Role of ionic strength in the thicknesses of the biopolymer fringes, spring constants, and Young's moduli of Pseudomonas putida

Abstract

Atomic force microscopy (AFM) was used to probe the mechanical properties of the soil microbe, Pseudomonas putida, as a function of the ionic strength (IS). By modeling the linear and nonlinear regimes of the AFM force-indentation data, the spring constant and the Young's modulus of the bacterial cell were estimated, respectively. In addition, the thickness of the biopolymer fringe that covers the bacterial surface was estimated as the transition distance between the linear and nonlinear regimes of a given force-indentation curve. The authors' results indicated that as the IS increased from 0.16 to 0.52 M, the bacterial spring constant remained unchanged, suggesting that the turgor pressure of the bacteria is unchanged in that range. This finding further suggests that P. putida arrives to osmotic equilibrium with its surroundings in the IS range of 0.16 and 0.52 M. Their results further indicated that as the IS increases from 0.02 to 0.52 M, the thickness of the biopolymer fringe decreased by ∼3-fold and this decrease was associated with a ∼6-fold apparent increase in the Young's modulus of the bacterial cell. Combined, their results suggest that enhanced soil-biofilm composite mechanical properties can be achieved via the application of higher IS solutions in soil. The use of low IS solutions should be avoided in applications where enhanced soil mechanics are sought because hydraulic conductivity is expected to be high.