Death of Bacillus subtilis gives rise to new cellular structures


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Death of Bacillus subtilis gives rise to new cellular structures.

Despite their simple structure, bacteria never cease to surprise scientists with peculiar mechanisms of survival and adaptation. Pili and flagella are the most well-known, and probably the most studied, bacterial structures. To quickly refresh your memory on the composition of a bacterial cell, pili are short, hair-like structures that are responsible for the adherence to surfaces. Some types of pili even participate in bacterial replication. Flagella, on the other hand, is a rigid structure that powers motility in many bacteria. But have you ever heard of nanotubes? They are found in bacteria Bacillus subtilis, and, unlike the other cellular appendages we just discussed, these structures are solely composed of lipids that make up a cytoplasmic membrane. Lipid composition is quite rare for an organelle, but it allows increased flexibility and quick assembly. 

Types of cellular appendages that are commonly found in bacteria. Source: 

There are two classes of bacterial nanotubes found specifically in B. subtilis. Extending nanotubes are responsible for increasing the surface area of a cell, which allows bacteria to uptake nutrients from the environment. Intercellular nanotubes create a link between cells of two bacterial species or can even connect bacteria with eukaryotic cells. 

Extending and intercellular nanotubes in B. subtilis. Intercellular nanotubes (green arrow) connect different species of bacterial cells or bacterial cells with eukaryotic cells. Extending nanotubes (black arrow) project outwards into the environment. Source: 

Pospíšil and his colleagues conducted a study, in which they examined genes and environmental conditions necessary for the formation of nanotubes in B. subtilis. From the word “tubes”, it would be easy to think that these structures are responsible for the transfer of nutrients or genetic materials. After all, bacteria live in communities, where cells share acquired resources. 

It turns out, however, that nanotubes are formed only under stress conditions. More importantly, these structures develop when cells are in the process of dying or even after death. They do not serve as channels of transportation; instead, they are a sign of disintegrating cells. This conclusion was made after the experiment, in which the researchers pressed a coverslip against bacteria to create a monolayer, a single layer that is one cell in thickness. Applying pressure from a coverslip is enough to induce a stress response in bacteria and initiate the process of assembly of nanotubes. But how much time does this process take? The answer may surprise you. It takes a matter of seconds for these nanotubes to assemble and appear! As soon as the researchers registered a decrease in signal coming from a cell membrane, there was an increase in nanotubes formation. Therefore, it was assumed that once cells start to die and fall apart, occurring weak spots serve as channels through which nanotubes emerge. 

To get to the bottom of the bacterial nanotubes mystery, the researchers studied genetic mechanisms that regulate the formation of these structures. When a bacterial cell experiences stress, it either adapts to these conditions or, when the stress is too severe, dies. In situations when a cell fails to adapt to stress conditions, transcription factor SigD becomes essential. With the help of the group of genes, called CORE element genes, this factor speeds up the process of gene transcription. In addition, as mentioned earlier, nanotubes are composed of lipids, not proteins like the rest of organelles in a cell. To weaken and break the cell wall, autolysins LytE and LytF are necessary. Autolysins are enzymes that break down the components of bacterial cells. 

The model of nanotubes formation. Cells under stress can either adapt to new conditions or die. During or after the cell death, the plasma membrane weakens, allowing extrusion of nanotubes. Source: 

Interestingly enough, there are other bacterial species that possess similar structures that emerge from dying cells (for example, another representative of the Bacillus genus Bacillus megaterium or Escherichia coli). And once again, these organisms require some kind of stress factor to induce the formation of nanotubes. While the authors of the article conclude that nanotubes of B. subtilis are a hallmark of dying or dead cells, they do not rush to assume the same for other bacterial species. They conclude that more research should be done to derive the physiological role of nanotubes in other organisms.

Link to the original post: Pospíšil, J., Vítovská, D., Kofroňová, O. et al. Bacterial nanotubes as a manifestation of cell death. Nat Commun 11, 4963 (2020).

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