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The structure of the cell wall of Spirillum serpens was studied with the electron microscope in shadowed preparations, in the negative-contrast media of phosphotungstic acid (PTA) and uranyl acetate, and in sections of both whole organisms and isolated cell wall fragments. The structured external surface layer described by Houwink in 1953 was confirmed and identified in both PTA preparations and in sections of isolated walls. It lies outside all the other layers (outer "unit" membrane and the inner dense layer) normally identified in sections of intact cells, it is the component characterized by Martin and Frank (1962) as lipoprotein and is quickly removed by 1% sodium lauryl sulphate or by water saturated with phenol. The exact site of the mucopeptide structural layer has not been identified.By comparing measurements and areas of staining (or permeation) by uranyl acetate, osmium tetroxide, and PTA the unit structure of the lipoprotein layer was partially clarified and a tentative model proposed. A six-spoked arrangement of 20 × 60 Å subunits around a hub forms 100 Å polygonal units with links of Y-form between them. The central "hole" in the unit is the hub and the prominent hexagon surrounding the hub is probably formed both by lack of resolution of contiguous parts and a small number of subunits superimposed on the basic network. The whole unit is embedded in a globule of substance to form the structures observed in shadowed preparations and these structures in turn are enclosed in a relatively delicate matrix, which completes and encloses this outermost zone of the cell wall. When these units are stripped away by lauryl sulphate and PTA in combination, a delicate, Δ-linked, pattern of the same spacing is revealed. Its exact place is uncertain.The packing of units is hexagonal (5:3:2 cubical symmetry) and topology demands the discontinuity provided by six pentagonal arrangements of units in order to cover the hemispherical surfaces at each pole of Spirillum. Search provided a few such pentagons.
R. G. E. Murray (Sat,) studied this question.