  Growing tissue and bacterial colonies are active matter systems where cell
divisions and cellular motion generate active stress. Although they operate in
the non-equilibrium regime, these biological systems can form large-scale
ordered structures such as nematically aligned cells, topological defects, and
fingerings. Mechanical instabilities also play an essential role during growth
by generating large structural folding. How active matter dynamics and
mechanical instabilities together develop large-scale order in growing tissue
is not well understood. Here, we use chain forming Bacillus subtilis, also
known as a biofilm, to study the direct relation between active stress and
nematic ordering. We find that a bacterial biofilm has intrinsic length scales
above which series of mechanical instabilities occur. Localized stress and
friction control both linear buckling and edge instabilities. Remarkably, these
instabilities develop nematically aligned cellular structures and create pairs
of motile and stationary topological defects. We also observe that stress
distribution across the biofilm strongly depends on the defect dynamics which
can further initiate the formation of sporulation sites by creating
three-dimensional structures. By investigating the development of bacterial
biofilms and their mechanical instabilities we are proposing a new type of
active matter system which provides a unique platform to study the essential
roles of nematics in growing biological tissue.
