Chromombacterium transports violacein within outer membrane vesicles to kill other bacteria

Bacteria firing toxic bubbles

Welcome to this week’s chapter of Sarah’s journey through the fantastic bacterial world 🙂

Have I made it clear yet that I am truly amazed by the mechanisms that bacteria evolved to overcome absolutely any hurdle?

Hot around here? No problem, bacteria found repair mechanisms that take care of any cell rubbish that might accumulate after too much heat.

Too cold? Naaah, bacteria know how to modify their cell membrane so the lipids in it won’t solidify…

Today I would like to tell you about another cool mechanism that bacteria use to defend themselves. 

Before talking about the actual study, I just want to explain another way of how bacteria send stuff into the environment. 

As you might be aware, bacteria come in one of two kinds. They can either have one or two cell membranes. If bacteria have one cell membrane, they are called Gram-positive. If they have two cell membranes, an inner and an outer membrane, they are called Gram-negative bacteria. 

The outer and inner membrane of Gram-negative bacteria are slightly different. Interestingly, the inner membrane of Gram-negative bacteria is the same as the one cell membrane of Gram-positive bacteria. But in Gram-positive bacteria, the one membrane has a lot of stuff on top to make it thicker and sturdier.

Anyway, the cool thing that Gram-negative bacteria can do, is that their outer membrane form “blebs“. These blebs, also called vesicles, eventually form round spheres that completely detach from the membrane and are released into the environment.

outer membrane vesicle formation in Gram-negative bacteria
Gram-negative bacteria can form blebs, which contain parts of the outer membrane.

As you can see, these vesicles are made from the outer membrane of Gram-negative bacteria, which is why they are called outer membrane vesicles. So, outer membrane vesicles are basically a double layer of lipids in form of a sphere with stuff inside. 

Within these vesicles, bacteria pack anything that they want to get rid of. This can be cell junk – you know when cell machines don’t work anymore and it is not worth repairing, you just get rid of it and throw it out.

However, these vesicles can also carry important proteins or enzymes that might actually help the bacterium during infections. But more about that another time.

Today, I want to tell you about the bacterium Chromobacterium violaceum and this one uses these vesicles to kill other bacteria.

Chromobacterium violaceum produces the antibiotic violacein. Violacein is a purple molecule and turns Chromobacterium colonies into purple dots. 

Chromobacterium violaceum colonies turn purple
Colonies of Chromobacterium violaceum on a chocolate agar plate. Picture taken from de Siqueira et al, 2005

Since violacein is an antibiotic, it can kill other bacteria. However, this antibiotic only kills Gram-positive bacteria, those with only one cell membrane.

The problem with violacein is, that it is a very hydrophobic molecule, so it is insoluble in water. This is why researchers were interested to find out how Chromobacterium transports violacein through water towards other bacteria to kill them. 

So, some researchers started to have a closer look at Chromobacterium cells. They could see that these bacteria also produce these outer membrane vesicles.

Chromobacterium violaceum produces outer membrane vesicles.
Chromobacterium violaceum produces outer membrane vesicles. Picture adapted from Choi et al, 2020.

The researchers then purified these outer membrane vesicles (picture at the bottom) and added them to Staphylococcus aureus, which is a Gram-positive bacterium. This killed Staphylococcus aureus, so the researchers thought that the violacein might probably be inside the vesicles.

Then they grew a Chromobacterium mutant that did not produce any violacein. This mutant however still produced outer membrane vesicles. But, surprisingly, these vesicles did not kill Staphylococcus aureus. 

From this the researchers concluded that violacein is transported within the outer membrane vesicles.

This meant that the researchers just found a new way that bacteria can use outer membrane vesicles  

a) to solubilise a very hydrophobic molecule

b) to transport a hydrophobic and toxic molecule towards other bacteria

c) as bacterial weapons

Chromombacterium transports violacein within outer membrane vesicles to kill other bacteria
Chromobacterium violaceum sends off outer membrane vesicles filled with the antibiotic violacein to kill other bacteria. Comic by Noémie Matthey.

Now, this concept gives us some interesting possibilities to apply outer membrane vesicles.

Maybe, one day we will find a way to include therapeutic molecules into such outer membrane vesicles and send them towards tumour cells or we just found a new way to deliver antimicrobial substances in general.

For sure, scientists will come up with some cool new ideas to use outer membrane vesicles in the clinic, but as always, that requires a lot more research 🙂

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