Ralstonia solanacearum is a rod-shaped phytopathogenic bacterium that
causes lethal wilt disease in many plants. On solid agar growth medium,
in the early hour of the growth of the bacterial colony, the type IV
pili-mediated twitching motility, which is important for its virulence
and biofilm formation, is prominently observed under the microscope. In
this study, we have done a detailed observation of twitching motility in
R. solanacearum colony. In the beginning, twitching motility in
microcolonies was observed as a density-dependent phenomenon that
influences the shape and sizes of the microcolonies. No such phenomenon
was observed in Escherichia coli, where twitching motility is absent. In
the early phase of colony growth, twitching motility exhibited by the
cells at the peripheral region of the colony was more prominent than the
cells towards the centre of the colony. Using a time scale photography
and merging those into a video, twitching motility was observed as an
intermittent phenomenon that progresses in layers in all directions as
finger-like projections at the peripheral region of a bacterial colony.
Each layer of bacteria twitches on top of the other and produces a
multi-layered film-like appearance. We found that the duration between
the emergence of each layer diminishes progressively as the colony
becomes older. This study on twitching motility demonstrates distinctly
heterogeneity among the cells within a colony regarding their dynamics
and the influence of microcolonies on each other regarding colony shape
and size.