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

An interesting question in active matter physics is what states of matter may arise in active matter and how different they are from thermal systems. Bacterial populations are particularly interesting in this context, as they are known to show a plethora of collective phases, yet it remains challenging to characterize dense states of bacteria, partly because it is experimentally difficult to realize a uniform growth condition for dense populations.

Here we overcome this by a recently developed membrane-based device [1] and report the emergence of glassy states in two-dimensional suspension of Escherichia coli [2]. 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 be relevant in broad contexts including biofilms and active rod systems in general.

[1] T. Shimaya et al., Commun. Phys. 4, 238 (2021) https://doi.org/10.1038/s42005-021-00739-5

[2] H. Lama et al., arXiv:2205.10436 https://doi.org/10.48550/arXiv.2205.10436