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To swim through a viscous fluid, a flagellated bacterium must overcome the fluid drag on its body by rotating a flagellum or a bundle of multiple flagella. Because the drag increases with the size of bacteria, it is expected theoretically that the swimming speed of a bacterium inversely correlates with its body length. Nevertheless, despite extensive research, the fundamental size–speed relation of flagellated bacteria remains unclear with different experiments reporting conflicting results. Here, by critically reviewing the existing evidence and synergizing our own experiments of large sample sizes, hydrodynamic modeling, and simulations, we demonstrate that the average swimming speed ofEscherichia coli, a premier model of peritrichous bacteria, is independent of their body length. Our quantitative analysis shows that such a counterintuitive relation is the consequence of the collective flagellar dynamics dictated by the linear correlation between the body length and the number of flagella of bacteria. Notably, our study reveals how bacteria utilize the increasing number of flagella to regulate the flagellar motor load. The collective load sharing among multiple flagella results in a lower load on each flagellar motor and therefore faster flagellar rotation, which compensates for the higher fluid drag on the longer bodies of bacteria. Without this balancing mechanism, the swimming speed of monotrichous bacteria generically decreases with increasing body length, a feature limiting the size variation of the bacteria. Altogether, our study resolves a long-standing controversy over the size–speed relation of flagellated bacteria and provides insights into the functional benefit of multiflagellarity in bacteria.
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This content will become publicly available on June 11, 2026
Nascent flagellar basal bodies are immobilized by rod assembly in Bacillus subtilis
ABSTRACT Flagella are complex, trans-envelope nanomachines that localize in species-specific patterns on the cell surface. Here, we study the localization dynamics of the earliest stage of basal body formation inBacillus subtilisusing a fluorescent fusion to the C-ring protein FliM. We find thatB. subtilisbasal bodies do not exhibit dynamic subunit exchange and are largely stationary at steady state, consistent with flagellar assembly through the peptidoglycan (PG). However, rare mobile basal bodies were observed, and the prevalence of mobile basal bodies is elevated both early in basal body assembly and when the rod is mutated. Thus, basal body mobility is a precursor to patterning, and we propose that rod polymerization probes the PG superstructure for pores of sufficient diameter to permit rod transit. Furthermore, mutation of the rod disrupts basal body patterning in a way that phenocopies mutation of the cytoplasmic flagellar patterning protein FlhF. We infer that rod synthesis and the cytoplasmic regulators coordinate flagellar assembly by interpreting a grid-like pattern of pores, pre-existent in the PG. IMPORTANCEBacteria insert flagella in a species-specific pattern on the cell body, but how patterns are achieved is poorly understood. In bacteria with a single polar flagellum, a marker protein localizes to the cell pole and nucleates the assembly of the flagellum at that site.Bacillus subtilisassembles ~25 basal bodies over the length of the cell in a grid-like pattern and lacks proteins required for their polar targeting. Here, we show thatB. subtilisbasal bodies are mobile soon after assembly and become immobilized when the flagellar rod transits the peptidoglycan (PG) wall. Moreover, defects in the flagellar rod lead to a more-random distribution of flagella and an increase in polar basal bodies. We conclude that the peritrichous patterning of flagella ofB. subtilisis different from the polar patterning of other bacteria, and we infer that theB. subtilisrod probes the PG for holes that can accommodate the machine.
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- Award ID(s):
- 2439964
- PAR ID:
- 10649218
- Editor(s):
- Salama, Nina R
- Publisher / Repository:
- American Society for Microbiology
- Date Published:
- Journal Name:
- mBio
- Volume:
- 16
- Issue:
- 6
- ISSN:
- 2150-7511
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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