Power Laws Describe Bacterial Viscoelasticity

Bacterial cells survive in a wide range of different environments and actively tune their mechanical properties for purposes of growth, movement, division, and nutrition. In Gramnegative bacteria, the cell envelope with its outer membrane and peptidoglycan are the main determinants of mechanical properties and are common targets for the use of antibiotics. The study of bacterial mechanical properties has shown promise in elucidating a structure-function relationship in bacteria, connecting, shape, mechanics, and biochemistry. In this work, we study frequency and time-dependent viscoelastic properties of E. coli cells by atomic force microscopy (AFM). We perform force cycles, oscillatory microrheology, stress relaxation, and creep experiments, and use power law rheology models to fit the experimental results. All data sets could be fitted with the models and provided power law exponents of 0.01 to 0.1 while showing moduli in the range of a few MPa. We provide evidence for the interchangeability of the properties derived from these four different measurement approaches.

Automated summary

Bacterial cells actively tune their mechanical properties for various functions, and the cell envelope is crucial for mechanical properties. This study used AFM to investigate the viscoelastic properties of E. coli cells and successfully fitted the experimental results with power law rheology models. The findings provide valuable evidence for the interchangeability of properties derived from different measurement approaches.