I talked a lot already about how bacteria adapt to different environmental conditions. And yet, at times the simplicity and efficiency of how bacteria sense their environment still blows my mind.

The problem for bacteria is, that they need to have some means of knowing what is going on around them.

Bacterial systems to sense the environment

So, here, I want to describe another, or rather two, systems that bacteria use to first sense and then adapt to changing environments.

When I say changing environments, I mean all environmental conditions. This includes temperature change, change in pH, presence or absence of nutrient, oxygen level or the presence of certain antibiotics…

Basically, bacteria use these systems to sense a specific signal. And they then respond to that signal. This is why these systems are the so-called response regulator systems.

The one-component system

The simplest of these systems is the ‘one component system‘ as this includes only one protein. This protein consists of two modules. One module is the sensor module (bright green in the picture below) which senses or measures a certain signal within the cell. The second one is the response module (dark green) that responds in a specific way to the measured signal.

The two modules are in close contact with each other. Thus, the response module knows exactly what the sensory module measures and how it is supposed to react.

In most cases, the sensory module measures the presence or absence of a certain component, for example oxygen or iron. This happens because the sensory module has a very high binding affinity for such component.

For example, if oxygen is present within the bacterial cell, it binds to the sensory module. If oxygen is not bound to the sensory module, this module will have a slightly different structure.

This difference in structure tells the response module that oxygen is missing. When this is the case, it responds by binding to another response module from another protein.

Now, a dimer of the same protein can bind to bacterial DNA at specific positions. It binds right next to a certain gene and this DNA binding can then lead to the production of certain proteins. These proteins will now help the cell to deal with the lack of oxygen.

Like this, the response module makes sure the cell gets the right set of proteins that are needed in this specific situation.

Okay, but this system only measures conditions within the cell. How does a bacterium know what is going on outside?

The two-component system

In this case, the sensory module and the response module are located within two separate proteins. These are the sensor protein and the response protein. Since now we have two proteins involved in this system, we call it the ‘two component system‘.

Here, the sensor protein has two modules and sits in the bacterial cell membrane. The sensor module is a similar module as in the ‘one-component system’. And the second module now has an enzymatic activity (orange in the figure below).

The response protein also has two modules and lives inside the bacterial cell. The receiver module (red) waits for specific interactions with the sensor protein. Interestingly, the second module, the response module, is the same as in the ‘one-component systems’.

Because the sensor module sits in the bacterial cell membrane, it can sense signals present in or near this bacterial compartment. For example, there are certain antibiotics that damage the cell membrane. Bacteria measure such membrane damage with specific sensory modules.

Other sensory modules can measure the temperature within a bacterium. The membranes of bacteria are made of fatty acids and you know how the consistency of fat changes when the temperature changes? Similarly, with rising temperatures, the membrane becomes more fluid. On the contrary, at low temperatures, it is more rigid. Interestingly, sensory modules exist that act like thermometers and measure the fluidity of the cell membrane.

Once a certain signal activates the sensor module in the membrane, its enzymatic module also gets activated. The enzymatic module can interact with the receiver module from the response protein and adds a little molecule (P) to the receiver.

This addition changes the structure of the response protein. Now, the response module can bind to another response module, similar to what happens in the ‘one-component system’.

Hence, the response is very similar. Proteins are now produced that cope with the detected signal, for example how to deal with the changing temperature or how to get rid of the antibiotic.

The two-component system is also part of the chemotaxis system which controls the movement of bacteria towards nutrient.

Bacteria know how to adapt to the changing environment

In all, bacteria found an amazingly simple but efficient way to know what is going on in their environment. Using these systems, they know exactly how to deal with the changes based on the information they receive. This simple trait, in my opinion, makes them super smart.

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