Physical mechanism reveals bacterial slowdown above a critical number of flagella

TL;DR

This study uses advanced hydrodynamic simulations to reveal that bacterial swimming slows down beyond a critical number of flagella due to load-dependent motor torque and flagellar cooperativity, with implications for bacterial adaptation.

Contribution

The paper introduces a comprehensive model considering the full torque-speed relationship, explaining bacterial slowdown at high flagella numbers and predicting environmental effects on flagella count.

Findings

Bacterial slowdown occurs beyond a critical flagella number.

Critical flagella number depends on fluid viscosity.

Model predictions align with empirical data for oli.

Abstract

Numerous studies have explored the link between bacterial swimming and the number of flagella, a distinguishing feature of motile multiflagellated bacteria. We revisit this open question using augmented slender-body theory simulations, in which we resolve the full hydrodynamic interactions within a bundle of helical filaments rotating and translating in synchrony. Unlike previous studies, our model considers the full torque-speed relationship of the bacterial flagellar motor, revealing its significant impact on multiflagellated swimming. Because the viscous load per motor decreases with flagellar number, the bacterial flagellar motor (BFM) transitions from the high-load to the low-load regime at a critical number of filaments, leading to bacterial slowdown as further flagella are added to the bundle. We explain the physical mechanism behind the observed slowdown as an interplay between the load-dependent generation of torque by the motor, and the load-reducing cooperativity between flagella, which consists of both hydrodynamic and non-hydrodynamic components. The theoretically predicted critical number of flagella is remarkably close to the values reported for the model organism \textit{Escherichia coli}. Our model further predicts that the critical number of flagella increases with viscosity, suggesting that bacteria can enhance their swimming capacity by growing more flagella in more viscous environments, consistent with empirical observations.