  Densely packed, motile bacteria can adopt collective states not seen in
conventional, passive materials. These states remain in many ways mysterious,
and their physical characterization can aid our understanding of natural
bacterial colonies and biofilms as well as materials in general. Here, we
overcome challenges associated with generating uniformly growing, large,
quasi-two-dimensional bacterial assemblies by a membrane-based microfluidic
device and report the emergence of glassy states in two-dimensional suspension
of Escherichia coli. As the number density increases by cell growth,
populations of motile bacteria transition to a glassy state, where cells are
packed and unable to move. This takes place in two steps, the first one
suppressing only the orientational modes and the second one vitrifying the
motion completely. Characterizing each phase through statistical analyses and
investigations of individual motion of bacteria, we find not only
characteristic features of glass such as rapid slowdown, dynamic heterogeneity
and cage effects, but also a few properties distinguished from those of thermal
glass. These distinctive properties include the spontaneous formation of
micro-domains of aligned cells with collective motion, the appearance of an
unusual signal in the dynamic susceptibility, and the dynamic slowdown with a
density dependence generally forbidden for thermal systems. Our results are
expected to capture general characteristics of such active rod glass, which may
serve as a physical mechanism underlying dense bacterial aggregates.
