Bacteria kill other bacteria to flourish in an environmental niche

Bacteria killing each other

Most people have only ever heard of those bad bacteria that make us sick and give us diarrhoea or pneumonia. However, few people actually know that those little bugs are all around us. They live in every corner of our body, they can be basically found everywhere in nature, be it in the forest, in the sea or in volcanoes. They are also amazing flatmates, sitting on your toilet, in the kitchen or in your bed.

Now for example, imagine an environment within our body which is full of nutrients and metals, like our gut. A real paradise for bacteria because everything we eat, arrives here. This is why we have thousands of different bacterial cells from different species and families in our gut and every little bacterium wants to get some of the goodies, wants to survive, grow and reproduce. So each single bacterium has to make sure it is the major inhabitant of this environment. And there is no other way than to kill these other bacteria.

Bacteria kill other bacteria to flourish in an environmental niche

To achieve this, some bacteria have developed an incredible toxic weapon to eradicate these annoying food-stealing bacteria. In the scientific world, this weapon is called the “type 6 secretion system“. However, this is not the only killing machine that bacteria use. But more about other ways bacteria kill each other later. 

For now, we will focus on this special and relatively newly discovered type 6 secretion system. You can imagine this as a tiny crossbow with a sharp arrow that sits on the outside of a bacterium. It just sits there, fully loaded and ready to be fired. When our bacterium with the crossbow meets another bacterium that could be a danger, it becomes aggressive which is when we call this an attacker. The other bacterium becomes the so called prey, because the attacker will shoot its arrow into the prey which will ultimately lead to its death. 

This tiny crossbow is supposed to kill other bacteria? Yes, and very efficiently indeed.

And all this due to the arrows. They are the real deal.

Let’s look at this in more detail. The arrow is clamped within the crossbow and has a very sharp spike and a long bolt. The spike of the arrow is needed to punch a hole into the prey bacterium. This ultimately leads to the delivery of the whole arrow into the prey. The interesting thing now is that toxins are glued onto the arrow. By firing the arrow out of the cell and into the prey, the glued toxin reaches the other bacterium as well. 

An attacking bacterium fires its T6SS arrow into a competitor

So what is this toxin? Toxins are enzymes that chew essential components of a bacterial cell, like its DNA or the membrane, or damage the cell by interfering with essential metabolic circuits. Without these components the prey bacterium cannot survive and will eventually die. There will be probably more on how bacterial toxins work in a later post. 

Important for now is, however, that whenever a bacterium produces a toxin, it also produces an immunity compound which neutralises the toxin. With this the toxin-producing cell is not damaged by its own toxin.

Now, another cool thing about some bacteria is that they can have more than one of these crossbows, some bacteria even up to six different ones. All of these crossbows can shoot different arrows, some species contain around ten of them! And to make it even more mind-blowing: Each of these arrows has its own specific toxin glued to it. And some arrows can carry multiple different toxins. 

So within one bacterial cell, there are many different toxins. Don’t forget that within a bacterial cell there is a bunch of other proteins and cellular compounds as well. So each toxin has its own patch with which it specifically recognises its complementary patch on its cognate arrow. Then, this equipped arrow can be loaded onto its designated crossbow. And again, it somehow needs to specifically find its pairing crossbow within this mess of a bacterial cell. However, the patch and the complement of each matching toxin-arrow pair still remain to be found by scientists.

This means, that when one bacterium meets a prey that it wants to kill, it can choose between its many crossbows and arrows and toxins. Each one of these combinations probably has its own advantage under different circumstances and against different preys. However, how a bacterium decides when to use which crossbow and which arrow and which toxin is still unclear. 

This is why scientists are working hard to find out how this fascinating system exactly works. One goal would then be to use this nano weapon in other applications as for example to kill those bacteria that we cannot fight with commercial antibiotics anymore. 

Take away from this week’s article

  • bacteria can kill each other to establish their own survival
  • they do that with a nano cross bow filled with a lethal arrow
  • one bacterium can have many of these crossbows shooting specific arrows

This is part 1 of a series of bacterial killing nanomachines. Read part 2 here and part 3 here.

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