Mitsugu MATSUSHITA
Department of Physics, Chuo University,
Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
The growth of bacterial colonies is a ``Treasure
Island'' of pattern formation. Apparently,
the patter-forming members (bacterial cells)
are strongly interesting, self-reproducible
and self-driven ``particles'', very different
from conventional physical systems. Some
colony patterns grown on the surface of agar
plates can be understood in terms of well-defined
models of pattern formation such as DLA,
but not in call cases.
Here we used a bacterial species {\it Proteus
mirabilis}. This species has been well-known
for more than a century to form a macroscopically
almost perfect concentric-ring like colony
with approximately equal spacing of terraces
on the surface of semi-solid substrate such
as an agar plate. The colony grows cyclically
with the interface repeating an advance (migration)
and a step (consolidation) alternately. We
distinguish three phases (initial lag phase,
the following migration and consolidation
phases that appear alternately) for the colony
growth. Even when we decreased the inoculum
cell density to some threshold value, the
interval of the lag phase was almost constant.
However, if the inoculum density was decreased
below th threshold, the lag phase became
prolonged. This clearly suggests that it
is necessary for the cell density to reach
some threshold value for the onset of migration.
When we cut a colony just behind a migrating
front shortly after, say, the fourth migration
started, the first migration after the cut
ended earlier and the following consolidation
lasted longer. However, the following cycles
were not influenced by the cut at all. We
also checked phase entrainment by letting
two colonies collide with each other and
found that it does not take place in macroscopic
scales.
All these experimental results suggest that
(1) no global chemical signals from the center
exist for the colony formation,
(2) the periodic growth cannot be explained
by the idea of interacting nonlinear
oscillators, and
(3) there may be two threshold values of
cell density, the higher threshold for the
onset of migration and the lower one for
the onset of consolidation.
In order to form periodic colonies {it Proteus
mirabilis} seems to use ``quorum sensing''
mechanism which is intrinsically multicellular.