Cell Rheology The mechanical properties of the bacterium and how they regulate cell growth Rico Rojas Huang 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. Morse Equation Gram negatives: P ~ 1 atm Gram positives: P ~ 10 atm

The bacterial cell wall is a cross-linked polymeric gel that encloses the cell. PolysaccharidesPolypeptides 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/l e

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. subtilisE. coli Highly non-linear osmoregulation in Gram-positive species

Too simple a model 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!