Ultrastructure of the cell walls of two closely related clostridia that possess different regular arrays of surface subunits

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Abstract
Cell walls of Clostridium thermohydrosulfuricum and C. thermosaccharolyticum have a 2-layered structure, consisting of a thin, lysozyme-sensitive murein layer and a surface (S) layer composed of hexagonally or tetragonally arranged subunits. The subunits can be removed from the murein layer by treatment with guanidine hydrochloride and urea. Intact S layers, obtained by lysozyme treatment of cell wall preparations, are composed of a fragile, pH-sensitive monolayer of macromolecular subunits. In both organisms the 1st stage of the cell division process involves only the plasma membrane and the murein layer. During the subsequent cell separation, a surplus of S-layer subunits appears at the site of division and consequently the newly formed cell poles remain completely covered by the S layer throughout the separation process. In autolyzed cells an additional layer of subunits assembles on extended areas of the inside of the mucopeptide layer. The biological function of the S layer apparently depends on its ability to maintain a complete covering of the cell surface at all stages of cell growth and division.