Out-of-balance growth enables cost-free synthesis of the flagellum and other proteins in a single bacterium.

In Escherichia coli single cells, flagellar gene activity is associated with an increase in growth rate, despite bulk measurements linking it to reduced growth.

• Why it matters: Bulk “balanced growth” laws may hide how stochastic growth and gene regulation shape the growth–gene-expression relationship within individual cells.
• What they did: The authors measured instantaneous growth and flagellar gene activity per cell, avoiding bulk-like aggregation, and validated generality using synthetic fluorescent-protein expression systems plus a computational model.
• The result: Out-of-balance growth boosts, likely from inherited surplus growth factors across division, reconcile the apparent Simpson’s-paradox contradiction and suggest a general mechanism for short-term cost mitigation.