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

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.