Spatial mapping of the conformational and mechanical properties of bacterial surface biopolymers.

Forces acting between an atomic force microscopy silicon nitride cantilever and the bacterial surface biopolymers of Escherichia coli or Pseudomonas putida were spatially probed in water. The interactions were fitted to a model of steric repulsion to estimate the bacterial surface biopolymer brush length and grafting density. The forces were further fitted to a Hertz model of contact mechanics modified by Sneddon et al. to quantify Young's modulus of elasticity for the cells. Contour plots of the quantified properties described above (i.e., the bacterial surface biopolymer brush length and grafting density, and Young's modulus of elasticity for the cells) based on the location coordinates on the bacterial surfaces were generated. Our contour plots indicated the bacterial cells organize their biopolymers uniquely to help them survive in the environment. Specifically, our results showed that the perimeter of a bacterial cell is characterized by a more flexible as well as longer biopolymer brush compared to those estimated at the center top of the cell. These results suggest that bacteria are likely to use their longer brushes on the edges to facilitate their adhesion by bridging surfaces. Also, they maintain their structural reinforcement by developing higher densities of grafted biopolymers and hence higher elasticities at their centers. Moreover, a stronger linear relationship was observed between the brush thicknesses and the grafting densities for the collapsed brush at the center of the cells when compared to the perimeter of the cells.