A new class of mutants of Salmonella typhimurium (IkyD mutants) are described. The mutants are defective in morphogenesis of the division septum, and are characterized by a failure of the outer membrane to invaginate despite normal ingrowth of the cytoplasmic membrane and murein layers of the growing septum. The cell envelopes of the mutants show a significant decrease in the bound form of murein-lipoprotein and a corresponding increase in the free form of the lipoprotein. This suggests that the morphogenetic defect may result from a defect in formation of covalent bonds between the free lipoprotein of the outer membrane and the murein of the nascent septum.In all cells that divide by binary fission, the process of cell division requires ingrowth of a septum at the mid-point of the cell in a sequence that must be controlled both spatially and temporally. In most cases, this occurs by the circumferential invagination of the components of the cell envelope and eventually leads to complete separation of the two daughter cells. The biochemical mechanisms responsible for this complex morphogenetic event are unknown.In Gram-negative bacteria, the cell envelope consists of three layers-cytoplasmic membrane, murein layer, and outer membrane (Fig. In). During normal cell division these components invaginate coordinately to form the new septum. In this paper, we describe a new class of bacterial mutants characterized by uncoupling of the normally coordinate ingrowth of these three layers during septum formation.

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