Inherited flagellar structures coordinate the onset of swimming among related E. coli cells

Bacteria commit to costly behaviors before they can benefit from them. In Escherichia coli, flagellar synthesis is expensive, yet the genes are expressed in stochastic pulses and only a small minority of pulses result in motility. Despite this noise, we find that lineage-related cells begin swimming at similar times, in groups of two to eight cells. Rare, high-amplitude pulses commit a single cell within one generation, but more commonly, pulses rise slowly and fail to trigger swimming before division. Rather than decaying, their output accumulates as cells inherit hook-basal bodies (HBBs) and continue to build new ones. This structural inheritance acts as a physical integrator, summing ongoing transcriptional activity across generations, so that an incomplete flagellar cascade is carried forward at division rather than lost. HBB accumulation sets when cells swim, and the anti-sigma factor FlgM sets how sharply. By holding late flagellar genes off until several HBBs have accumulated, FlgM makes motility depend steeply on early flagellar gene expression, so that relatives inheriting similar sets of HBBs become motile together. Without FlgM, this sharp dependence is largely lost, and activations of the full cascade do not reliably produce swimming. Together, our results show how the accumulation of a molecular machine integrates fluctuations, and how a checkpoint on its assembly sharpens them into a decision, setting both the speed and heritability of a behavioral trait.