Artwork: Dan Nowakowski/Nicholas Taylor



The bacterial flagellar engine has an automatic gearshift

Physics of Living Systems Seminar, EPFL


Navish Wadhwa
Harvard University

slides for this talk: navishwadhwa.com/talks

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

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?

Instantaneously

Reversibly

Controllably

Electrorotation allows full control on motor load

Instantaneous

Reversible

Controllable

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

Hypothesis
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



Acknowledgements

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




K99/R00: GM134124