Artwork: Dan Nowakowski/Nicholas Taylor

The bacterial flagellar engine has an automatic gearshift

Navish Wadhwa

Harvard University


Machines perform specific tasks for us

Cells use molecular machines for specific tasks

ATP synthase



Do cells have smart machines too?

Yes, they do.

We use the bacterium E. coli as a model organism

Inhabits guts of mammals

Some strains are harmful, most harmless

Multiple flagella per cell

Turner et al., J. Bacteriol., 2000

Bacteria are the smallest self-propelled swimmers

Many bacteria swim by rotating helical flagella

Slowed down 20 times

Turner et al., J. Bacteriol., 2000

Reversal of flagellar rotation enables changes in the swimming direction

Turner et al., J. Bacteriol., 2000

A nanoscale motor powers flagellar rotation

Changes in the rotor enable bidirectional rotation

Torque production differs between CCW and CW

Chen and Berg, Biophys. J., 2000
Yuan et al, PNAS, 2010

Automatic gearshift in cars allows the engine to adapt to changing terrains

Automatic gearshift in E. coli allows the motor to adapt to changing loads

Lele et al., PNAS, 2013

What is the physical and molecular mechanism underlying this automatic gearshift?

How can we change motor load?




Electrorotation allows full control on motor load




Electrorotation allows full control on motor load

A change in load triggers stepwise changes in motor speed

Wadhwa et al., PNAS, 2019

The stator remodels in response to load change

Wadhwa et al., PNAS, 2019
Lele et al., PNAS, 2013
Nord et al., PNAS, 2017

Remodeling kinetics vary with electrorotation speed

Wadhwa et al., PNAS, 2019

Higher electrorotation speed leads to lower torque

Stator remodeling depends on torque

A quantitative model for stator assembly

We extracted the on rate ($k_+$) and the off rate ($k_-$) from the data

Wadhwa et al., PNAS, 2019

The off-rate decreases with torque

Free energy of the bound state decreases with torque

The off-rate decreases exponentially with torque

Molecular mechanism for torque-dependent unbinding rate

Low torque

High torque

Torque anisotropy allows us to test the model

Collapse of CCW and CW data validates the model

Wadhwa et al., PNAS, 2021

Conclusions and perspective

Cars and bacteria use different approaches

Cars adapt the transmission while bacteria adapt the engine itself

What is the ecological relevance of autonomous motor remodeling?

Credit: Eshel Ben-Jacob and Ingham Colin

Berne et al., Nat. Rev. Microbiol. 2018


Howard Berg (Harvard)
Yuhai Tu (IBM)
Rob Phillips (Caltech)
Ethan Garner (Harvard)
Nicholas Taylor (U. Copenhagen)
Marc Erhardt (Humboldt U.)
Alberto Sassi (IBM)

K99/R00: GM134124