Bacteria and virus mechanics ; AFM

Passive and active response of bacteria under mechanical compression

The ability to maintain positive turgor pressure, by means of higher osmolarity of the cell interior than the exterior, is a requirement for proper metabolism

E. coli imaged with the atomic force microscope

in walled microbial cells. Turgor pressure is sensitive to changes in external osmotic conditions and is drastically increased upon osmotic downshock, together with cell volume. Bacteria prevent lysis caused by excessive osmotic pressure through mechanosensitive (MS) channels: membrane proteins that release solutes (ions) in response to mechanical stress. The exact mechanism of channel gating in the natural setting, however, has been elusive due to the lack of experimental methods appropriate for the small dimensions of prokaryotes. Our lab uses Atomic Force Microscope techniques to study MS gating in E Coli in vivo. We extract turgor pressure fluctuations associated with MS activity through the mechanical response of single cells.


An analytical model describing the E. coli cell wall

Bacteria have tough shells that allow them to withstand large turgor pressures. In gram-negative bacteria,such as E. coli, shell toughness stems from the ~4 nm thick peptidoglycan layer cushioned between inner and outer lipid membranes. Under physiological conditions, the mechanical situation is analogous to that of a thin-walled pressurized elastic balloon. Using numerical and analytical approaches we study passive mechanical responses to different loads. In general –given the molecular structure of the peptidoglycan layer– the coarse-grained elastic models we use are highly non-linear and anisotropic.

Sketch of a highly pressurized spherocylindrical capsule constricted in a box. The system mimics the typical contact forces that a rod-shaped bacterium may experience during biofilm formation or in micro-manipulation experiments. Despite the large deformations involved, the elastic non-linearity, and anisotropies, the system is tractable and provides a novel method to study turgor pressure and cell shape homeostasis.


Material properties and dynamics of assembly and filling of virus particles

Viruses are intriguing intermediates between the inanimate and the animate world. They are built very simply out of few structural building blocks, typically arranged in crystalline shell arrays (icosahedra and related shapes), and their genetic information is coded in RNA or DNA. These are Nature’s true nanomachines that far outperform any manmade nanotechnology, which, after all, generally plays at micrometer length scales.

AFM images of WT capsids. (Upper) Direct images. (Lower) Derivative images obtained from the direct images that show the shapes more clearly. (A) Empty capsid, adsorption on a twofold site. (B) Full capsid, adsorption on a fivefold site. (C) Full capsid, adsorption on a threefold site. The loading forces were ≈100 pN.

Experimentally testing the material properties of virus shells is becoming feasible and it will be possible to directly link to exact microscopic calculations and to simulations based on X-ray structures with atomic resolution. The combination, then, of two-dimensional crystalline shells with DNA or RNA in confined and possible ordered states will make for rather complex and interesting material properties. These are, of course, closely connected to the functions of the virus since they are designed by evolution for their specific purpose.



Wilts, Bodo D., Iwan AT Schaap, and Christoph F. Schmidt. “Swelling and softening of the cowpea chlorotic mottle virus in response to pH shifts.” Biophysical journal 108.10 (2015): 2541-2549

Michel, J. P., et al. “Nanoindentation studies of full and empty viral capsids and the effects of capsid protein mutations on elasticity and strength.” Proceedings of the National Academy of Sciences 103.16 (2006): 6184-6189.