Haem binds at the outside of a bacterium and is transported inside by a barrel like transporter

What the presence of iron tells a bacterium about its location

All living cells require iron to live, this includes bacteria and human cells, because iron has essential functions in the cellular metabolism. The problem is, that free iron is actually toxic, so that each organism needs to make sure iron is bound by a special vehicle.

In the human body there are many different iron-carrying vehicles. Most of the iron in our body is stored in a special molecule which is called haem. It looks like the ring structure in the figure below with the iron (Fe) in the middle.

Image result for heme

Haem is part of a protein called haemoglobin which is located inside our red blood cells. The iron inside the haem inside the haemoglobin is the one that binds oxygen and carries it to all our cells.

Interestingly, when haem is bound to oxygen, it turns red, which is the reason why our blood is red. Without the oxygen, the haem complex is blue-red.

Anyway, most of our iron inside the body is stored within these haem molecules. And many pathogenic bacteria developed strategies to survive within the body by stealing our haem and thus our iron.

To do this, they produce special transporters that sit at the outside of a bacterial cell. This transporter resembles a barrel, like the blue one in the figure below, that tightly binds haem and transports it through the membrane into the bacterium. After they imported the haem, they degrade the haem molecule to set the iron free and use the iron in their metabolism.

Haem binds at the outside of a bacterium and is transported inside by a barrel like transporter

A new study just found that my beloved bacterium of interest, Pseudomonas aeruginosa, actually contains three of these transporters that import iron into the cell. In this study, the researchers tried to understand why this bacterium would need three of these haem system, since they all seemed to have the same function – to find and import haem and thus iron.

The first thing they saw was that the bacterium only produces any of these transporters when it lives in a surrounding where there is only little iron available. If a bacterium finds lots of iron, it takes it up easily without the need of these specialised transporters that cost a lot of energy.

Next, the researchers grew the bacteria with either no haem, a little haem or lots of it, while always keeping the iron low. Now they wanted to test under which conditions the bacteria would produce any of these three transporters.

When the cells grew with no or only a little haem, the bacteria produced one of the three systems, the so called Phu system. By producing this system even in the absence of haem, the bacteria are prepared for when haem comes along. Then they can immediately take it up and use it.

However, the activity of the Phu system decreased when the bacteria grew with lots of haem. In this case, the other two systems were highly produced. These are called the Hxu and the Has systems.

These two systems are special transporter systems that belong to those that I explained in my article “Bring in the iron”. They contain the same barrel as I showed above but the transporters are also involved in information sensing and messaging. This means that they not only take up the haem but they also tell the bacterium that there is haem on the outside.

So this is one possibility for the bacterium to know that it is located inside a host, like the human body. Because nowhere else would a bacterium find haem and especially in this amount.

Since the bacterium knows that it made its way into a host, it can produce all the heavy machinery to invade the host, make it sick and all the other things I explained in this article “How bacteria get (too) attached“.

But why does the bacterium need two of these haem transporting systems? Likely as a safety net in case one of them fails.

The researchers indeed found that the two systems can replace each other if one of them was missing. So with this the bacterium makes sure that in any case the haem will be taken up and the iron used.

Now, the next step for the researchers will be to try to find drugs that can inhibit any of these transporters. This would cut off the messaging process and the bacterium would not know that it is currently inside a host so that it would not activate its heavy machinery to make us sick.

In all, this study told us a lot about how bacteria learn about their surrounding and showed us a new alternative to fight these nasty bugs!

Take away from this week:

  • Bacteria learn about the presence of haem in their surroundings to know they found a host
  • Bacteria can use our haem to extract iron for their metabolic processes
  • These transporters represent promising targets for anti-microbial drugs

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