  The volume occupied by unconstrained bacterial DNA in physiological solutions
exceeds 1000 times the volume of the cell. Still, it is confined to a well
defined region of the cell called the nucleoid, which occupies only a fraction
of the cell volume. There is still no general agreement on the mechanism
leading to the compaction of the DNA and the formation of the nucleoid.
However, advances in in vivo sub-wavelength resolution microscopy techniques
have recently allowed the observation of the nucleoid at an unprecedented level
of detail. In particular, these observations show that the compaction of the
nucleoid is not static but is instead a highly dynamic feature, which depends
on several factors, like the richness of the nutrient, the cell cycle stage,
temperature, the action of an osmotic shock or antibiotics, etc. After a short
description of the electrolyte content of the cytosol and a brief overview of
the different mechanisms that may lead to the formation of the nucleoid, this
paper reviews some of the most fascinating recent results of in vivo
sub-wavelength resolution microscopy. It is furthermore argued that these
observations provide converging indications in favor of a model that describes
the cytosol as an aqueous electrolyte solution containing several
macromolecular species, where demixing and segregative phase separation occur
between DNA and RNA (essentially rRNA and mRNA involved in translation
complexes, but also the large amounts of rRNA synthesized at the rrn operons of
cells growing in rich media). It is also pointed out that crowding may play a
crucial role through its synergy with electrostatic forces. By constraining
macromolecules to remain at short distances from one another and feel
electrostatic interactions in spite of the strong screening exerted by
electrolyte species, crowding favors stronger DNA/RNA demixing and nucleoid
compaction.
