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Bacterial motility: machinery and mechanisms

Abstract

Bacteria have developed a large array of motility mechanisms to exploit available resources and environments. These mechanisms can be broadly classified into swimming in aqueous media and movement over solid surfaces. Swimming motility involves either the rotation of rigid helical filaments through the external medium or gyration of the cell body in response to the rotation of internal filaments. On surfaces, bacteria swarm collectively in a thin layer of fluid powered by the rotation of rigid helical filaments, they twitch by assembling and disassembling type IV pili, they glide by driving adhesins along tracks fixed to the cell surface and, finally, non-motile cells slide over surfaces in response to outward forces due to colony growth. Recent technological advances, especially in cryo-electron microscopy, have greatly improved our knowledge of the molecular machinery that powers the various forms of bacterial motility. In this Review, we describe the current understanding of the physical and molecular mechanisms that allow bacteria to move around.

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Fig. 1: Flagellum-driven swimming motility in bacteria.
Fig. 2: Swarming motility on surfaces.
Fig. 3: Surface-associated twitching motility.
Fig. 4: Gliding motility over surfaces.

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Acknowledgements

The authors thank K. Fahrner for critical reading of the manuscript. N.W. is supported by the National Institute of General Medical Sciences of the US National Institutes of Health under award number K99GM134124. H.C.B. has been supported by the US National Institute of Allergy and Infectious Diseases, the US National Science Foundation and the Rowland Institute for Science.

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Glossary

Reynolds number

A dimensionless parameter in the equations of motion of fluids that indicates the relative importance of inertial forces, forces that accelerate fluids and viscous forces, which shear fluids.

Twitching

Movement across surfaces that occurs when thin filaments are extended outwards from the cell, stick to the substratum at their distal tips and then are disassembled at their base.

Gliding

Movement across surfaces that occurs when adhesins, driven along tracks fixed to the cell wall, adhere to the substratum.

Flagellar filaments

Long, thin and rigid components of the bacterial flagellum, subject to changes in crystal polymorphic form which, when rotated at their base, generate thrust that pushes the cell forward.

Bundles

Groups of flagellar filaments rotating in parallel that push a peritrichously flagellated cell forward.

Polar

Flagellation with filaments that appear at one or the other pole: monotrichous, single filaments; lophotrichous, a tuft of filaments.

Brownian motion

Random motion of an object suspended in a fluid caused by collisions with the molecules of the fluid.

Chemotaxis

A behavioural response to chemical gradients whereby cells move towards regions that contain more nutrients (or nutrient homologues) and away from noxious regions.

Random walk

Migration by stepping in directions chosen at random. The walk is biased if steps in a particular direction are longer or more frequent.

Tethered cells

Cells fixed to glass by a single flagellar filament and, instead of rotating the flagellar filament, the motor rotates the cell body.

Stator units

Also known as force-generating units, torque-generating units or MotA–MotB complexes, an assembly of five MotA proteins and two MotB proteins supporting two transmembrane ion channels that powers flagellar rotation.

Proton motive force

The electrochemical gradient of protons across a membrane due to a combination of the membrane potential and the concentration gradient of protons. Protons driven down this electrochemical gradient energize various cellular processes, including flagellar rotation, ATP synthesis and ion transport.

Viscosity

Denoted by either μ or η, a parameter indicating the magnitude of the force required to shear a fluid. Water has a relatively small viscosity, molasses has a relatively large viscosity.

Newtonian fluid

A fluid in which the viscosity does not depend on the rate of shear (for example, water and solutions of Ficoll). Solutions containing long unbranched chains, such as mucus, hyaluronic acid, methyl cellulose or polyvinylpyrrolidone, are not Newtonian fluids.

Osmolytes

Small organic compounds that are synthesized by the cell to affect intracellular or extracellular osmolarity.

Surfactants

Compounds that when added to a liquid lower its surface tension. Soap and detergent are common examples.

Surface tension

The tendency of liquid surfaces to minimize the surface area and resist extension. Liquids with lower surface tension spread more easily.

Pinion

A round gear that transmits motion to a larger gear or a rack.

Rack

A linear gear that engages with the pinion and translates rotational motion into linear motion.

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Wadhwa, N., Berg, H.C. Bacterial motility: machinery and mechanisms. Nat Rev Microbiol 20, 161–173 (2022). https://doi.org/10.1038/s41579-021-00626-4

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