On the Plasma Membrane of Some Bacteria and Fungi

The contents of bacterial cells can be reversibly pulled away. (plasmolyzed) from the wall by strong salt solutions. Bacterial “protoplasts”—living cells divested of their wall—shrink or swell with changing concentrations of solutes in their environment and burst when this concentration falls below a critical level. These observations have long suggested that the cytoplasm of bacteria is covered by a selectively permeable plasma membrane. In recent years electron microscopy has revealed the existence of a narrow profile at the surface of the cytoplasm of bacteria. In some bacteria, these profiles have the dimensions of sections of the “unit membrane” found in the interior and at the surface of plant and ainimal cells. In the place of a unit membrane, single tracks of densely stained granules are found in many bacteria. New electron micrographs confirm the observation of Young and Fitz-James (1959) that the first spore membrane is derived fromn an infolding of the plasma membrane of the mother bacterium. Illustrations are provided of the plasmna membrane of B. mycoides and of the yeast Lipomyces lipofer.