26 November 2020

Bacteria colonies of billions invade new territories without traffic jams – we now know how


An international collaboration between researchers at the Niels Bohr Institute, University of Copenhagen, Oxford University, and University of Sheffield has revealed that colonies of slow moving bacteria can expand significantly quicker than their fast moving counterparts. The result is now published in Nature Physics.

Computer simulation of bacterial competition. The fast bacteria (green) get stuck in rosettes-like structures, grinding to a halt, while the slow bacteria keep moving. Credit: Oliver J. Meacock and William Durham 

The researchers combined genetics, experiments, custom image analysis algorithms and theoretical physics to investigate the efficiency of the bacterial invasion. It turned out that bacteria move slowly and prudently in order to avoid crashes and jams, making them capable of moving efficiently in dense and massive, multimillion population crowds. The result may have implications for how we treat infections in a future in which super bacteria, immune to antibiotics, pose a threat to human health. 

Pathogenic bacteria, Pseudomonas ariginosa, move around by grabbing the surfaces with tiny feet called pili. The researchers at Oxford Zoology and Sheffield University set up an experiment in which they modified the individual bacteria by simply adding the number of feet. No other features or properties were changed. The individual bacteria were now able to move approximately two times faster than before, and the researchers asked the question if this enhancement of the individual’s abilities would also enhance the population’s ability to invade new territories. “We wanted to understand the behavior of the bacteria, both on an individual level and on the level of the collective”, Amin Doostmohammadi from the Niels Bohr Institute says.

The tortoise beats the hare - again

The researchers literally made a race between the fast moving and the slow moving colonies, and very counterintuitively, the genetically enhanced, fast population was overtaken by the slower moving wildtype bacteria population. In the beginning, the fast population was ahead, but was, over the course of few hours, overtaken by the slower moving, but apparently more efficient wildtype population. The researchers also put the two different types of bacteria together, to have them compete directly, and again the slower moving, wildtype bacteria population ended up being better at expanding their population. “We find that a mutant that produces a larger number of pili could move more rapidly than the wildtype on an individual level, but in large groups they tended to crash into one another at high speeds. These collisions rotate the mutant cells vertically and trap them in place. As a result, the slower moving wildtype cells can move past them and ultimately win the race into new territory.”

Basic physics is at play in a colony of bacteria

By characterizing the orientation of bacteria the researchers found that collisions take place at specific locations: singular points in cell alignment in the form of aster-like structures that are known as +1 topological defects in physics. “Considering how much biology goes into the machinery of bacteria and their behavior, it is striking that we were able to recreate almost exactly the same patterns by using basic physics principles and modelling them in a computer. In other words, the bacteria are obeying a simple, physical principle that limits their pace as individuals, but still allows for a very efficient invasion of a colony. Evolution seems to have put a fundamental speed limit on bacteria: if they move faster than certain amount, they collide together and get trapped in structures of their own creation”.

Infections may in the future be dealt with in other ways than with antibiotics

Control of an infection typically means adding a drug to the bacteria colony to influence the individual bacteria – slow it down or kill the individuals in a population with antibiotics, but the surprising new discovery seems to show that speeding up the pace of the crowd of bacteria may actually cause the infection to self-destruct. If the bacteria population, through evolution, have solved the problem of crowding by gaining new territory at a very specific speed, if you then turn up the speed dial, the infection “crashes” and dies out. “From the physics perspective, we may be able to say what property exactly we need to control in a bacteria population, and from the genetics perspective they (biologists) may say ‘we know how to control that’, and then we can move on to do so. It is a quite new way of thinking, linking different fields of expertise together. Understanding how to control the crowd, rather than the individual in an infection, we hope, will lead to new strategies to control infections in the future” Amin Doostmohammadi says.

Interdisciplinarity was key to the success of the experiment

It was impossible to do this work without a continuous cross talk between researchers within different fields: the expertise in genetic modifications, image analysis, and bacterial ecology from Dr. Oliver Meacock and Dr. William Durham at Sheffield, and Professor Kevin Foster at Oxford Zoology, was combined with theoretical expertise on topological defects from Professor Julia Yeomans in Oxford Physics, and Dr. Amin Doostmohamamdi at the Niels Bohr Institute to discover and explain a counterintuitive phenomenon of how nature has favored slow individuals to make fast collectives.

Link to the scientific article: https://www.nature.com/articles/s41567-020-01070-6