this year: construct a dynamical model

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P ~ (Cin − Cout )

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˙ C in ~ − P − P0( )

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˙ ε ~dPdt

+ kd P ⎛ ⎝ ⎜

⎞ ⎠ ⎟I. Constitutive Equation

II. Morse’s Equation

III. Osmoregulation

{{

GrowthElasticity

Things to do: test the model

Model

Data

Things to do: test the model

1. Ion channel knockouts2. Deprive cells of compatible solutes3. Knockout/over-express/purify endopeptidase4. Compare different morphologies, species,

kingdoms

ω ω

φ

? ?η

Things to do: address the relationship between synthesis and mechanics

Garner et al., 2011

MreB Motion

Things to do: apply external mechanical force

Other ideas:1. Functionalized microcapillary2. MEMS device

Optical Trap

Squashed Cells -Kian

Thanks!