More than just a phase: the search for membraneless organelles in the bacterial cytoplasm

The bacterial cytoplasm, once thought to be a relatively undifferentiated reaction medium, has now been recognized to have extensive microstructure. This microstructure includes bacterial microcompartments, inclusion bodies, granules, and even some membrane-bound vesicles. Several recent papers suggest that bacteria may also organize their cytoplasm using an additional mechanism: phase-separated membraneless organelles, a strategy commonly used by eukaryotes. Phase-separated membraneless organelles such as Cajal bodies, the nucleolus, and stress granules allow proteins to become concentrated in sub-compartments of eukaryotic cells without being surrounded by a barrier to diffusion. In this review, we summarize the known structural organization of the bacterial cytoplasm and discuss the recent evidence that phase-separated membraneless organelles might also play a role in bacterial systems. We specifically focus on bacterial ribonucleoprotein complexes and two different protein components of the bacterial nucleoid that may have the ability to form subcellular partitions within bacteria cells.