Depletion interactions play a crucial role in crowded environments such as the one inside a human cell. While spherical depletants like polyethylene glycol (PEG) have been widely used to induce depletion, our study employs duplex DNA as depletants. We report on a surprising phase separation in a suspension of
Bacillus subtilis, leading to aggregation when DNA is present at micromolar concentrations. In contrast, polyethylene glycol, a common depletant, causes a phase separation and aggregation of the bacteria cells at millimolar concentrations. We explain these striking results using simple arguments showing that rods are extremely strong depletants when compared to spheres. Through optical microscopy, dynamic light scattering (DLS), and turbidity measurements, we also quantify the aggregation kinetics and structural organization of bacterial suspensions under varying depletant concentrations. We find that the depletant shape has pronounced effects on bacterial clustering, phase behavior, aggregate morphology, and mobility.
Overall, these insights deepen our understanding of depletion-driven bacterial organization and highlight the fundamental role of depletant shape in modulating bacterial interactions. Crucially, as DNA is present in vivo and, in most contexts, wherein bacteria are present, our study is of practical relevance in describing phenomenology pertaining to bacterial systems.
