In the world of bacteria, these prey become the predators… of their predators
When you hear the word “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 certain 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 the world of bacteria, prey can become the predators… of their predators, as if the gazelle were to start hunting the lion.
Marie Vasse, University of Montpellier
The world as we know it is home to a multitude of other worlds that are less easily accessible, largely because they are invisible to the naked eye. I am 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 one another. Between 2019 and 2021 at the Swiss Federal Institute of Technology in Zurich (ETH), colleagues and I conducted a laboratory project to evolve bacterial communities and attempt to understand how their interactions change over generations.
Some strange results…
In 2021, after conducting one of the project’s experiments twice, I left ETH to join the CNRS and left it to one of my colleagues to repeat the experiment in question two more times. In research, one way to validate observations is to repeat an experiment to ensure that the results do not change between repetitions. Except that this time, the results weren’t just slightly different—they were completely reversed!
The experiment involved bringing together a bacterium described as a predator and a bacterium described as prey to assess the effectiveness of predation. During the first two trials, as well as in previous experiments we had conducted 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 proliferating but also that M. xanthus had completely disappeared from the plastic dishes (called Petri dishes, which contain the culture media) in which we were conducting the experiments.
After much questioning and lengthy discussions, we realized that the difference between her approach to the experiment and mine was that she left the petri dishes containing P. fluorescens on the lab bench—and thus at room temperature—instead of incubating them at 32 °C like M. xanthus, due to a lack of space in the incubator. It is important to note that the two species do not grow at the same rate, and therefore, before studying their interactions, they must be grown separately.
We were really surprised and eager to learn more. So we formulated a new research question: Can the temperature at which these bacteria grow determine which is the prey and which is the predator? We began by verifying that temperature was indeed the determining factor by growing P. fluorescens at 22°C and 32°C, then exposing it to the other species at 32°C, and we estimated the number of M. xanthus present after interaction.
The former prey kills and feeds on its predator!
When P. fluorescens was grown at 32 °C, we found several million M. xanthus in the plates; but when it was grown at 22 °C, we could not detect a single living cell of that 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 is not enough to kill; one must also be able to feed on one’s prey. Since it is difficult to observe a bacterium having lunch, we assessed the microbe’s ability to eat another by measuring the effects of the interaction on population sizes: 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 conducted 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 significantly reduced the number of P. fluorescens by consuming these bacteria. At 22 °C, however, M. xanthus was wiped out by P. fluorescens, and on average, we found twice as many P. fluorescens as when only the saline solution was present: the predator-prey relationship was reversed, and the former prey killed and fed on its predator!
We then sought to understand how P. fluorescens became the predator. By growing this species in a liquid medium, we discovered that at 22 °C, but not at 32 °C, it produced one (or more) molecule(s) and secreted them into the environment. It is this secreted molecule that kills M. xanthus.
In our study, P. fluorescens produces the molecule involved in predation even before interacting with the other bacterium. It is therefore likely that, at 22 °C, P. fluorescens uses this molecule for purposes we do not yet understand, and that killing M. xanthus is a side effect of this process.
Finally, we wanted to know whether this molecule could kill other bacteria or if it was specific. We therefore exposed seven other bacterial species to the secretions of P. fluorescens that had been grown at 22 °C. No other bacterial species was completely eradicated like M. xanthus, but in the presence of P. fluorescens secretions, the number of Bacillus bataviensis was reduced by an average of 10% and that of Micrococcus luteus by 50%. M. xanthus is therefore not the only bacterium that can be killed by P. fluorescens when the latter has grown at 22 °C.
We do not yet know exactly what kind of molecule in P. fluorescens enables it to prey on M. xanthus. Further experiments will be needed to discover this, and they are currently underway. What our study does indicate, however, is that, unlike the lion and the gazelle (has anyone ever seen a gazelle devour a lion?), altering a single parameter in the bacteria’s growth conditions can have radical consequences on their roles in these interactions. If bacteria that have never been described as predators can eradicate their predator simply by growing in a slightly cooler environment, it is highly likely that many predator-prey interactions among bacteria are currently unknown to us. Another implication of this study is that it is difficult to categorize bacteria simply as prey or predators. On the contrary, it is by studying their interactions that we can learn more about how these fascinating microscopic worlds function!
Marie Vasse, Researcher in evolutionary biology, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.