Dynamic viscoelastic behavior of individual Gram-negative bacterial cells

We have used atomic force microscopy (AFM) to measure the viscoelastic properties of individual Escherichia coli K12 cells under fully hydrated conditions by collecting AFM force-indentation and force-time curves. Spherical colloidal tips were used to reduce the local strain ensuring that the measurements were performed in the linear viscoelastic regime. We find that the cells exhibit a time-dependent viscoelastic response to the constant compressive force applied by the AFM tip. The results are interpreted using the standard solid model which describes both an instantaneous and a delayed elastic deformation. The values obtained for the three viscoelastic parameters were used to generate a full description of the dynamic viscoelastic behavior of the cells over a wide range of frequencies derived from the differential equation governing the standard solid model. The results obtained using this methodology are compared to experimental data obtained for the E. coli K12 cells and these results are compared to previously reported viscoelastic data on Pseudomonas aeruginosa PAO1 cells. The differences between the viscoelastic behaviors of these two types of Gram-negative bacterial cells are discussed in terms of their structural differences.