When people ask me about my research, I have to make a quick decision. Do I want to convince that I’m doing something “useful”, like our work on bacterial infections, or describe something relatable from nature documentaries, like our work on meerkats and birds? Even though meerkats and bacteria make an unusual combination; to an evolutionary biologist it makes sense. All life on earth is shaped by the same evolutionary processes and in my research group we are interested in the processes responsible for social behaviour. For this it is often very helpful to think across taxonomic boundaries.
People tend to assume that it is much easier to study the behaviour of a bacterial cell cultured in a lab than the behaviour of a wild animal roaming the desert. But not if you are interested in adaptation. We can go to the Kalahari and watch animals forage, interact, defend themselves; we can experience the environment in more or less the same way as they do. Now try the same exercise for a bacterial cell that is infecting a human lung. How do bacterial experience life inside a lung? Why do only some move? Why are some covered with slime? This is why microbiologists call bacteria in their lab “isolates”, because to observe their behaviour we isolate them from their natural environment - a bit like trying to understand a polar bear from its behaviour in a zoo.
A breakthrough came from finding the clues we needed in the bacterial genome. The genome carries mutations that have been favoured inside the host, and so contain clues about selection pressures faced by bacteria in their natural environment. By analysing the sequences of hundreds of isolates, sampled over decades of infection from patients, we have been able to identify signatures of social interactions between bacterial cells. As an infection progresses, cells become increasingly slow growing and lose the ability to contribute to the shared supply of secreted products they depend on for growth and survival. Our analyses revealed that this pattern is a result of a cheat invasion: cells that do not contribute to the pool of resources exploiting more cooperative neighbours to extinction, and becoming more poorly adapted to the lung environment as a result. This discovery provides a rare insight into how the environment inside our bodies shapes behavioural changes in infection; and shows how research into social behaviour of animals such as meerkats can help us understand social interactions across the tree of life.