The T6SS, an all-rounder of many - but not all - bacteria.

The type VI secretion system (T6SS) is a molecular mechanism that enables certain bacteria to kill competitors, manipulate host cells and absorb nutrients. Who would want to be without such superpowers? In fact, this secretion apparatus is widespread among bacteria. At the same time, however, there are many bacterial species that completely lack this feature. Even among strains of the same species, some have a T6SS and others do not. What could this be related to?

Both in nature and in humans, bacteria are exposed to situations where several bacteria compete for the same space or the same nutrients. Having a mechanism such as the T6SS to kill the opponent and take up additional nutrients can be a decisive advantage. For example, there is evidence that cholera bacterial strains of the 7th pandemic wave differ from strains of previous cholera pandemics because of their T6SS (see press release). So, summarizing and evaluating the current research on this once in a fundamental way can be a starting point for exciting further research questions and considerations.

Scientists from the Max Planck Institute for Evolutionary Biology, Kiel University, and the Max Planck Institute for Terrestrial Microbiology published a review article this week in which they do just that. In it, they not only lay out the current state of research on T6SS, but also offer thoughts on what factors determine which bacteria are equipped with a T6SS and which are not.

As the findings show, several areas appear to be relevant. While the T6SS is found in completely different ecosystems, e.g., both in the ocean and in the soil, it is more common in bacteria that are dependent on another organism (such as in the human microbiome) than in "free-living" bacteria.
On the other hand, many pathogenic bacteria possess a T6SS, for example, often those that belong to the multidrug-resistant germs.
It was also found that there are differences depending on the local origin of a particular bacterium. Bacteria of the same species may have a T6SS in one location but not in another. The fact that both variants often coexist also shows that the T6SS does not only bring advantages, because the energetic costs that a bacterium has to spend for the secretion apparatus are not negligible.

Thus, this analysis of the current state of research makes it clear that research on the T6SS secretion system is not yet at an end point. Rather, it will be worthwhile to look more closely at different bacterial species to find out which bacteria use the T6SS - and for what purpose.