Emergence of bacterial glass: two-step glass transition in 2D bacterial suspension

Bacteria show a plethora of collective phases, such as a swarming state and an orientationally ordered phase. Understanding how their collective states result from individual activities, such as growth, motion, and interactions, is a crucial facet of physical and biological studies on bacteria, with far-reaching implications including colonies and biofilms. Dense bacterial populations are expected to show particularly rich emergent phases, which however remain largely unexplored. A difficulty was to realize a uniform growth condition for dense populations. Here we overcome this by a recently developed membrane-based device and report the emergence of glassy states in two-dimensional (2D) 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 freedom of bacteria, and the second one vitrifying the bacteria completely. We also characterize individual motion of bacteria, and find spontaneous formation of micro-domains of aligned cells. This leads to collective motion, which results in unusual behavior of characteristic quantifiers of glass. Our model experiment of dense bacteria may impact broad contexts including biofilms and active rod systems in general.