Bacteria sensing their environment 3.0

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 environments still blows my mind. 

In previous articles I already described chemotaxis, which is the movement of a bacterium towards or away from a chemical substance or nutrient. To read more about chemotaxis, check out the article towards the goodies

I also talked about how bacteria sense the iron content of their immediate environment. In article bring in the iron I describe how this system works and in the article how iron informs a bacterium about its location you will find how bacteria use this system to adapt to the human body. 

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 different environmental conditions like temperature change, change in pH, presence or absence of nutrient, oxygen level or the presence of certain antibiotics…

Since bacteria use these systems to sense a specific signal and then respond to it, these systems are called response regulator systems. 

The simplest of these systems is called ‘one component system‘ as they include only one protein. This protein consists of two modules. One module is the so-called 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 mediates a specific response 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 one 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 has the ability to 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 protein that is needed in this specific situation.

Schematic of a one-component system. The sensor module measure a specific signal within the bacterial cell which leads to a structural change of its response module. The response module now can dimerize and bind to DNA at specific target genes.

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

In this case, the sensory module and the response module are located within two separate proteins, the sensor protein and the response protein. Since now we have two proteins involved in this system, it is called ‘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 systems’ and a 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 and 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. Such membrane damage can be measured by specific sensory modules. 

Other sensory modules are involved in temperature measurement 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 while at low temperatures it is more rigid. Interestingly, sensory modules exist that act like thermometers by measuring the fluidity of the cell membrane. 

Once the sensor module got activated by a certain signal 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 in a way that the response module can now 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.

Schematic of a two-component system. The sensor module of the sensor proteins measures a specific signal within the bacterial cell membrane which leads to activation of its enzymatic module. This adds a little phosphate group to the receiver module of the response protein which leads to a structural change of its response module. The response module now can dimerize and bind to DNA at specific target genes.

If you want to find out how bacteria do get rid of antibiotics, check out my article about antimicrobial resistance and their problems

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 which in my opinion makes them super smart.

Take away message from this article 

  • Bacteria measure their environment and couple it directly to a specific response
  • Signals can derive within the bacterium or or outside the cell
  • The response helps the bacterium to adapt to the new environmental condition 

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