In bacteria, prey become predators... of their predators

When you think of predation, you probably think of a lion chasing a gazelle or a lynx pouncing on a hare in the snow. But did you know that some bacteria also kill and feed on their prey? In an article published today in the scientific journal Plos Biology, we explain how we observed that, in bacteria, prey can become predators... of their predators, as if the gazelle were hunting the lion.

Marie Vasse, University of Montpellier

The predatory bacterium Myxococcus xanthus (left) decimates its prey (right). Black dots are aggregates of predators. Nicola Mayrhofer, ETH, Provided by the author

The world as we know it is home to a multitude of other worlds that are less easily accessible, not least because they are invisible to the naked eye. I'm interested in these microscopic worlds and the organisms that inhabit them. I study how these microorganisms interact, how they cooperate to access resources for example, how they fight, how they communicate and even how they kill and eat each other. Between 2019 and 2021 at the Swiss Federal Institute of Technology Zurich (ETH), colleagues and I have been running a laboratory project to evolve communities of bacteria and try to understand how their interactions change over generations.

Strange results...

In 2021, after having carried out one of the project's experiments twice, I left ETH to join CNRS and left it to one of my colleagues to repeat the experiment in question twice more. When you're doing research, one of the ways of validating observations is to repeat an experiment to ensure that the results don't change between repetitions. Except that this time, the results weren't just a little different, they were completely reversed!

The experiment consisted in bringing together a bacterium described as predator and one described as prey, in order to estimate the efficiency of predation. In the first two replicates, and in our previous experiments with these two bacterial species, Myxococcus xanthus killed and fed on Pseudomonas fluorescens. It was therefore clear that M. xanthus was the predator and P. fluorescens the prey. During the third repetition, my colleague observed not only that P. fluorescens was on the increase, but also that M. xanthus had completely disappeared from the plastic dishes (called Petri dishes and containing the culture media) in which we were conducting the experiments.

After many questions and lengthy discussions, we realized that the difference between her way of conducting the experiment and mine was that she left the dishes in which P. fluorescens was growing on the laboratory bench, and therefore at room temperature, instead of incubating them at 32°C like M. xanthus, for lack of space in the incubator. It's important to remember that the two species don't grow at the same speed, so before studying their interactions, we need to grow them independently.

We were really surprised and eager to find out more. So we formulated a new research question: can the temperature at which these bacteria grow determine who is the prey and who is the predator? We began by checking that temperature was indeed the determining factor by growing P. fluorescens at 22°C and 32°C, before bringing it into contact with the other species at 32°C, and estimated the number of M. xanthus present after interaction.

The former prey kills and feeds on its predator!

When P. fluorescens had grown at 32°C, we found several million M. xanthus in the dishes; but when it had grown at 22°C, we couldn't detect a single living cell of this species! These results under controlled temperature conditions corroborated our previous observations: the "prey" could exterminate its "predator". It should be noted, however, that to be a predator, it's not enough to kill, you must also be able to feed on your prey. As it is difficult to observe a bacterium eating its lunch, we assessed the ability of the microbe to eat another by measuring the effects of the interaction on population size: if P. fluorescens kills and feeds on M. xanthus, we would expect to see fewer living M. xanthus and more P. fluorescens, the latter having been able to reproduce thanks to the nutrients derived from predation.

We therefore carried out a new experiment in which we grew P. fluorescens at 22°C and 32°C and then added either M. xanthus in a saline solution, or the saline solution alone. At 32°C, the presence of M. xanthus greatly reduced the number of P. fluorescens eating these bacteria. At 22°C, on the other hand, M. xanthus was exterminated by P. fluorescens, and we found on average twice as many P. fluorescens as in the presence of the saline solution alone: the predator-prey relationship was reversed, and the former prey killed and fed on its predator!

We then set out to understand how P. fluorescens became the predator. By growing this species in a liquid medium, we understood that at 22°C, but not 32°C, it produced one (or more) molecule(s) and secreted them into the environment. It is this secreted molecule that exterminates M. xanthus.

In our study, P. fluorescens produced the predatory molecule even before interacting with the other bacterium. This means that, at 22°C, P. fluorescens uses this molecule for purposes unknown to us, and that this molecule has the side-effect of killing M. xanthus.

Finally, we wanted to know whether this molecule could kill other bacteria, or whether it was specific. We therefore exposed seven other bacterial species to secretions of P. fluorescens grown at 22°C. No other bacterial species was totally exterminated like M. xanthus, but in the presence of P. fluorescens secretions, the number of Bacillus bataviensis was reduced by 10% on average, and Micrococcus luteus by 50%. M. xanthus is therefore not the only bacterium that can be killed by P. fluorescens when grown at 22°C.

We don't yet know the exact nature of the P. fluorescens molecule that enables it to act as a predator of M. xanthus. Further experiments will be needed to find out, and are currently underway. What our study does indicate, however, is that unlike the lion and the gazelle (who ever saw a gazelle devour a lion?), altering a single parameter in the growth conditions of bacteria can have radical consequences for their roles in interactions. If bacteria that have never been described as predators can eradicate their predator simply by growing in a slightly colder environment, it's highly likely that many prey-predator interactions between bacteria will escape us. Another consequence of this study is that bacteria can hardly be pigeonholed as either prey or predator. On the contrary, it's by taking an interest in their interactions that we can learn more about the workings of these fascinating microscopic worlds!

Marie Vasse, Researcher in evolutionary biology, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.