  Microcolonies are aggregates of a few dozen to a few thousand cells exhibited
by many bacteria. The formation of microcolonies is a crucial step towards the
formation of more mature bacterial communities known as biofilms, but also
marks a significant change in bacterial physiology. Within a microcolony,
bacteria forgo a single cell lifestyle for a communal lifestyle hallmarked by
high cell density and physical interactions between cells potentially altering
their behaviour. It is thus crucial to understand how initially identical
single cells start to behave differently while assembling in these tight
communities. Here we show that cells in the microcolonies formed by the human
pathogen Neisseria gonorrhoeae (Ng) present differential motility behaviors
within an hour upon colony formation. Observation of merging microcolonies and
tracking of single cells within microcolonies reveal a heterogeneous motility
behavior: cells close to the surface of the microcolony exhibit a much higher
motility compared to cells towards the center. Numerical simulations of a
biophysical model for the microcolonies at the single cell level suggest that
the emergence of differential behavior within a multicellular microcolony of
otherwise identical cells is of mechanical origin. It could suggest a route
toward further bacterial differentiation and ultimately mature biofilms.
