Schematic of Contact dependend growth inhibition between bacteria. The attacker releases a toxin which binds on the surface of the prey bacterium.

Death on a stick

This week will be all about another one of my favourite killing devices bacteria invented.

This one is called CDI for contact-dependent growth inhibition, which means bacteria kill (inhibit the growth of) other bacteria when they are close to each other (in contact). Or as it was called recently as well : Death on a stick!

Recently, researchers were actually able to see that stick on a bacterium. Imagine, how tiny a bacterium is and they managed to even see that tiny stick sticking out of the bacterium! Major effort!

Image of a bacterium with a stick showing contact dependend growth inhibition
image description

This system basically consists of three components. Three components that can have such a huge impact as I tried to sketch in the figure below.

At the bottom of the figure is the attacking bacterium with its stick in the membrane. The attacker bacterium uses its stick to inhibit the other bacterium which is shown at the top with the two membranes.

Merely, there are proteins A, B and an immunity protein in an attacker’s bacterium.

Protein B (brown) makes a hole into the membrane of the bacterium so that protein A can fit through it to the outside.

Protein A is huge and has many different parts with different functions (hence the different colours in the figure). At the very end of protein A is a toxic part (red) that is responsible for the killing function of the protein.

Really important: whenever a bacterium produces a toxic component which could eventually harm the cell itself, it needs to make an immunity protein (grey). This immunity protein binds the toxic component within the cell and makes sure the toxic part cannot harm the producing bacterium itself.

So the big A protein with the toxic part lives within the attacker bacterium where it cannot do anything. But how does it reach its target, the other bacterium?

Schematic of Contact dependend growth inhibition between bacteria. The attacker releases a toxin which binds on the surface of the prey bacterium.

Basically, proteins A and B work together here. B brings A out of the cell through its hole step by step.

A part of the big A now sits outside of the cell and forms a rigid stick (blue) while the rest of the protein is still inside the cell.

At the end of the stick sits a part (green) that specifically recognises another bacterium.

As soon as the green part comes in close contact with another bacterium, the rest of A is pulled to the outside as well.

The membrane insertion part of A (in yellow) now attaches itself into the membrane of the target bacterium.

This allows the last bit of A to travel through the other bacterium’s membrane and to cut itself lose from A.

Now the translocation part (orange) binds a specific transporter in the inner membrane of the target bacterium.

This transporter then imports the toxic domain (red).

If the target bacterium does not have the matching immunity protein (grey) for this toxin domain, the imported toxin domain can go crazy and damage the bacterium as it wishes. 

As so often in microbiology, this is a highly complex mechanism and not every step is a hundred percent understood yet. But researcher are currently heavily investigating this system. This could allow us eventually to use this system for our own advantage.

This system is also super specific. This means it can only recognise bacteria from the same species. Hence an E. coli bacterium (the one from your gut) can only connect with another E. coli bacterium, but not with a P. aeruginosa bacterium for example. 

The interesting thing is that there are many different E. coli strains, which are like subfamilies. And usually each of these strains have a different toxic part within their A protein. This means that different strains have different “killing powers”.

So why did bacteria evolve such a cool system with different killing powers?

One theory is, that bacteria use this system to distinguish between self- and non-self bacteria. This allows them to only connect with bacteria from the same species or family.

So with this, they could form a very exclusive network of only like-minded (or -powered) bacteria and all the other bacteria that do have a different toxin (and obviously do not belong to the same family) will be excluded from the community

However, these so far are theories and we are still trying to understand bacteria and their behaviour better!

I am positive that we will hear a lot more about this fascinating system in the near future. It would be one promising weapon that we maybe learn to modify and use to our own advantage, namely in the fight of all those superbugs! so stay tuned!

3 thoughts on “Death on a stick

  1. Awesome post! Great and clear explanation … but I have one question. Do the bacteria have a “choice” (ie. need specific conditions/proteins to be present) about whether they use their stick of death or is it an automatic, instinct-type response to being in contact with another bacterium?

    Liked by 1 person

    1. Hey, thanks for response. That’s a great question and I’m not too sure that has been found out yet. But I guess it is rather an instinct as in there is another bacterium, let’s kill it…


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