Scientists have uncovered evidence that a fundamental evolutionary trade-off in plant pathogenic bacteria has persisted for more than 200 years — and possibly for millions of generations — helping explain why the “strongest” microbes do not always dominate in nature.

In a new study published in the Proceedings of the National Academy of Sciences (PNAS), researchers from the laboratories of Talia Karasov (University of Utah) and Hernán A. Burbano (UCL Biosciences), together with collaborators from several other institutions, analysed modern and historical genomes of the plant pathogen Pseudomonas viridiflava. 

By sequencing bacteria preserved in centuries-old herbarium plant specimens, the team discovered that bacterial strains face a long-term evolutionary dilemma: becoming better at killing competitors comes at the cost of being worse at infecting plants.

To uncover the origins and persistence of this trade-off, the researchers combined experiments in modern bacterial strains with analyses of more than a thousand naturally occurring isolates and historical genomes spanning two centuries.

“This study shows that evolution doesn’t always find a perfect solution,” said co-first author Talia Backman of the University of Utah. “Some biological conflicts can persist for incredibly long periods of time because improving one trait inevitably weakens another.”

“These findings suggest that some evolutionary conflicts may be far more difficult to resolve than we previously thought,” added co-first author Emma Caullireau of the University of Utah.

The researchers focused on specialised bacterial weapons called tailocins — microscopic structures derived from ancient viruses that bacteria use to attack rival strains. Tailocins act like molecular syringes, puncturing neighboring bacteria and killing them.

Some strains carried especially powerful tailocins capable of killing most competitors. Yet surprisingly, those highly aggressive strains remained rare in natural populations.

“To understand why the most lethal strains never took over, we had to look beyond short-term competition,” said co-first author Jiajun Cui of University College London. “The answer turned out to be a classic evolutionary trade-off.”

The key lies in a bacterial surface structure called the O-antigen. Bacteria lacking this structure became more resistant to attack from competitors, giving them an advantage in microbial warfare. But those same bacteria were significantly worse at colonising and infecting plants. In other words, bacteria could either become better fighters or better colonisers — but not both.

To determine whether this trade-off was temporary or evolutionarily stable, the team analysed DNA from 49 historical bacterial genomes recovered from dried Arabidopsis thaliana plant specimens collected across Europe between 1817 and 2015. The centuries-old samples revealed that the same genetic variants associated with the trade-off have persisted continuously over time.

“Using historical DNA allowed us to watch evolution across centuries instead of weeks in the lab,” said Burbano. “What surprised us most was how stable these competing strategies remained, despite enormous opportunities for evolution to break the trade-off.”

The findings suggest that some evolutionary constraints may be deeply embedded in the physical structure of biological systems, making them difficult or impossible to overcome completely. The researchers say the work could eventually help scientists design more sustainable antimicrobial strategies by exploiting natural evolutionary weaknesses in harmful bacteria.

“Understanding the limits of bacterial evolution may help us predict how pathogens respond to treatment,” said Karasov. “Instead of fighting evolution, we may be able to work with these long-standing constraints.”

The study also highlights the growing scientific value of museum and herbarium collections, which preserve biological snapshots spanning centuries.

“Historical collections are becoming powerful tools for studying evolution in real time,” said Burbano. “These specimens let us ask questions that would otherwise be impossible.”

The article, titled “Persistent trade-offs balance competition and colonization across centuries,” is currently available on PNAS.

 

List of authors

Talia Backman, Jiajun Cui, Emma Caullireau, Ella Bleak, Ilja Bezrukov, Patricia Girardi, Aubrey Hawks, Jesse R. Lasky, Sergio M. Latorre, Joel M. Erberich, Lua Lopez, Manuela Neumann, Allison M. Perkins, Efthymia Symeonidi, Parastoo Azadi, Martin P. Horvath, Artur Muszyński, Patricia L. M. Lang, Talia L. Karasov, and Hernán A. Burbano.

 

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