Myxococcus xanthus, a social bacterium, forms colonies that spread into a thin layer of cells on a flat and nutrient-rich substratum. In contrast, as the nutrients deplete, they aggregate into three-dimensional droplet-like fruiting bodies, which are crucial for their survival. This morphological transition is driven by consistent cell influx toward certain locations where the aggregation happens. To achieve this, do the cells need to coordinate globally or not? Our experiments show that where the cells move out of a mono-layer and form multiple layers is strongly affected by their local orientational order. Their tendency or ability to move out of a mono-layer, on the other hand, is controlled by the polar order of the system. Each cell in the colony has an intrinsic polarity: when nutrient-rich, it reverses its direction of motion frequently, but when nutrient-poor, this reversal frequency is significantly reduced. More polar cells lead to more consistent cell flows and generate stronger mechanical forces inside the colony, which enhance out-of-plane cell motion. As a result, a mechanically-driven morphological transition emerges as M. xanthus cells change the way they move, and no long-range coordination is required, at least at the early stage of fruiting body formation.
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