Marine bacteria approach the limit of chemotactic precision

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The trajectories of bacteria as they move towards the area where a nutrient has been released.

Recreating microscopic features of the ocean shows that marine bacteria have pushed food-finding behaviour almost to its theoretical limit.

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Bacteria in the ocean detect food sources and navigate towards them with extremely high precision. This discovery was made by an international team led by Dr Douglas Brumley from the University of Melbourne’s School of Mathematics and Statistics and Dr Francesco Carrara from ETH Zurich.

The research team studied how Vibrio ordalii, a common marine bacterium, navigates towards food hotspots. The bacteria can detect changes in nutrient concentration around them and move towards more favourable regions using a process called chemotaxis.

Previous research into chemotaxis mostly studied bacteria in steady environments with a single nutrient source at medium to high concentrations. But wild bacteria often live in rapidly changing environments – due to ocean mixing, for example. They might encounter several nutrient sources at much lower concentrations. Bacteria also have to contend with different types of ‘noise’, or interference, which affect their ability to accurately sense the nutrient gradient.

The research team recreated microscopic features of the ocean under a microscope. They recorded how V. ordalii bacteria responded to a pulse of the common nutrient glutamate. The release of glutamate into the water was controlled to match the amount of food that leaks out when marine algae break down in the ocean. As the glutamate spread through the water, the researchers measured how the nutrient gradient changed. They also tracked how the bacteria responded, recording the speed and direction of more than one million individual movements.

Watch: Simulation of bacteria with ideal chemotactic precision

The simulation shows that each bacterium moves towards the food source (at the centre of the screen).

Based on these recordings, the researchers built a mathematical model of bacterial chemotaxis. The model allowed them to simulate how chemotaxis changes in environments with levels of noise that the bacteria might encounter in the ocean.

The researchers used the results of these simulations to determine the chemotactic precision of V. ordalii – that is, how accurately it estimates the nutrient gradient.

They found that V. ordalii operates close to the theoretical limit. This limit is determined by an equation that considers the size of the bacteria, the accuracy of the estimated nutrient gradient, the local nutrient concentration, and how quickly the nutrient diffuses through the water.

Next steps

The researchers plan to study how V. ordalii movement changes in response to multiple food sources. They will also vary the amount of time the food is available and the number of competing bacteria.

Funding

ARC Discovery Early Career Researcher Award

The Mechanisms Driving Microbial Navigation In Marine Systems (DE180100911)

Publication

Brumley DR et al (2019) Bacteria push the limits of chemotactic precision to navigate dynamic chemical gradients. Proceedings of the National Academy of Sciences USA 116(22): 10792–10797. doi: 10.1073/pnas.1816621116

Image: Douglas Brumley

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