Surface Viscoelasticity of Individual Gram-Negative Bacterial Cells Measured Using Atomic Force Microscopy

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Surface Viscoelasticity of Individual Gram-Negative bacterial Cells Measured Using Atomic Force Microscopy

V. Vadillo-Rodriguez, T J. Beveridge, and T R Dutcher Journal of Bacteriology, Vol. 190, No. 12, June 2008, p. 4225-4232

Keywords: Viscoelasticity, creep

Summary

Bacteria can be divided into two types: Gram-negative and Gram-positive, depending on the characteristics of their cell wall. Gram-negative bacteria, like the Pseudomonas aeruginosa used in this study, have two cell membranes separated by a thin layer called a periplasm. This flexible layer is a peptidoglycan, which consists of stiff polysaccharides connected by flexible protein cross-linkers. The qualitative mechanical properties of large numbers of cells and the elastic behavior of single cells have both been previously investigated, but this is one of the first works to look at the full viscoelastic properties of an individual bacterium. To accomplish this, the authors examine the force-displacement curves of an AFM-tip pushed into the cell surface, as well as the deformation over time at a constant force. The authors find that the time-scales for these processes, on the order of a couple seconds, is likely optimal for the cells. It allows the cell to maintain its integrity while growing, while still preventing the magnification of potentially hazardous local stresses.

Soft Matter Concepts

The main feature of this paper is its focus on viscoelastic, rather than purely elastic behavior. An elastic material will deform proportional to an applied stress, whereas a viscous material will flow at a rate proportional to the stress. In the case of the cells, there is an immediate deformation of the cells, followed by a slow approach to some equilibrium strain.