Bacterial shape: two-dimensional questions and possibilities

Abstract

Events in the past decade have made it both possible and interesting to ask how bacteria create cells of defined length, diameter, and morphology. The current consensus is that bacterial shape is determined by the coordinated activities of cytoskeleton complexes that drive cell elongation and division. Cell length is most easily explained by the timing of cell division, principally by regulating the activity of the FtsZ protein. However, the question of how cells establish and maintain a specific and uniform diameter is, by far, much more difficult to answer. Mutations associated with the elongation complex often alter cell width, though it is not clear how. Some evidence suggests that diameter is strongly influenced by events during cell division. In addition, surprising new observations show that the bacterial cell wall is more highly malleable than previously believed and that cells can alter and restore their shapes by relying only on internal mechanisms.

Figure 1

Composition of the division and elongation machinery in E. coli. The division apparatus (on the left) and the cell elongation apparatus (on the right) are anchored by polymers of the FtsZ and MreB cytoskeleton proteins, respectively (blue circles). Each is joined by additional cytoplasmic proteins (pink trapezoids), membrane associated proteins (light blue rectangles) and membrane bound or soluble periplasmic enzymes (green and purple circles). Not all involved proteins are pictured. The identities and functions of these and other accessory proteins are described elsewhere (10, 11, 13, 32, 60, 75, 92). The bottom figure represents an elongated E. coli cell just prior to the beginning of septation. The elongation apparatus is active along the side wall in the bulk of the cell cylinder, whereas the division apparatus is active only at the cell center. Two inhibitors of FtsZ ensure that the division ring (blue circle) assembles only at the cell center. When the cell grows to a sufficient length, the concentration of MinC protein (in a gradient represented by the red triangle) becomes low enough to allow assembly of the FtsZ ring. A second FtsZ inhibitor, SlmA, binds to the bacterial chromosome and inhibits FtsZ assembly in an area around the nucleoid (central dark blue zone).