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Journal of Bacteriology, June 2008, p. 4225-4232, Vol. 190, No. 12
0021-9193/08/$08.00+0     doi:10.1128/JB.00132-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

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

Virginia Vadillo-Rodriguez,1,2,3 Terry J. Beveridge,2,3 and John R. Dutcher1,3*

Department of Physics,1 Department of Molecular and Cellular Biology,2 Advanced Foods and Materials Network, Networks of Centres of Excellence (AFMnet), University of Guelph, Guelph, Ontario, Canada N1G 2W13

Received 24 January 2008/ Accepted 4 April 2008

The cell envelope of gram-negative bacteria is responsible for many important biological functions: it plays a structural role, it accommodates the selective transfer of material across the cell wall, it undergoes changes made necessary by growth and division, and it transfers information about the environment into the cell. Thus, an accurate quantification of cell mechanical properties is required not only to understand physiological processes but also to help elucidate the relationship between cell surface structure and function. We have used a novel, atomic force microscopy (AFM)-based approach to probe the mechanical properties of single bacterial cells by applying a constant compressive force to the cell under fluid conditions while measuring the time-dependent displacement (creep) of the AFM tip due to the viscoelastic properties of the cell. For these experiments, we chose a representative gram-negative bacterium, Pseudomonas aeruginosa PAO1, and we used regular V-shaped AFM cantilevers with pyramid-shaped and colloidal tips. We find that the cell response is well described by a three-element mechanical model which describes an effective cell spring constant, k1, and an effective time constant, {tau}, for the creep deformation. Adding glutaraldehyde, an agent that increases the covalent bonding of the cell surface, produced a significant increase in k1 together with a significant decrease in {tau}. This work represents a new attempt toward the understanding of the nanomechanical properties of single bacteria while they are under fluid conditions, which could be of practical value for elucidating, for instance, the biomechanical effects of drugs (such as antibiotics) on pathogens.

* Corresponding author. Mailing address: Department of Physics, 50 Stone Road East, University of Guelph, Guelph, Ontario, Canada N1G 2W1. Phone: (519) 824-4120, ext. 53950. Fax: (519) 836-9967. E-mail: dutcher{at}physics.uoguelph.ca

{triangledown} Published ahead of print on 11 April 2008.

Journal of Bacteriology, June 2008, p. 4225-4232, Vol. 190, No. 12
0021-9193/08/$08.00+0     doi:10.1128/JB.00132-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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