Periplasmic gel: new concept resulting from the reinvestigation of bacterial cell envelope ultrastructure by new methods

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Abstract
Bacterial cell envelope ultrastructure was investigated both by the progressive lowering of temperature embedding technique and freeze-substitution, using conventional and scanning transmission electron microscopy. Comparison with standard embedding procedures revealed a new aspect of cell envelope structure in specimens at low temperatures. The envelope was delimited by an electron-dark layer, beneath which was a uniform matter-containing layer lying between the outer and inner membranes. There was no empty periplasmic space. Buoyant densities of isolated peptidoglycan obtained in Percoll (1.02 to 1.07 g ml-1) and CsCl2 (1.44 g ml-1) led to a calculated hydration of the peptidoglycan which was more than was previously assumed. Peptidoglycan therefore possibly fills the entire space between the inner and outer membranes in the form of a periplasmic gel. The new model of cell envelope organization is discussed with respect to the current knowledge on bacterial cell wall structure and function.