Best evidence across studies

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 Long Explanation

Evidence linking motility, cost, and fitness

Flagella require both a one-time biosynthetic investment and continuing energy input during operation. Reported estimates illustrate the scale: E. coli was estimated to spend 2.32 × 10⁸ ATP-equivalents constructing its flagellum and 8.08 × 10⁸ ATP-equivalents operating it; across the broader dataset, flagellar costs spanned roughly 0.1–40% of cellular energy expenditure. These are model-based estimates, not direct measurements in every species. Flagellar energy budget

Why motility can increase fitness

Motility can improve the probability of reaching nutrients, escaping physical traps, navigating viscosity, and exploiting surfaces. In Vibrio, the polar flagellum is sodium-driven, whereas lateral flagella are proton-driven and are induced on surfaces or in viscous conditions; the polar system also participates in surface sensing and swarmer differentiation. Vibrio energy and surface response

Where heterogeneity comes from

Cells need not express identical flagellar numbers, lengths, motor states, or swimming patterns. Regulatory checkpoints couple assembly to transcription: in polar flagellates, MS-ring/rotor/fT3SS assembly is required for downstream rod and hook transcription, while FlhF/FlhG perturbation changes flagellation. In Salmonella, changing the FliA–FlgM balance increased flagellar number two- to threefold. Assembly-regulated heterogeneity Flagellar-number regulation

Fitness interpretation: heterogeneity can be advantageous when environments fluctuate: some cells may pay the cost of strong motility while others conserve energy or adopt surface-associated states. Consistent with this trade-off, a 1,839-genome analysis found reduced prevalence of 12 of 53 flagellar genes in host-associated Pseudomonadota, although genome size, uncertain lifestyle labels, undetected divergent homologs, horizontal transfer, and inability to distinguish polar from lateral systems limit causal interpretation. Comparative gene loss

Bottom line: fitness is expected to depend on the environment, not merely on swimming speed. The supplied evidence supports a cost–benefit trade-off and regulatory heterogeneity, but does not directly measure reproductive fitness or ATP flux for matched motile and non-motile cells. A decisive test would jointly measure single-cell energy use, motility, survival or growth, and competitive success across controlled viscosity, nutrient, surface, and density conditions.