New insight into self-defence at the nano-level.
Bacteria have to watch out for themselves- mean amoeba like to snack on them, catching them through their pseudopodia to be digested at a later date. Not the nicest of fates. And thankfully, scientists have now discovered some bacteria are well equipped to defend themselves.
Scientists from ETH Zurich have found a mechanism that they assume is crucial for the survival of Amoebophilus inside the amoeba. The bacterium has devices to shoot micro-daggers. It can use the daggers to pierce the amoeba from inside and thus escape digestion.
Amoebophilus was discovered by researchers at the University of Vienna a few years back. It doesn’t only survive inside amoeba but is so well adapted to thrive in there that they’ve have become this species’ favorite habitat.
To find the bacteria’s secret, the researchers used a novel method developed at ETH that’s currently in use only in a handful of labs worldwide. They froze amoeba that had absorbed bacteria at minus 180°C (-292°F). Then João Medeiros, the paper’s co-author, used a
focused ion beam to cut away at the specimens
, much like archaeologists chip away at soil and rock around an artifact with chisels. After milling away the amoeba and most of the bacteria, Medeiros was able to extract the bug’s defensive mechanism and produce a three-dimensional electron tomogram of it.The mechanism consists of several sheaths affixed onto the bacterium’s inner membrane by a baseplate and an anchoring platform. Their purpose is to shoot molecular-sized “daggers” at the amoeba. Each sheath “is spring-loaded and the micro-dagger lies inside it,” explains João Medeiros, paper co-author.
The scientists present for the first time the complete spatial structure of a shooting mechanism inside a cell in its natural context. They also show for the first time details of the baseplate and membrane anchor. “In the past, cell biologists investigated the function of such systems and structural biologists elucidated the structure of individual components,” says Pilhofer. “With the cryo-focused ion beam milling and electron cryo-tomography technologies that we have established at ETH Zurich, we can now close the gap between cell biology and structural biology.”
The mechanism was found to comes in really handy for the tiny Amoebophilus, as amoeba surround their pray into a stomach-like digestive vesicle (a pocket of membrane inside the amoeba filled with digestive fluids). The daggers shred through these vesicles and allow the bacteria to escape the very unfriendly conditions of the digestive bubble. But it doesn’t escape all the way — once safe, Amoebophilus stays inside the amoeba and takes advantage of its host to feed and even multiply.
The team also used genomic comparisons to peek into how Amoebophilus evolved its daggers. Their results show that “the relevant genes are very similar to bacteriophage injection systems,” which led the team to believe that these genes are originated from an ancestor of today’s bacteriophages that left bits of its code in the bacteria’s genome long ago. The results also suggest that many other bacterial strains in at least nine of the most important groups should have similar dagger arsenals. It’s not yet known whether they serve the same purpose or not, but the team says their next steps will be to find out.
The researchers further, intend to look at other complex molecular systems using this new method of cryo-focused ion beam milling. “The technique could help to address many other questions in cell, infection and structural biology. We are already working with other research groups and offering them our expertise,” says Medeiros.
