SUMMARY: The structure of surface layers of the bacterial form and different types of the L-form of was studied by electron microscopy of thin-sectioned organisms. Morphological data confirm the distinction between the cell wall-less protoplast-L-form and the wall-containing spheroplast-L-form. Organisms of the protoplast-L-form have only one surface-integument, presumably the cytoplasmic membrane. These forms never revert to the bacterial form. The spheroplast-L-form comprises reversible forms (unstable-sphero-plast-L-form) as well as non-reverting strains (stable spheroplast-L-form). In both spheroplast-L-forms two surface integuments are always present: a cytoplasmic membrane and a superposed cell wall. In sections of isolated cell walls of normal Proteus bacteria and in wall material spontaneously dissociating from damaged cell walls of spheroplast-L-forms the triple-layered ‘unit membrane’ is a prominent feature. Thin sections of isolated cell wall lipopolysaccharide identify the unit membrane as a specific structure of this polymer. The thickness of the murein (syn. mucopeptide, mucopolymer) in isolated murein layers (‘murein sacculi’, Weidel & Pelzer, 1964) from cell walls of normal Proteus bacteria was found to be approximately 20–25 å.


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