There are about 40 trillion bacteria living in and on you, which means things are pretty crowded at the microscopic level. With all that competition between microbes, some have developed a novel way to deal with unwanted neighbors: a tiny speargun that injects lethal poisons through a mechanism known as a type VI secretion system (T6SS). Now researchers at the University of Basel in Switzerland have figured out just how the deadly weapon works.

About two years ago, UB professor Marek Basler determined the atomic structure of the bacterial spearguns in their post-firing state, after they were contracted. The new work provides the molecular nitty gritty on their pre-firing state, when they are fully extended.

By using a technique called cryo-electron microscopy, which involves a frozen specimen that is beamed with electrons, Basler and his team imaged the speargun sheath in its extended position. They found that the sheath has about 200 protein rings arranged around the internal rigid spear. These rings act like cogs to contract the sheath and twist the spear outwards like a drill into neighboring cells or bacterial competitors.

"During the sheath contraction, ring after ring turns and gets closer to the previous ring, while the ring diameter expands and thus releases the spear," explains Basler. "This combination of sheath shrinking and turning results in drilling a hole into the target cells. Within less than two milliseconds, the T6SS sheath contracts to half of its length and at the same time the toxic spear spirals out like a screw. Therefore, the bacteria have an extremely powerful drill."

After bacteria use the T6SS system to destroy their neighbors, the speargun stays in its contracted state until it gets disassembled. Then a new one is created from the old material, ready to fire again. But, says Basler, no one really understood why the contracted sheath was chemically degraded while the extended one was left intact. The new research solved the mystery.

It turns out that the answer lies with a protein. When the sheath is extended, the protein is hidden, but when it contracts it becomes visible to other proteins that come in and disassemble it.

Some of the bacteria that employ the speargun method can cause causing cholera or pneumonia, so understanding the method by which they attack and defend themselves could lead to new ways to fight or disable them.

Now that Basler and his team have unraveled the mysteries of how the speargun works, they will continue investigating the mechanism, trying to find out – among other things – how the speargun in anchored in the bacteria.

"One of our projects is dedicated to the question of how the T6SS is embedded in the bacterial cell envelope," he says. "As the speargun is fired with such a high force, it must be firmly anchored, otherwise firing would not work properly or could be also fatal for the weapon-carrying bacteria themselves."

The research has been published in the journal Nature Biology.

The following video shows the molecular speargun in action.