Bacteria in biofilms evolve slower on rough surfaces

What is it about?

We show that bacterial biofilms - clusters of microorganisms adhering to surfaces - grow very differently on rough surfaces compared to flat surfaces, and that this affects biological evolution in such biofilms. We use a microfluidic device to grow E. coli biofilms on corrugated surfaces and find that corrugations prevent the expansion of "clonal sectors" - populations of bacteria that originate from a single cell within the biofilm. If a new mutation occurs in the biofilm, corrugations limit how much the new mutation will be able to spread, therefore limiting the speed of biological evolution. This is true even for mutations that are fitter than the parent bacterial strain. In particular, we show that the spread of an antibiotic-resistant mutant bacterium can be arrested on corrugated surfaces. We attribute this effect to the unique velocity field within the biofilm on corrugated surfaces, which limits cell movement and mixing. Our work highlights the critical role of mechanical interactions, such as adhesion and friction, in microbial evolution.

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Photo by CDC on Unsplash

Photo by CDC on Unsplash

Why is it important?

Biofilms play crucial roles in environments ranging from dental plaques to industrial pipelines. Typically, biofilm studies have used flat surfaces, but many naturally-occuring biofilms relevant for medicine and industry live on rough surfaces. Examples of such surfaces range from urinary catheters, which can become rough due to mineral deposits, to porous beads with rough surfaces used in biofilters, to the interior of pipelines affected by corrosion. All these situations increase the risk of bacterial colonization but the effect of surface roughness on biological evolution in biofilms remains unclear. A growing biofilm is not only difficult to remove but it also can lead to the emergence of new, undesired variants of bacteria, such as bacteria resistant to antimicrobials. This is an example of Darwinian evolution: naturally occurring mutations, which endow bacteria with antibiotic resistance, are selected for when the biofilm is exposed to antibiotics e.g. during therapy. These new resistant variants spread due to faster growth in the presence of the antibiotic and replace the parental bacteria sensitive to the antibiotic. We have shown that, on corrugated surfaces, the spread of antibiotic-resistant mutants is strongly suppressed. Surface corrugation required to see this effect can be small and do not have to physically isolate different regions of the biofilm. Rather, isolation occurs because surface corrugations redirect biofilm growth in a way which prevents bacteria to mix and invade their neighbours. Our research offers insights into controlling bacterial evolution by manipulating surface geometry. This may lead to new strategies in medicine and industry to prevent the spread of harmful bacterial traits.

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