Exercising restraint: how growing slower can allow bacteria to dominate


A scientist at the University of Sheffield has developed new insights on how different species of bacteria compete with one another in the porous environments where most bacteria live.

In the microbial world rapid growth has long been thought to be the key to success. However, new research shows that in porous environments, such as within soil, sediments, and rock, microbes such as bacteria can actually gain a competitive advantage by growing more slowly. Since more than 95% of bacteria on Earth live in these porous habitats, this new work provides new tools to understand how natural bacterial communities function, as well to engineer them for important functions, like cleaning up polluted drinking water or enhancing oil extraction.

Most of what scientists know about microbial life comes growing cells in devices like test tubes. In these typical laboratory conditions cells that grow more rapidly dominate over those that grow more slowly. Research published this week by Dr William Durham of the University’s Department of Physics and Astronomy and Imagine: Imaging Life centre finds that this concept does not always hold within more natural porous habitats, where cells rely on fluid flow to supply them with nutrients.

Dr Durham explained, “In porous environments most bacteria live attached to the surfaces of soil and rock, where they form communities called biofilms. It is incredibly hard to visualize how biofilms growing in these opaque environments affect patterns of flow, so we boiled this problem down to a much simpler model that still captures the fundamental physics. We found that bacteria living in porous substrates face a fundamental challenge: they need to reproduce but not so fast that they divert the flow that nourishes them.”

Durham began this project while working at University of Oxford and led a multidisciplinary team to develop experiments and mathematical models of competition between bacteria that grow at different rates. They found that when the rate at which flow travels through a porous environment is large, faster growing biofilms have the competitive advantage, as previously predicted. However, when flow was relatively weak, fast growing biofilms divert their nutrient supply to slower growing bacteria, allowing the latter to gain the upper hand. This seemingly paradoxical result stems from the fact that flow always takes the path of least resistance: when fast growing cells begin to divert flow, it reduces the rate at which they detach from the surface, which then causes them to further increase their resistance to flow. This positive feedback ultimately leads to the fast growing bacteria fully blocking their supply of nutrients: they bite the hand that feeds them.

The predictions of Durham and his international team of collaborators may ultimately allow us to better engineer bacterial communities to perform important functions. This study found that while strong and weak flow favour fast and slow growing bacteria respectively, intermediate rates of flow allow cells with different growth rates to maintain access to flow over long periods. In many situations, such the clean-up of harmful chemicals within porous reactors, maintaining diverse assemblages of bacteria that grow at different rates allows them more efficiently degrade harmful compounds. On the other hand, bacterial biofilms are also used to stifle the spread of pollutants that have spilled into the water table: in such situations it is clearly advantageous to design bacterial communities that block their pore spaces to inhibit flow that can spread contaminants to wells that supply drinking water.

This project was funded by the Engineering and Physical Sciences Research Council, the European Research Council and the Human Frontier Science Program, and was published this week in the Proceedings of the National Academy of Sciences (USA).

Image caption: Two different types of bacteria, one labelled red and the other green, compete in a microfluidic device that simulates soil (credit: Katharine Coyle, Roman Stocker, and William Durham).

For further information, please contact: William M. Durham, Lecturer of Biological Physics, University of Sheffield, 0114 222 4537, w.m.durham@sheffield.ac.uk

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