Should I kill or should I go? Bacteria making decisions

Remember my favourite bacterial killing machine?

You probably know the weapon called type 6 secretion system by now from my previous articles Bacteria killing each other, Bacteria firing powerful spikes and Bacteria firing powerful spikes with hats.

But having this powerful weapon, how would bacteria know when to use it or not? 

They can’t just fire these arrows randomly all the time. Producing the machine as well as the arrows costs a lot of energy. Hence, bacteria need to make sure they only produce and fire this weapon when required.

Two recent studies (here and here) looked at four different bacterial species from the Vibrio family and tried to understand whether the species would regulate the production and firing of this weapon in a similar manner.

They focused on Vibrio cholerae, which is the causative bacterium for choleric diarrhoea; on Vibrio parahaemolyticus, which is a seafood-borne cause for gastroenteritis; Vibrio alginolyticus, another seafood-poisoning cause, and the squid symbiont Vibrio fischeri.

All of these bacteria live in marine environments and can live in or infect fish or seafood, which is why they are being studied in depth. They also contain at least one of the T6SS killer weapons.

In the two studies, that I also explain in depth in this article, the researchers used different methodological approaches, but they came to similar conclusions.

They found that in the tested Vibrio species two proteins are mainly controlling the production of the T6SS weapons. Here, we will call these two controllers protein X and protein Y.

Interestingly, in all tested species, protein Y also controls the motility of the bacterial cell and protein X controls an activity that is called competence. 

Motility is the ability to swim using a bacterial rotor, similar to what I described here.

Bacterial competence means that the bacterial cell produces a special machine on the cell surface that picks up free DNA from the environment. Once a bacterium takes up external DNA, it can integrate the new DNA into its own DNA and become an evolved bacterium with new “powers” depending on what the new DNA is for.

In the bacterium Vibrio cholerae it is already known that both proteins X and Y control the T6SS weapon. Researchers showed that as soon as Vibrio cholerae killed a neighbouring bacterium, it activates its competence machine and takes up the DNA of the dead bacterium. This behaviour allows Vibrio cholerae not only to kill competitors, but also to advance with the new DNA and thus to better adapt to challenging environments.

It was also suggested that if Vibrio cholerae realises it does not have a chance against its competitor, protein Y activates motility and allows the bacterium to swim away and thus escape the predator.

The questions, both studies asked whether control of the T6SS weapon would work similar in the other Vibrio species.

The two controller proteins X and Y control the activities of the T6SS weapons in the bacteria Vibrio cholerae, Vibrio fischeri, Vibrio parahaemolyticus and Vibrio alginolyticus. X and Y also activate motility and competence.
This figure was adapted from Wettstadt (2019).

Vibrio parahaemolyticus has two T6SS weapons and while one, the so-called T6SS1, is active at lower temperatures (23 °C), the other one, the T6SS2, is active at warmer temperatures (30 °C). Both studies also showed that proteins X and Y activate the T6SS2. Interestingly, they showed that the T6SS1 is activated by protein Y and even deactivated by protein X. 

In Vibrio fischeri, there is only one T6SS weapon and that one is activated by protein X, but protein Y does not have any impact on it.

Vibrio alginolyticus also contains two T6SS weapons, the T6SS1 and the T6SS2. Here, the T6SS1 is specifically activated by protein Y and the T6SS2 by protein X.

Now, the question is, why would very similar proteins have different impacts on the T6SS weapons in different bacteria?

As there is currently no clear answer to this, my guess would be that it depends on the killing power of the T6SS weapon. Some T6SS weapons can kill other bacteria, so that it then makes sense to have the competence machinery activated and the attacker bacterium can take up the dead bacteria’s DNA. 

However, other T6SS weapons are supposed to kill higher species, like fungi or amoeba, whose DNA a bacterium could not use. In this situation, the competence machinery is not needed. In such situation, motility would be more appropriate to trigger an escape mechanism.

However, these are guesses and much more research is currently needed to fully understand this fascinating killing machine.

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