No abstract

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 proteins-lipid content increases. The higher protein content increases the rigidity and the number of hydrophobic nano-domains per surface area.

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.

Setup for a reliable cell-mineral interaction at the single-cell level, (a) study of the mineral by a sharp tip, (b) study of the bacterial modified probe by a characterizer, (c) cell-mineral interaction, (d) subsequent check of the modified probe.

<p>Soil particle wettability controls the water dynamics of dry and unsaturated soil and has an impact on many processes where water is involved. Pure soil minerals are usually wettable, but under environmental conditions they are easily covered by organic compounds, changing their surface properties and potentially making them water repellent. Besides organic compounds such as alkanes, fatty acids, free lipids and waxes, research also indicates a direct influence of bacterial cells on the development of soil water repellency. In a series of stress experiments with different Gram-negative and Gram-positive strains of bacteria we could show that cell surface wettability measured in terms of contact angle is affected by cell stress response caused by hypertonic or drought environmental conditions. The changes in wettability were found to be accompanied by changes in physicochemical surface properties and surface elemental composition of the cells, as indicated by X-ray photoelectron spectroscopy. Furthermore, coverage of minerals by cells caused significant changes in particle wettability, rendering originally wettable minerals water repellent, with the effect being more pronounced for cell-mineral associations (CMA) formed with stressed cells. To investigate the physical stability of these CMA, we conducted an incubation experiment with CMA formed by quartz particles and <em>Bacillus subtilis</em> cells either grown under physiological or hypertonic conditions. The CMA were incubated at different water potentials (pF 2.5 and 4.2) and part of them subjected to wetting-drying cycles. The results showed that the quartz–<em>B. subtilis</em> CMA formed with stressed cells remained significantly more water repellent than those formed with unstressed cells during the whole incubation time of 80 days and independent of the incubation conditions. Furthermore, we observed a slight tendency of increasing contact angle with increasing incubation time. Besides the generally lower wettability of the stressed cells, the lower wettability of the CMA formed with stressed cells can be related to a higher degree of microbial coverage, as indicated by higher surface C content and lower surface O/C and Si/C ratios compared to the CMA formed with unstressed cells. The higher microbial coverage can probably be explained by attachment conditions being more favorable in case of the stressed cells, as suggested by interaction free energies calculated using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. In summary, the results indicate that the hydrophobizing effect of hypertonic stress on <em>B. subtilis </em>was stable over time and support the assumption that stress-related changes in cell surface properties remained also in necromass and their effect on surface properties of CMA can persist.</p>