THE CELL WALL AND CELL DIVISION OF GRAM-NEGATIVE BACTERIA

Abstract

The cell wall of Gram-negative bacteria is generally complex and multilayered. Many, exemplified by Escherichia coli and Spirillum serpens in this study, show a constrictive division in which die cell appears to be pinched in the middle and septa are not seen. It was found that sections of E. coli and S. serpens show true septum formation in a high proportion of dividing cells when they were grown and fixed at 45°. Cells grown at 45° still showed some septum formation if cooled to 20° before fixation. However, septa were demonstrated in cells grown and fixed at 30° when the buffer used in the Ryter and Kellenberger fixative was diluted 1:6; it would appear that the tonicity of the fixing environment is critical. These septa involve an annular invagination of the plasma membrane and the concomitant synthesis of an extremely thin cell wall septum, which appears to consist of the mucopeptide layer alone. Centripetal splitting of this layer and synthesis of the outer layers of the wall follow but are sufficiently delayed that a complete septum was often visible. This form of septum has been known as the normal habit fur some and can now be considered normal for most, if not all, Gram-negative bacteria. The reason for the appearance of constriction in dividing cells fixed by commonly accepted methods remains obscure but attention is drawn to the problems of artefact in the preservation and study of dynamic structures at high resolution.