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The Evolution of Bacterial Warfare

Periodic Reporting for period 2 - MicroWars (The Evolution of Bacterial Warfare)

Reporting period: 2020-04-01 to 2021-09-30

Many bacteria are extremely aggressive. They assemble poisoned molecular spears to stab neighbours, they release protein machines that punch holes in competitors and, moreover, some cells will commit suicide in order to launch their attack. While there is a large literature on the evolution of combat and competition in animals, this has focused on why animals are generally reluctant to fight. The evolution of extreme aggression in bacteria, therefore, challenges our current understanding of competition in biology. My project has been developing bacterial warfare as a model for competitive behaviour by asking three key questions that each build in complexity: 1) Why do weapons evolve? 2) What tactics do bacteria use during combat and why? 3) How does ecological complexity influence the value and use of weapons? These questions are important for our fundamental understanding of bacteria and conflict, but also for the growing recognition that we need to be able to predict when and how particular bacteria will invade and grow if we are going to manage the human microbiome, which is so central to our health and wellbeing. We are aiming to show that, understanding the microbiome, will need a new focus on the evolution, and ecology, of bacterial warfare.
We have been examining our three key questions theoretically and testing our predictions using two bacterial model species, Escherichia coli and Pseudomonas aeruginosa, which possess very different levels of weaponry. In the first part of the project we have made progress in answering all of our questions, although there is still more to be done. In particular, we have shown the importance of how bacteria are arranged on fine spatial scales for when weaponry is beneficial via the development of bacterial 3D printing using E. coli. In addition, we are studying a wide range of different weapons systems in P. aeruginosa, which is revealing how some weapons are better when a cell first arrives in a new community, while others are better once a strain has established itself with many cells. We are also finding evidence from multiple angles that a key tactic employed by bacteria during combat is reciprocation or tit for tat, often they will only attack in force if they themselves are attacked, mirroring examples in animal behaviour. Finally, we are studying how bacteria fight in complex communities with other species and find conditions where the best thing to do is to make a very broad spectrum toxin that harms many species around them. All of these insights are combining to give us a set of principles to understand when and why bacteria fight and, moreover, what defines the difference between success and failure.
The novelty of our work comes from the combination of methods we employ to questions on the evolution and ecology of bacteria. These include a range of modelling methods, including complex systems theory and agent based modelling, along side advanced methods in microbiology including molecular microbiology, time lapse confocal microscopy that captures single cell behaviours and, most novel, our 3D bacterial printing method that allows us to patten bacterial battlegrounds with micron scale precision. These methods have allowed us to publish a set of papers in the first part of the project that each offer new insights into bacterial warfare and how and why it evolves.