Video: An Aureispira bacteria grabs victim cells. The second video sequence shows cross sections and a 3D model that illustrate the penetration of the “firing system” into the prey. © Yun-Wei Lien
The “pirates” of the microbial world also apparently use grappling hooks and cannons to catch prey: Researchers have gained insights into the weapon system of a predatory marine bacterium. The rabid tiny creatures catch and fixate their microbial victims using hooked appendages. They are then killed and exploited using a kind of bolt gun system. The researchers say that the predatory bacteria or their sophisticated weapon system could potentially be used for environmental management or medicine.
The focus is usually on the predators of the animal world: from lions to sharks, creatures from the world’s different ecosystems have produced fascinating prey-catching concepts. In addition to the animal predator-prey systems, there are also versions that are comparatively difficult to recognize: even in the microcosm there are hunters and hunted. It is known that certain types of bacteria prey on other microbes using sometimes sophisticated strategies. Researchers have already discovered a concept called ixotrophy in predatory bacteria that live in the sea: Microscopic examinations showed that the victims appear to stick to the predators – like insects to a flycatcher. They are then eaten by the hunter. How exactly this mechanism works, however, has so far remained unclear.
On the trail of the weapon system
In order to investigate the concept in more detail, researchers led by Yun-Wei Lien from the Swiss Federal Institute of Technology in Zurich (ETH) have now set their sights on the predatory bacterium Aureispira as a model. It is a thread-like microbe that preys on, among other things, so-called Vibrio bacteria in the sea. The team examined the Aureispira bacteria as they hunted these victims using multiple imaging techniques. In addition to light microscopy, cryo-electron microscopy was also used. This method makes it possible to fix and analyze tiny structures in a cellular context. With further development of the process, insights into the fine construction of the molecular protein structures that make up the functional units of the bacterium were also possible.
First, the team was able to use the microscopic images to document that the Aureispira bacteria move around their hunting grounds like pirate ships: if they have “sighted” Vibrio bacteria, they move towards them, dock, hold onto the normally mobile victims and kill them them finally off. As more detailed investigations revealed, this process is actually characterized in detail by “pirate techniques”: At Aureispira, the researchers discovered molecular structures that look like grappling hooks and are used accordingly. The bacterium also has a kind of bolt-action system that functions like cannon fire. The researchers explain that this is a so-called contractile injection system that has already been discovered in other bacteria.
On a privateering trip with grappling hooks and cannons
The “grappling hooks” are thread-like structures that extend from the cell surface of the Aureispira bacteria and have an anchoring structure at the end. When “hijacking” these structures take hold of the victim’s locomotor organs – the fine flagella. This binds the prey and means it can no longer escape, the researchers explain. Aureispira then fires its “cannons”: Using cryo-electron microscopy, the researchers were able to document in detail how the bolt-action units punch holes in the victims’ shells. Through further analysis, they were also able to demonstrate how the predators exploit the Vibrio bacteria: They consume the escaping cell substances as food. “The whole scene resembles a pirate attack on another ship,” says senior author Martin Pilhofer from ETH.
However, the “piracy” among microbes is born out of necessity, according to further results from the researchers: As it turns out, the Aureispira bacteria only go on raids when they are hungry. However, if the supply of nutrients in their environment is sufficient, they even forego equipping themselves with grappling hooks and cannons. The researchers found that this facultative predatory lifestyle is also reflected in the activity of certain genes. The team was also able to detect these genetic traits, on which the ixotrophy concept is apparently based, in other marine bacteria. “Ixotrophy may thus play an overlooked but crucial role in the nutrition of bacteria in heterogeneous aquatic environments and in the formation of microbial populations,” the authors write.
In addition to this importance for basic research, the results could also have practical application potential, say the researchers. Certain predatory bacteria are known to hunt single-celled cyanobacteria, which can cause toxic blue-green algae blooms in water bodies. It is possible that the pirate bacteria can be used to prevent mass proliferation of problematic microbes. There is also further potential in the bolt-action system: There are already ideas for loading these weapons with active ingredients. So the microbes could potentially inject them into cells on behalf of medicine.
Source: ETH Zurich, specialist article: Science, doi: 10.1126/science.adp0614