What do you do when you are hungry?

You go to the fridge and get something to eat or you call your favourite pizza delivery service and order your favourite pizza.

What do bacteria do when they are hungry?

It is a bit more difficult because bacteria can’t really “see” what is in their fridge. Even though their fridge might be fully stocked with all the deliciousness, they need other means of getting the stuff from the fridge.

To do this, they send out special food-catching vehicles.

What does this look like?

They send out an empty pizza box to find and catch a delicious pizza. Once the pizza box found and captured a pizza, it goes back to the cell and feeds the pizza to the bacterium.

Now, imagine, the bacterium is within a human body and the pizza that the bacterium wants is iron. Iron is an essential metal for all living organisms. Most organisms need iron for cellular respiration which is the key process to keep a cell alive.

However, iron is not floating around everywhere, because free iron is actually highly toxic for cells. This is why all living organisms keep iron bound to a specific iron vehicle, like its very own pizza box.

One of these iron vehicles for example is the haemoglobin within our blood.

So all the iron is kept away in pizza boxes, which makes it quite difficult for bacteria to find any iron within our body. But bacteria found a pretty clever way to use these pizza boxes from our body and sense that there is iron around.

Bacteria stealing our pizza (or bacteria sensing iron)

Bacteria produce their own highly specific iron vehicles (pizza boxes) that catch iron, which is the brown box with the red Fe in the figure below.

These pizza boxes find iron in the deepest corners of our body and bring it back to the bacterium. A specific transporter (the grey tube) then transports the pizza box with the iron into the cell.

The bacterium now uses this transport process to inform the inner of the cell that there is iron on the outside. Otherwise, the cell would not know about this. Remember, bacteria have no eyes and don’t know that there is pizza on the outside.

The transporter kind of works as eyes in this case, because it is like a tube that goes through the outer membrane. On the inside, the transporter connects to another component, which looks like the little red bean.

This red bean is like a messenger that hangs around between the two cell membranes. Here, it waits to receive the signal to go and find its counterpart – the pink bean – on the inner membrane.

This pink bean is bound to the yellow pacman that sits on the other side of the inner membrane. The yellow pacman is actually an inhibitor and is called the anti-sigma factor. It has this name because it binds and keeps hold of the sigma factor, the green worm thingy.

So now the pink bean gets the signal from the red bean that there is iron on the outside. And it activates the dismounting of the yellow pacman. Dismounting the anti-sigma factor/yellow pacman means that the sigma factor is set free. So, now it can go into the cell and bind its target machinery (in blue).

Sensing iron leads to making more transporters

The blue machine is RNA polymerase, which is the cell factory. This one is responsible to initiate protein production from DNA. The sigma factor is essential in this whole process. It helps the RNA polymerase to find specific genes that are required at a specific moment. In our example, the sigma factor shows the RNA polymerase the gene for the iron transporter.

Then the cell produces more iron transporters and sends them to the outer membrane. Here, the transporter waits for more iron-filled pizza boxes to transport into the cell. And with this regulatory circuit, bacteria sense iron in their environment.

A complex game with one goal – find iron!

With this rather complicated mechanism, bacteria sense iron and they know that iron is around. If this is not the case, it makes sure it only produces iron transporters when needed as they cost a lot of energy to produce. And the cell also assures that it captures as much iron as possible when it is actually available.

So this is how bacteria make sure that they know exactly what is going on in their surrounding. While we call the pizza delivery service to bring us a pizza box filled with pizza, bacteria send out some empty pizza boxes and hope that some of them come back filled with delicious pizza.

It is probably not more efficient than our way, but what can you do when you don’t have a phone to directly order your pizza.