23 November 2020

The tortoise and hare: how moving slower allows groups of bacteria to spread across surfaces

Scientists have found that bacterial groups spread more rapidly over surfaces when the individuals inside them move slowly, a discovery that may shed light on how bacteria spread within the body during infections.

Computer simulation showing bacteria moving and colliding
Simulations show fast-moving bacteria become trapped after colliding with each other because they are forced to stand on end. Credit: Oliver Meacock

  • Similar to the fable of the tortoise and the hare, scientists found that bacteria engineered to individually move faster actually lose  the race against slower strains when traveling  in densely packed groups
  • Experiments and mathematical models revealed that the fast moving bacteria tend to crash into one other at high speeds, causing them to rotate vertically and become stuck in place
  • The findings suggest that bacteria have evolved slow, restrained movement to benefit the group as a whole, rather than individual cells
  • This study  may provide new insights into how bacteria spread within the body during infections

Scientists have found that bacterial groups spread more rapidly over surfaces when the individuals inside them move slowly, a discovery that may shed light on how bacteria spread within the body during infections.

Researchers from the University of Sheffield and the University of Oxford studied Pseudomonas aeruginosa, a species of bacteria responsible for deadly lung infections, which moves across surfaces using tiny grappling hook-like appendages called pili. Similar to the fable of the tortoise and the hare, they found that bacteria engineered to individually move faster actually lost the race against slower strains when moving in densely packed groups.

Using a combination of genetics, mathematics, and sophisticated tracking algorithms that can simultaneously follow the movement of tens of thousands of cells, the researchers demonstrated that collisions between the fast-moving bacteria cause them to rotate vertically and get stuck.

In contrast, slower-moving cells remain lying down, allowing them to keep moving. The slower-moving cells therefore win the race into new territory, acquire more nutrients, and ultimately outcompete the faster moving cells. This research suggests that bacteria have evolved slow, restrained movement to benefit the group as a whole, rather than individual cells.

We routinely experience gridlock in our own lives while traveling by foot or in cars. These traffic jams often occur because individuals have prioritised their own movement over that of their neighbours. In contrast, bacteria have evolved to move carefully and effectively in crowds.

Dr William Durham

Lecturer in Biological Physics at the University of Sheffield

The findings have been published in the journal Nature Physics.

Dr William Durham, a Lecturer in Biological Physics at the University of Sheffield, said: “We routinely experience gridlock in our own lives while traveling by foot or in cars. These traffic jams often occur because individuals have prioritised their own movement over that of their neighbours. In contrast, bacteria have evolved to move carefully and effectively in crowds, likely because their neighbours tend to be genetically identical, so there is no conflict of interest. Bacteria accomplish this by moving more slowly than their top speed.”

To understand these phenomena, the researchers used theory originally developed to study materials known as liquid crystals.

Dr Oliver Meacock, a postdoctoral researcher at the University of Sheffield and lead author of the study, said: “Liquid crystals are everywhere around us, from smartphone screens to mood rings. Although we initially didn’t expect that the mathematical tools developed to understand these man-made materials could be applied to living systems, our findings show that they can also shed light on the challenges faced by microbes."

Patterns of collective movement that occur in flocks of birds and schools of fish have long been a source of fascination to onlookers. This new research shows that similarly spectacular forms of collective movement also occur in the microscopic world.

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