The mechanical properties of Rhodococcus RC291 were measured using force spectroscopy equipped with a bacterial cell probe. Rhodococcal cells in the late growth stage of development were found to have greater adhesion to a silicon oxide surface than those in the early growth stage. This is because there are more extracellular polymeric substances (EPS) that contain nonspecific binding sites available on the cells of late growth stage. It is found that EPS in the late exponential phase are less densely bound but consist of chains able to extend further into their local environment, while the denser EPS at the late stationary phase act more to sheath the cell. Contraction and extension of the EPS could change the density of the binding sites, and therefore affect the magnitude of the adhesion force between the EPS and the silicon oxide surface. By treating rhodococcal EPS as a surface-grafted polyelectrolyte layer and using scaling theory, the interaction between EPS and a solid substrate was modelled for the cell approaching the surface which revealed that EPS possess a large capacity to store charge. Changing the pH of the surrounding medium acts to change the conformation of EPS chains.

ÐÐÐÐÐÐÐÐÐWe consider the behaviour of single molecules on surfaces and, more generally, in confined environments. These are loosely split into three sections: single molecules in biology, the physics of single molecules on surfaces, and controlled (directed) diffusion. With recent advances in single molecule detection techniques, the importance and mechanisms of single molecule processes such as localised enzyme production and intracellular diffusion across membranes has been highlighted, emphasising the extra information that cannot be obtained with techniques, which present average behaviour. Progress has also been made in producing artificial systems that can control the rate and direction of diffusion, and because these are still in their infancy (especially in comparison to complex biological systems), we discuss the new physics revealed by these phenomena. Introduction: single molecule diffusionThe motion of the very small has been studied for a long time, [1,2] beginning with the initial microscopic observations of pollen on water in 1827 and continuing in the present day with a vast array of molecular diffusion studies. Initially such studies concentrated on the average -2 -motion of an ensemble of molecules as techniques such as fluorescence recovery after photobleaching [3] (FRAP) were not sensitive enough to observe an individual particle. Despite this, averaging techniques have been, and continue to be, very successful in probing protein dynamics [4][5][6] and protein-protein interactions, [7,8] as well as determining average diffusion coefficients of molecules within a small region. [9,10] Recent advances in experimental techniques have allowed the motion and interactions of single molecules to be studied with improved accuracy. Some of these techniques, such as atomic force microscopy, [11,12] total internal reflection fluorescence (TIRF) imaging, [13,14] and super-resolution imaging, [15][16][17] have allowed direct images to be produced which provide insight into the orientation, [18] clustering, or changes to a molecule within a system. Time-stop imaging techniques have also been used to determine the kinetic properties of a system. [19] However these are somewhat limited by the equipment in terms of exposure time, acquisition rate, and size of detection region. In general, imaging is useful as it provides visual confirmation of the region under study. We show in Figure 1 data exemplifying why single molecule imaging reveals more information than ensemble averaging techniques; here molecular motion is smeared out of the signal when the data are averaged (box 4 in Figure 1), but time-stop imaging reveals a complex molecular trajectory (box 3 in Figure 1).Techniques designed to examine the diffusive properties of molecules within a sample, such as neutron spin echo, [20][21][22] dynamic light scattering, [23,24] fluorescence correlation spectroscopy (FCS), [25][26][27][28] and single mode optical fibre detectors, [29] generally do not involve imaging in real space, but require spectroscopic determ...

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