If you thought there was only a handful of ways how bacteria can kill each other, you were completely wrong 🙂
Bacteria and other microorganisms like viruses and fungi found endless ways to make each other’s life as painful as possible. In an evolutionary context, this makes perfect sense. Everyone/everything just wants to survive and spread its own genetic material. So in some cases they need to make sure they are the only living being in a certain environment.
I don’t know why I am so fascinated by the way bacteria interact with each other. I guess it is about the fact that these single cell organisms have some kind of conscience – or whatever you want to call it; some kind of mechanism that makes the decision for them: nice neighbour or killer bacterium?
In this blog post I want to introduce another strategy of how bacteria combat in the bacterial warfare. By using this strategy, bacteria seem to be more social than we previously thought.
This strategy can be used only by some bacteria and involves the production of a so called bacteriocin. These bacteriocins are toxic proteins that either kill or inhibit the growth of closely related bacteria.
When a bacterium produces a bacteriocin, it also always produces an immunity protein. This makes sure that the producing bacterium itself is safe from the toxicity of the bacteriocin.
Additionally, bacteria produce a lysis protein. This lysis protein destroys the cell membrane of the producing bacterium.
Thus, once a lot of the bacteriocin has been produced, all of it is being released into the surrounding. This bacteriocin is then having a toxic effect on nearby bacteria.
Hence, by one bacterium committing suicide many more neighbouring bacteria are being killed.
Obviously, production of these three proteins needs to be very well regulated. This means, a bacterium has to make sure that it is not all the time producing a toxin and then just dying from its release.
However, it is still quite unclear how exactly this regulation process works and why bacteria sometimes spontaneously lyse and release this toxin. Now a recent study showed that bacteria can use this strategy also to communicate with each other and send alarm signals.
For this, researchers looked at how two colonies of the same bacterium interact when both colonies produce different bacteriocins. A bacterial colony is the accumulation of millions of bacterial cells. Due to the high amount of cells, we can actually see such colony with the eye.
The researchers started by putting bacteria in two spots right next to each other, let them grow and took pictures with the microscope. In the above figure, none of the bacteria actually produce a bacteriocin, which is why there are big fat circles or spots of bacterial colonies; so everyone is happily growing.
The bacteria in the left spot do not produce bacteriocins but instead have a so called reporter gene. This reporter gene gets activated when the bacterium WANTS to produce a bacteriocin. Activating the reporter gene turns a bacterium green.
However, this is not the case here. The bacteria from the right spot do not produce a bacteriocin and thus, the bacteria from the left spot do not feel like producing a bacteriocin.
Now, the bacteria in the right spot produce a bacteriocin. The bacteria in the left spot are the same as above with the reporter but no bacteriocin.
You can see two things: there is some kind of clearance at the interface between the two spots. Bacteria in the left spot are killed by the released bacteriocins from bacteria in the right spot.
Further, in the left spot there is a very green region next to the killing zone. Thus, the remaining bacteria in this region are getting angry and turn into green little Hulks. This means they would very much like to counterattack that right spot – however, they can’t because they don’t have a bacteriocin.
Next, bacteria in the left spot have the reporter AND they produce a bacteriocin. So now, they turn green and are able to actually fight back. Which is exactly what they do.
The clearing zone is now present in both spots which means that bacteria in both spots are killed.
Also, almost the whole left spot turned green now. In the above figure, only a small zone close to the killing zone was green while now the whole left spot is ready to counterattack that right spot.
So what does all of this mean?
It seems that bacteria have some kind of competition-sensing mode. As soon as bacteria realise that there is danger, they communicate with each other and start fighting off the predator together. Basically, someone angers Hulk; he turns green and assembles all the other Avengers so they can put an end to it together.
In here, researchers showed that bacteria have some kind of social behaviour. This was somewhat unexpected and tells us that bacteria are a lot smarter than we previously thought.
(All of the figures here used were adapted from Mavridou et al.)