The capacity of fungi to serve as vectors for the dispersion of pollutant-degrading bacteria was analyzed in laboratory model systems mimicking water-saturated (agar surfaces) and unsaturated soil environments (glass-bead-filled columns). Two common soil fungi (Fusarium oxysporum and Rhexocercosporidium sp.) forming hydrophilic and hydrophobic mycelia, respectively, and three polycyclic aromatic hydrocarbon degrading bacteria (Achromobacter sp. SK1, Mycobacterium frederiksbergense LB501TG, and Sphingomonas sp. L138) were selected based on the absence of mutual antagonistic effects. It was shown that fungal hyphae act as vectors for bacterial transport with mobilization strongly depending on the specific microorganisms chosen: The motile strain Achromobacter sp. SK1 was most efficiently spread along hyphae of hydrophilic F. oxysporum in both model systems with transport velocities of up to 1 cm d(-1), whereas no dispersion of the two nonmotile strains was observed in the presence of F. oxysporum. By contrast, none of the bacteria was mobilized along the hydrophobic mycelia of Rhexocercosporidium sp. growing on agar surfaces. In column experiments however, strain SK1 was mobilized by Rhexocercosporidium sp. It is hypothesized that bacteria may move by their intrinsic motilitythrough continuous (physiological) liquid films forming around fungal hyphae. The results of this study suggest that the specific stimulation of indigenous fungi may be a strategy to mobilize pollutant-degrading bacteria leading to their homogenization in polluted soil thereby improving bioremediation.

Knowing how motile bacteria move near and along a solid surface is crucial to understanding such diverse phenomena as the migration of infectious bacteria along a catheter, biofilm growth, and the movement of bacteria through the pore spaces of saturated soil, a critical step in the in situ bioremediation of contaminated aquifers. In this study, a tracking microscope is used to record the three-dimensional motion of Escherichia coli near a planar glass surface. Data from the tracking microscope are analyzed to quantify the effects of bacteria-surface interactions on the swimming behavior of bacteria. The speed of cells approaching the surface is found to decrease in agreement with the mathematical model of Ramia et al. [Ramia, M., Tullock, D. L. & Phan-Tien, N. (1993) BiophysJ. 65,755-778], which represents the bacteria as spheres with a single polar flagellum rotating at a constant rate. The tendency of cells to swim adjacent to the surface is shown in computer-generated reproductions of cell traces. The attractive interaction potential between the cells and the solid surface is offered as one of several possible explanations for this tendency.In a homogeneous fluid medium, peritrichously flagellated bacteria such as Escherichia coli and Salmonella typhimurium execute random walks as they alternate between two phases of motion: running (motion in essentially straight paths) and tumbling (changes in direction while remaining in place) (1, 2).This behavior is similar to molecular diffusion except that changes in direction are due to reversal of flagellar rotation and not molecular collisions. In many natural systems, the characteristic motion of swimming bacteria is modified by the presence of solid surfaces. Examples of such systems include the motion of bacteria in saturated soil (3-5), the migration of bacteria through small-diameter capillary tubes (6), and the migration of infectious bacteria along medically implanted surfaces, such as prostheses and catheters (7).Bacterial transport rates in the presence of solid boundaries are different from transport rates in the absence of such boundaries. Depending on the system studied, the migration of bacteria can either be enhanced (6, 7) or attenuated (5, 8) by solid surfaces. In this study we have quantified the change in the characteristic motion of bacteria when cells approach solid surfaces by measuring the speed as a function of the separation distance. The experimental results were compared with solutions of the mathematical model of Ramia et al. (9) based on hydrodynamic forces to determine the validity of the application of this model. We also observed bacteria swimming in circles parallel to solid surfaces, as previously seen in other studies (6, 10). We illustrate the motion exhibited by both wild-type cells (cells that change direction by tumbling) and smooth-swimming cells (cells that do not tumble) through three-dimensional projections of cell traces near solid surfaces.The publication costs of this article were defrayed in part by page charge...

The initial events in bacterial adhesion are often explained as resulting from electrostatic and van der Waals forces between the cell and the surface, as described by DLVO theory (developed by Derjaguin, Landau, Verwey, and Overbeek). Such a theory predicts that negatively charged bacteria will experience greater attraction toward a negatively charged surface as the ionic strength of the medium is increased. In the present study we observed both smooth-swimming and nonmotile Escherichia coli bacteria close to plain, positively, and hydrophobically coated quartz surfaces in high-and low-ionic-strength media by using total internal reflection aqueous fluorescence microscopy. We found that reversibly adhering cells (cells which continue to swim along the surface for extended periods) are too distant from the surface for this behavior to be explained by DLVO-type forces. However, cells which had become immobilized on the surface did seem to be affected by electrostatic interactions. We propose that the "force" holding swimming cells near the surface is actually the result of a hydrodynamic effect, causing the cells to swim at an angle along the glass, and that DLVO-type forces are responsible only for the observed immobilization of irreversibly adhering cells. We explain our observations within the context of a conceptual model in which bacteria that are interacting with the surface may be thought of as occupying one of three compartments: bulk fluid, near-surface bulk, and near-surface constrained. A cell in these compartments feels either no effect of the surface, only the hydrodynamic effect of the surface, or both the hydrodynamic and the physicochemical effects of the surface, respectively.The goal of this work was to determine the force or forces controlling reversible adhesion of motile bacterial cells to surfaces. Reversible bacterial adhesion is operationally defined here as a situation in which a bacterium remains very close (within the same plane of focus for a light microscope) to a surface for a period of several minutes. Reversibly adhering bacteria are presumed to retain their ability to move laterally along the surface (37), by swimming or Brownian motion, and these cells may also eventually leave the vicinity of the surface. Cells behaving in this manner have been observed in numerous experiments and will often spend long times (Ͼ1 min) swimming near the surface (5,12,22,25,41). In irreversible adhesion, by contrast, bacteria adhering to the surface do not move, either by swimming or Brownian motion, for the duration of observation (36). In general, bacteria that have become immobilized on the surface are described as irreversibly adhered to the surface, while cells that can still swim along the surface are described as reversibly adhered. Cells may also become tethered to the surface, when a flagellum adheres to the surface but the cell body still rotates freely. Figure 1 illustrates the definitions of these terms.Adhesion of individual cells to a surface is the first step in the formation of biof...

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