The periseptal annulus: An organelle associated with cell division in Gram-negative bacteria.

Evidence is presented that the site of cell division in Salmonella typhimurium is flanked by two circumferential zones of cell envelope differentiation, the periseptal annuli, which separate the division site from the remainder of the cell envelope. Each annulus is composed of a continuous structure in which the membranous elements of the cell envelope are closely associated with the murein cytoskeleton. The paired annuli appear early in the division process and the region between them defines a new cellular domain, the periseptal compartment, within which the division septum is formed.