  The unconstrained genomic DNA of bacteria forms a coil, which volume exceeds
1000 times the volume of the cell. Since prokaryotes lack a membrane-bound
nucleus, in sharp contrast with eukaryotes, the DNA may consequently be
expected to occupy the whole available volume when constrained to fit in the
cell. Still, it has been known for more than half a century that the DNA is
localized in a well defined region of the cell, called the nucleoid, which
occupies only 15% to 25% of the total volume. Although this problem has focused
the attention of many scientists for the past decades, there is still no
certainty concerning the mechanism that enables such a dramatic compaction. The
goal of this Topical Review is to take stock of our knowledge on this question
by listing all possible compaction mechanisms with the proclaimed desire to
clarify the physical principles they are based upon and discuss them in the
light of experimental results and the results of simulations based on
coarse-grained models. In particular, the fundamental differences between
psi-condensation and segregative phase separation and between the condensation
by small and long polycations are highlighted. This review suggests that the
importance of certain mechanisms, like supercoiling and the architectural
properties of DNA-bridging and DNA-bending nucleoid proteins, may have been
overestimated, whereas other mechanisms, like segregative phase separation and
the self-association of nucleoid proteins, as well as the possible role of the
synergy of two or more mechanisms, may conversely deserve more attention.
