  Liquid-liquid phase separation is emerging as a crucial phenomenon in several
fundamental cell processes. A range of eukaryotic systems exhibit liquid
condensates. However, their function in bacteria, which in general lack
membrane-bound compartments, remains less clear. Here, we used high-resolution
optical microscopy to observe single bacterial aggresomes, nanostructured
intracellular assemblies of proteins, to undercover their role in cell stress.
We find that proteins inside aggresomes are mobile and undergo dynamic
turnover, consistent with a liquid state. Our observations are in quantitative
agreement with phase-separated liquid droplet formation driven by interacting
proteins under thermal equilibrium that nucleate following diffusive collisions
in the cytoplasm. We have discovered aggresomes in multiple species of
bacteria, and show that these emergent, metastable liquid-structured protein
assemblies increase bacterial fitness by enabling cells to tolerate
environmental stresses.
