The Escherichia coli chromosome is organized with the left and right chromosome arms in separate cell halves.

We have developed a system for the simultaneous labelling of two specific chromosomal sites using two different fluorescent ParB/parS systems. Using this, we demonstrate that the two chromosome arms are spatially arranged in newborn cells such that markers on the left arm of the chromosome lie in one half of the cell and markers on the right arm of the chromosome lie in the opposite half. This is achieved by reorganizing the chromosome arms of the two nucleoids in pre-division cells relative to the cell quarters. The spatial reorganization of the chromosome arms ensures that the two replication forks remain in opposite halves of the cell during replication. The relative orientation of the two reorganized nucleoids in pre-division cells is not random. Approximately 80% of dividing cells have their nucleoids oriented in a tandem configuration.