Researchers have meticulously examined the unique set of tools that a predatory marine bacterium possesses, which may one day find applications in medicine.
The oceans are home to countless bacteria, all confronting a significant challenge: the nutrients they require for growth are sparse and unevenly distributed across vast waters. While some areas may be rich in nutrients, many others are quite deficient. Consequently, some bacteria have evolved into adept hunters, seeking out new food sources from other microorganisms.
Despite the effectiveness of this hunting strategy, only a handful of predatory bacterial species have been discovered thus far. One well-known species is the soil bacterium Myxococcus xanthus, while another is Vampirococcus, which consumes its prey as if it were a vampire.
In a recent investigation, researchers from ETH Zurich, led by Professor Martin Pilhofer from the Department of Biology, together with colleagues Yun-Wei Lien and Gregor Weiss, introduced another rare predatory bacterium: the filamentous marine species Aureispira.
The researchers revealed that Aureispira possesses molecular structures resembling grappling hooks, used for catching its prey. Additionally, it has a mechanism similar to a bolt gun, which it employs to incapacitate its victims.
Like a pirate ship hunting for a target, Aureispira quickly moves along surfaces in pursuit of its prey, such as Vibrio bacteria. If its target is floating nearby, it waits patiently. Once in close range, the grappling hooks latch onto the victim’s flagella, preventing escape.
In a matter of seconds, Aureispira fires its internal cannons to create openings in the membrane of the Vibrio bacterium, allowing it to absorb the leaking cell components as nourishment. “The whole scene resembles a pirate raid on another ship,” Pilhofer jokes.
Aureispira only turns to a predatory lifestyle when nutrient levels in its environment drop. When there is enough nutrient supply, this bacterium refrains from hunting and disarms its biological weapons. However, once it is deprived of nutrients, it becomes motivated to hunt, leading it to reconstruct its cannons and hooks. This type of selective predation is referred to as ixotrophy. Alongside Martin Polz’s group from the University of Vienna, researchers found that this hunting behavior is not only observable in lab settings but also in actual marine samples.
Innovative imaging techniques provide new insights
The research team utilized various imaging technologies, including light microscopy and cryo-electron microscopy, to examine the function and molecular makeup of the grappling hooks and cannons.
This approach allowed for the preservation and analysis of molecular structures without distortion and in their natural cellular context. An advanced version of these methods can even reveal the molecular arrangements of the proteins that comprise the bacterium’s weapons. “All these imaging techniques are accessible at ETH Zurich’s ScopeM competence centre, which made this research possible,” Weiss notes.
What are the implications of these findings? “Primarily, this research stems from our natural curiosity,” Pilhofer explains. For the past decade, he and Weiss have focused on understanding contractile injection systems — the term used for the pirate bacteria’s internal cannons.
In other predatory bacteria, these contractile injection systems often contain toxins that can quickly kill their prey. It is possible that such bacterial bolt guns could be engineered to carry active substances for targeted injection into individual cells using a molecular mechanism.
Some predatory bacteria are known to feed on cyanobacteria, commonly known as blue-green algae. This suggests their potential use in managing algal blooms or controlling the proliferation of Vibrio bacteria. “These bacterial predators excel at their task,” Weiss remarks.
