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A systematic investigation into the effect of surface chemistry on bacterial adhesion was carried out. In particular, a number of physicochemical factors important in defining the surface at the molecular level were assessed for their effect on the adhesion ofListeria monocytogenes, Salmonella typhimurium,Staphylococcus aureus, and Escherichia coli. The primary experiments involved the grafting of groups varying in hydrophilicity, hydrophobicity, chain length, and chemical functionality onto glass substrates such that the surfaces were homogeneous and densely packed with functional groups. All of the surfaces were found to be chemically well defined, and their measured surface energies varied from 15 to 41 mJ · m−2. Protein adsorption experiments were performed with3H-labelled bovine serum albumin and cytochromec prior to bacterial attachment studies. Hydrophilic uncharged surfaces showed the greatest resistance to protein adsorption; however, our studies also showed that the effectiveness of poly(ethyleneoxide) (PEO) polymers was not simply a result of its hydrophilicity and molecular weight alone. The adsorption of the two proteins approximately correlated with short-term cell adhesion, and bacterial attachment for L. monocytogenes and E. coli also correlated with the chemistry of the underlying substrate. However, for S. aureus and S. typhimurium a different pattern of attachment occurred, suggesting a dissimilar mechanism of cell attachment, although high-molecular-weight PEO was still the least-cell-adsorbing surface. The implications of this for in vivo attachment of cells suggest that hydrophilic passivating groups may be the best method for preventing cell adsorption to synthetic substrates provided they can be grafted uniformly and in sufficient density at the surface.

letters to nature 312 NATURE | VOL 398 | 25 MARCH 1999 | www.nature.comComparing Figs 2a, b and 3, we see that the initially thin but highly strained layer of lattice expansion produces the broad, lowintensity lines at early time delays. As the strain wave propagates away from the surface, the layer of surface expansion thickens and these lines correspondingly become narrower and more intense, but diffract at smaller angular deviations due to the weaker average strain. In parallel, the compression wave leading the propagation into the bulk contributes to the slight shift to higher angles seen in the main Ka lines. At the longest delays, the strain wave has largely moved beyond the depths probed by the X-ray pulse, and we see diffraction lines broadened and shifted slightly to lower angles due to the exponential surface strain of the relaxed lattice. The strain for these late times is simply proportional to the temperature distribution. Finally, the theoretical angle-integrated diffraction signal similarly reproduces the monotonic increase and plateau behaviour seen in the experimental data.Additionally, we performed an iterative genetic-algorithm inversion, obtaining the strain from the measured data. The angle-and time-resolved diffraction curves corresponding to the retrieved strain are shown in Fig. 2c. Spectral and geometrical broadening of the diffraction lines signi®cantly limits the uniqueness of the result. Nevertheless, we obtain qualitatively similar strain behaviour to that shown in Fig. 3. We retrieve an exponential surface strain, with 8-mA Ê peak strain, and a unipolar, 3-mA Ê strain pulse which propagates into the bulk at several thousand metres per second. Although the physical model that we used is fully consistent with the measured data (to within our experimental resolution), the spectral and geometrical broadening mentioned above mean that it is not a unique interpretation of that data.This work, using a table-top laboratory apparatus, demonstrates with a simple crystalline material the direct observation of millia Êngstro Èm atomic motion on the picosecond timescale. Further developments of the technique should permit direct observation by ultrafast X-ray diffraction of the ultrafast atomic motions accompanying a wide variety of physical, chemical, and perhaps biological processes.M

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