The variable T model for gram-negative morphology.

Gram-negative micro-organisms possess only a very thin murein sacculus to resist the stress caused by the internal hydrostatic pressure. The sacculus consists of at most one molecular layer of peptidoglycan in an extended conformation. It must grow by the insertion and cross-linking of new murein to the old before the selective cleavages of the stress-bearing murein are made which allow wall enlargement. Since insertion of new murein occurs all over the surface of Escherichia coli (even in completed poles), the internal pressure would tend to force the cells into a spherical shape and prevent both cylindrical elongation and cell division. Of course, Gram-negative bacteria do achieve a variety of shapes and do divide. Because prokaryote cells, unlike eukaryotic cells, do not have cytoskeletons and contractile proteins to transduce biochemical free energy into the mechanical work needed to achieve aspherical shapes and to divide, this paradox seems to be resolvable only by postulating that the details of the biochemical mechanism for wall growth vary in different regions of the surface, affecting the work required to enlarge the wall locally. Depending on the degree and rate of change in the biochemical energetics, it is possible to account for rod and the other more complex shapes of Gram-negative bacteria. Division occurs in Gram-negative organisms by the development of constrictions that progressively invade the cytoplasm. The work to cause these morphological processes must ultimately derive from the biochemical process of the stress-bearing wall formation. A biophysical basis for cell division in these prokaryotic organisms is proposed.