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

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.

Changes in cell surface properties of Pseudomonas fluorescens by adaptation to NaCl induced hypertonic stress.

Determination of the effect of water stress on the surface properties of bacteria is crucial to study bacterial induced soil water repellency. Changes in the environmental conditions may affect several properties of bacteria such as the cell hydrophobicity and morphology. Here, we study the influence of adaptation to hypertonic stress on cell wettability, shape, adhesion, and surface chemical composition of Pseudomonas fluorescens. From this we aim to discover possible relations between the changes in wettability of bacterial films studied by contact angle and single cells studied by atomic and chemical force microscopy (AFM, CFM), which is still lacking. We show that by stress the adhesion forces of the cell surfaces towards hydrophobic functionalized probes increase while they decrease towards hydrophilic functionalized tips. This is consistent with the contact angle results. Further, cell size shrunk and protein content increased upon stress. The results suggest two possible mechanisms: Cell shrinkage is accompanied by the release of outer membrane vesicles by which the protein to lipid ratio increases. The higher protein content increases the rigidity and the number of hydrophobic nano-domains per surface area.

Water Stress-Driven Changes in Bacterial Cell Surface Properties.

Increased drought intensity and frequency exposes soil bacteria to prolonged water stress. While numerous studies reported on behavioral and physiological mechanisms of bacterial adaptation to water stress, changes in bacterial cell surface properties during adaptation are not well researched. We studied adaptive changes in cell surface hydrophobicity (CSH) after exposure to osmotic (NaCl) and matric stress (polyethylene glycol 8000 [PEG 8000]) for six typical soil bacteria (Bacillus subtilis, Arthrobacter chlorophenolicus, Pseudomonas fluorescens, Novosphingobium aromaticivorans, Rhodococcus erythropolis, and Mycobacterium pallens) covering a wide range of cell surface properties. Additional physicochemical parameters (surface chemical composition, surface charge, cell size and stiffness) of B. subtilis and P. fluorescens were analyzed to understand their possible contribution to CSH development. Changes in CSH caused by osmotic and matric stress depend on strain and stress type. CSH of B. subtilis and P. fluorescens increased with stress intensity, R. erythropolis and M. pallens exhibited a generally high but constant contact angle, while the response of A. chlorophenolicus and N. aromaticivorans depended on growth conditions and stress type. Osmotically driven changes in CSH of B. subtilis and P. fluorescens are accompanied by increasing surface N/C ratio, suggesting an increase in protein concentration within the cell wall. Cell envelope proteins thus presumably control bacterial CSH in two ways: (i) by increases in the relative density of surface proteins due to efflux of cytoplasmic water and subsequent cell shrinkage, and (ii) by destabilization of cell wall proteins, resulting in conformational changes which render the surface more hydrophobic. IMPORTANCE Changes in precipitation frequency, intensity, and temporal distribution are projected to result in increased frequency and intensity of droughts and heavy rainfall events. Prolonged droughts can promote the development of soil water repellency (SWR); this impacts the infiltration and distribution of water in the soil profile, exposing soil microorganisms to water stress. Exposure to water stress has recently been reported to result in increased cell surface hydrophobicity. However, the mechanism of this development is poorly understood. This study investigates the changes in the physicochemical properties of bacterial cell surfaces under water stress as a possible mechanism of increased surface hydrophobicity. Our results improve understanding of the microbial response to water stress in terms of surface properties, the variations in stress response depending on cell wall composition, and its contribution to the development of SWR.