  Formation of spatial patterns of cells is a recurring theme in biology and
often depends on regulated cell motility. Motility of M. xanthus depends on two
motility machineries: the S-engine and A-engine. Moving M. xanthus cells can
organize into spreading colonies or spore-filled fruiting bodies depending on
their nutritional status. To understand these two pattern formation processes
and the contributions by the two motility machineries, as well as cell
reversal, we analyze spatial self-organization in 3 strains: i) a mutant that
moves unidirectionally without reversing by the A-motility system only, ii) a
unidirectional mutant that is also equipped with the S-motility system, and
iii) the wild-type that, in addition to the two motility systems, reverses its
direction of movement. The mutant moving by the A-engine illustrates that
collective motion in the form of large moving clusters can arise in gliding
bacteria due to steric interactions of the rod-shaped cells, without the need
of invoking any biochemical signal regulation. The two-engine strain mutant
reveals that the same phenomenon emerges when both motility systems are
present, and as long as cells exhibit unidirectional motion only. From the
study of these two strains, we conclude that unidirectional cell motion induces
the formation of large moving clusters at low and intermediate densities, while
it results into vortex formation at very high densities. These findings are
consistent with what is known from self-propelled rods which strongly suggests
that the combined effect of self-propulsion and volume exclusion interactions
is the pattern formation mechanism leading to the observed phenomena. In
addition, we learn that when cells reverse, as observed in the wild-type, cells
form small but strongly elongated clusters and self-organize into a mesh-like
structure at high enough densities.
