Cellular DNA in bacteria is localized into nucleoids enclosed by cytoplasm. The forces which cause condensation of the DNA into nucleoids are poorly understood. We suggest that direct and indirect macromolecular crowding forces from the surrounding cytoplasm are critical factors for nucleoid condensation, and that within a bacterial cell these crowding forces are always present at such high levels that the DNA is maintained in a condensed state. The DNA affected includes not only the preexisting genomic DNA but also DNA that is newly introduced by viral infection, replication or other means.
