Atomic Force Microscopy Imaging and Force Spectroscopy of Microbial Cell Surfaces
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Characterization of the structural and physical properties of microbial cell surfaces is a continuously expanding field of microbiology. Studying cell surfaces is important not only in basic research to elucidate their functions (cell shape, protection, molecular sieve, molecular recognition, cell adhesion, and cell aggregation), but also in medicine (fouling of implants, microbial infections) and biotechnology (cellular interactions in fermentation technology, removal of heavy metals). Electron microscopy has long been the technique of choice for probing cell surface structure down to the molecular level. Yet the requirement of vacuum conditions may raise the question of whether the information obtained is always relevant to the real, hydrated state. In parallel, a variety of approaches have been developed to probe the cell surface physical properties, including chemical analysis of wall constituents, specific binding studies, selective degradation by enzymes, cell wall mutants, modifications by antibiotics, and surface analysis using various physical techniques. Many of these techniques provide averaged information obtained on a large ensemble of cells; that is, they are not suited for mapping the nanoscale distribution of physical properties on individual cells.
Since the late 1980s, atomic force microscopy (AFM) has been increasingly used in biology and it is now established as a versatile tool to address the structure, properties, and functions of biological structures, going from single molecules to lipid membranes and living cells. Basically, AFM offers two main advantages over conventional microscopy techniques. First, it provides images of the specimen with nanometer (subnanometer) resolution, in real-time and under physiological conditions. Second, because the instrument works by sensing the force between a very sharp tip and the sample surface, this principle can be exploited to measure molecular interactions and physical properties on a local scale. These capabilities provide a range of novel opportunities in microbiology. For example, the following questions can now be addressed with AFM. Does the surface nanostructure of untreated living cells correlate with that observed by electron microscopy? How does surface structure change with time during dynamic processes such as cell growth and cell division? How do external agents such as enzymes and antibiotics affect the cell surface architecture? Do surface properties such as hydrophobicity, charge, and elasticity vary across the surface of a single cell? What are the intermolecular forces involved in molecular recognition and cellular interactions? What is the elasticity of single-cell surface macromolecules? The intent of this article is to survey recent achievements brought by AFM in microbiology, particularly emphasizing studies on whole cells. Rather than providing an exhaustive review of the literature in the area, the paper discusses imaging and force spectroscopy applications using a selection of recent data.