Elasticity of Pseudomonas putida KT2442 surface polymers probed with single-molecule force microscopy

Single-molecule force microscopy was used to study the effect of solvent polarity and ionic strength on the elasticity of bacterial surface polymers. Adhesion forces were measured between Pseudomonas putida KT2442 bacterial cells and silicon nitride tips of an atomic force microscope (AFM). Force-extension profiles were analyzed to determine the elasticity of the polymer chains in several solvents (water, formamide, methanol, 0.1 M KCl, and 0.01 M KCl). Adhesion peaks were fit to entropic-based, statistical mechanical, random walk formulations (the freely jointed chain (FJC) and the wormlike chain (WLC) models). The experimental data showed better agreement with the FJC (average R-2 = 0.86 +/- 0.20) than the WLC model (average R-2 = 0.76 +/- 0.22). The segment length was 0.18-1.0 nm in all five solvents using the FJC model, with about 60% of the chains having a segment length of 0.154-0.20 nm. The persistence length was 0.18-0.83 nm using the WLC model, with about 78% of chains having a persistence length of 0.154-0.20 rim. The WLC model was not able to represent polymer properties for chains of < 11 nm (4% of the data), since persistence lengths shorter than the C-C bond were obtained. The WLC model also failed to predict the high-magnitude adhesion forces in KCl solutions and methanol. The extensible freely jointed chain model (FJC+) was considered since the latter accounts for enthalpic effects neglected in the FJC model, but it did not represent the data better than the FJC model. Adhesive interactions between the biopolymer and the AFM tip were compared in these solvents. Adhesion was highest in the least polar solvent, methanol. Adhesion forces in water and formamide were about the same and less than forces observed in methanol. Although biopolymer contour lengths varied over a wide range (tens to hundreds of nanometers) in all solvents, shorter lengths were observed when salt was present, indicating that the polymer chains were less extended in the presence of salt.