The emergence of phase separation as an organizing principle in bacteria

Recent investigations in bacteria suggest that membraneless organelles play a crucial role in the subcellular organization of bacterial cells. However, the biochemical functions and assembly mechanisms of these compartments have not yet been completely characterized. This Review assesses the current methodologies used in the study of membraneless organelles in bacteria, highlights the limitations in determining the phase of complexes in cells that are typically an order of magnitude smaller than a eukaryotic cell, and identifies gaps in our current knowledge about the functional role of membraneless organelles in bacteria. Liquid-liquid phase separation (LLPS) is one proposed mechanism for membraneless organelle assembly. Overall, we outline the framework to evaluate LLPS in vivo in bacteria, we describe the bacterial systems with proposed LLPS activity, and we comment on the general role LLPS plays in bacteria and how it may regulate cellular function. Lastly, we provide an outlook for super-resolution microscopy and single-molecule tracking as tools to assess condensates in bacteria. Statement of SignificanceThough membraneless organelles appear to play a crucial role in the subcellular organization and regulation of bacterial cells, the biochemical functions and assembly mechanisms of these compartments have not yet been completely characterized. Furthermore, liquid-liquid phase separation (LLPS) is one proposed mechanism for membraneless organelle assembly, but it is difficult to determine subcellular phases in tiny bacterial cells. Thus, we outline the framework to evaluate LLPS in vivo in bacteria and we describe the bacterial systems with proposed LLPS activity in the context of these criteria.