Hypertonic stress induced changes of Pseudomonas fluorescens adhesion towards soil minerals studied by AFM

Abstract

Studying bacterial adhesion to mineral surfaces is crucial for understanding soil properties. Recent research suggests that minimal coverage of sand particles with cell fragments significantly reduces soil wettability. Using atomic force microscopy (AFM), we investigated the influence of hypertonic stress on Pseudomonas fluorescens adhesion to four different minerals in water. These findings were compared with theoretical XDLVO predictions. To make adhesion force measurements comparable for irregularly shaped particles, we normalized adhesion forces by the respective cell-mineral contact area. Our study revealed an inverse relationship between wettability and the surface-organic carbon content of the minerals. This relationship was evident in the increased adhesion of cells to minerals with decreasing wettability. This phenomenon was attributed to hydrophobic interactions, which appeared to be predominant in all cell-mineral interaction scenarios alongside with hydrogen bonding. Moreover, while montmorillonite and goethite exhibited stronger adhesion to stressed cells, presumably due to enhanced hydrophobic interactions, kaolinite showed an unexpected trend of weaker adhesion to stressed cells. Surprisingly, the adhesion of quartz remained independent of cell stress level. Discrepancies between measured cell-mineral interactions and those calculated by XDLVO, assuming an idealized sphere-plane geometry, helped us interpret the chemical heterogeneity arising from differently exposed edges and planes of minerals. Our results suggest that bacteria may have a significant impact on soil wettability under changing moisture condition.

Figure 1

AFM Height (top) and Peak Force Error (bottom) maps of (a) goethite, (b) kaolinite, (c) montmorillonite and (d) quartz, obtained in KNO₃ solution with sharp tips. The scale bar is 1.5 µm for all images.

Figure 2

Correlative AFM/ESEM characterizations of tips modified with four different minerals shown for each mineral in one row: (a) ESEM pictures of the cantilevers modified with the minerals with insets showing 3D inverse AFM images of the respective modifying mineral, (b) closer AFM images of the mineral clusters at the top of the glue spot, (c) insets of local areas inside (b) with the highest z extension, and (d) local AFM maps (scale bar of 200 nm) of the set of 5 probes later used for the cell-mineral interactions and graphs presenting their tip area (left) and R_q (right) as a function of height (or deformation depth).

Figure 3

Exemplary images of the same unstressed bacterial cultures in 10 mM KNO₃ solution for one mineral with the same scale in each row made by (a) a sharp tip before the cell-mineral interactions, CMI, (b) a mineral modified tip during the CMI with the marks indicating the positions of FD curve acquisition, and (c) a sharp tip after the CMI.

Figure 4

Frequency distributions of adhesion pressures of cell-mineral interactions between cells of two independent unstressed and three independent stressed P. fluorescens cultures and five individual functionalized tips for each mineral (Fig. 2d) in 10 mM KNO₃ solution. A set of ~ 30 FD curves for each tip with 1 s contact time and 5 nN applied force was made. The solid arrows highlight significant differences in adhesion pressures between the minerals (with p vales for unstressed and stressed separated by “,”) while the dashed arrows show the effect of stress on adhesion pressure.

Figure 5

The work of adhesion was calculated from CA data as described in Traini et al. (Eq. 8). Adhesion pressure values are the mean of the medians extracted from the boxplots (Supplementary Fig. S-I 14) and thus just approximate values to show the general relationship. In contrast to the energy profiles, the negative values found for the CMI with unstressed cells indicate repulsive interactions.

Figure 6

(a) sketch of a water drop on a kaolinite surface (top) demonstrating the preferential flat orientation of the particles on the glue surface with the magnification (bottom) showing the interaction between water molecules and the basal planes, (b) interaction between a flat kaolinite particle and a planar cell surface with the magnification (c) which illustrates how the basal plane with low atomic packing and edge plane with high atomic packing contact the cell surface, (d) same as (a) for montmorillonite, (e) interaction between a porous domain of a montmorillonite particle and a planar cell surface with the magnification (f) showing the basal/edge stacking units that get in contact with the cell surface. The crystal pattern is made by Avogadro free software (version 2.0) and it is general for clay minerals. The overlapping basal/edge stacking units of montmorillonite are pure imagination based on our AFM results and the literature^,.