Changes in the morphology of and were examined with the electron microscope during growth in liquid media in shaken cultures. Cells at the early stage of exponential growth are characterised by a surface layer of fibrillar material consisting of cell-wall lipopolysaccharide, by the peripheral distribution of both single and clustered ribosomes, and by a fine network of nuclear filaments.

Late exponentially growing cells show cell walls overlaid with an amorphous lipopolysaccharide material which differs chemically from the fibrillar layer. The ribosomes at this stage are scattered throughout the cytoplasm.

Fibrillae, possibly ribonucleic acid in nature, interconnect single ribosomal particles during all stages of exponential growth.

In stationary-phase cells the nuclear filaments are concentrated in the central portion of the cytoplasm. Fine networks and bundles of fibrils, varying in size, become detached from the cell walls. After maximal growth, ribosomal structures disappear and the plasma membrane becomes detached from the inner surface of the cell wall, as an initial sign of plasmolysis.

Morphological differences between the cell-wall structure of and that of are noted near the stage of maximal growth. In the former species, the cell wall consists of a dense thick layer, possibly containing large amounts of free lipid. Cell walls in the latter species are composed of a thin dense layer which is separated from the plasma membrane by a moderately dense layer.


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