Scanning tunneling microscope imaging of hoops from the cell sheath of the bacteria methanospirillum hungatei and atomic force microscope imaging of complete sheathes

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Abstract
This paper describes techniques used to image the cell sheath of the archaebacterium methanospirillum hungatei. Scanning tunneling microscope (STM) studies were done on hoop-shaped components separated from the cell sheath by chemical treatment. High-quality images were only obtained when the hoops were dispersed on highly oriented pyrolytic graphite and overcoated with a conductor, although hopping scans occasionally could image uncoated sheath fragments. Atomic force microscope (AFM) studies of the entire sheath showed the characteristic surface corrugation that was also seen in STM and in electron microscope images. We suggest that the STM imaging of any large biological structures may be limited principally by low electrical conductivity, although tip-sample interactions and poor adhesion to the substrate also contribute to difficulties in the technique.