Nanomechanical Measurements of Viscoelastic Properties of Bacterial Cells: Effect of Antimicrobial Peptides and Challenges Associated with Measurements in Buffer
The effectiveness of antimicrobial compounds can be easily screened; however, their mechanism of action is much more difficult to determine. Many compounds act by compromising the mechanical integrity of the bacterial cell, and we have developed an atomic force microscopy (AFM)-based creep-deformation technique to evaluate changes in the time-dependent mechanical properties of bacterial cells upon exposure to antimicrobials. Measurements revealed large changes to the viscoelastic parameters including a distinctive signature for the loss of integrity of the bacterial cell envelope. This technique provides unique insight into the kinetics and action of antimicrobials on bacteria. Initial experiments were performed in Milli-Q water on Pseudomonas aeruginosa PAO1 bacteria, which can withstand large osmotic pressures. Subsequent experiments addressed the challenges of performing AFM creep deformation experiments on live cells in PBS buffer. We describe measurements of, and improvements to, the stability of the experimental set-up in an effort to facilitate these challenging experiments.