A bacterium and an archaeon hand in hand.


Breaking down the microbiology world one bite at a time

A bacterium and an archaeon hand in hand.

Archaea and bacteria have been living on our planet since the very first forms of life appeared in the history of evolution. They have colonized every environment, from the deepest ocean to the highest mountains, from hot springs to cold glaciers, and even in places as unexpected as clouds!

You probably already have an idea of what a bacterium is. You know some of them cause diseases and you probably heard that others live in our bowels, but you may never have heard about archaea. Yet, they are no less interesting than their bacterial cousins: despite looking extremely alike (one could not physically differentiate a bacterium from an archaeon under a microscope), they display a stupendous range of abilities. They can survive in very salty brines or breathe methane instead of oxygen; therefore, they are often referred to as “extremophiles”.

Archaea were first discovered in extreme environments, such as volcanic hot springs. Credits:Wikicommons

In these extreme environments where archaea live, we can also find some species of extremophilic bacteria. They can live apart from each other, each consuming a specific resource, or they can also live in syntrophy, that is, they cooperate at the level of nutrition to mutually benefit each other. This syntrophy can take many forms, but the one we are studying today is quite surprising.

In their study, Shimoyama and collaborators investigated the syntrophy between a bacterium called Pelotomaculum thermopropionicum (that we will call PT and refer to as a “she”) and an archaeon named Methanothermobacter thermautotrophicus (that we will name MT and refer to as a “he”). 

The bacterium PT is a fermenter: like yeasts we use to produce beer or wine, she produces her energy by transforming sugar into alcohol in the absence of oxygen. On the other hand, the archaeon MT is a “methanogen”: he breathes carbon dioxide (as we breathe oxygen) to produce his energy and turns it into methane (as we turn oxygen into carbon dioxide). 

In this case, PT’s fermentation results in the production of carbon dioxide which is directly used by MT for his respiration. This is not an unknown phenomenon for scientists: previous work already showed such syntrophy between bacteria and archaea in environments without oxygen. Then, what is so special about this one?

Bacteria can have one or multiple flagella.
Credits: Wikicommons

Well, it appears that PT attaches to MT via… her flagellum! You might already have heard this term: it is the spaghetti-like structure that spermatozoids use to swim until they reach the female’s egg. A lot of bacterial species use one (or more) flagella to move in their environment (figure).  

So why does PT use her flagellum for a completely different purpose? Apparently, it seems to have two roles: first to ensure proximity with her partner MT, and second to synchronize their metabolism. 

This synchronization is necessary because when PT is fermenting, she produces carbon dioxide which MT needs to use in a short matter of time, otherwise he may lose access to it. But then you may ask, how do we know this syntrophy isn’t just a coincidence? 

Well, that’s what Shimoyama’s team investigated. They first analyzed the composition of the filaments formed by PT and MT when they live together, and they found that these filaments were in majority composed of flagellin, which is the principal protein component of the bacterial flagellum as seen in the figure.

A flagellum is composed of a big protein called flagellin, which itself is composed of several protein subunits (4 in this example). Credits: Johann Bauerfeind (2015), on iGEM.org

They also tested the affinity of this protein and found it attaches only to MT and another archaeon also known to form syntrophy with PT. It means that evolution “shaped” the flagellum of PT to specifically adhere to MT, and give PT an advantage. But it has to give an advantage to MT as well, otherwise evolution would have given MT a defense mechanism against this flagellum. Bingo: Shimoyama’s team found that MT’s respiration was more efficient when PT was attached through her flagellum. After the adhesion, some genes in MT are more expressed and methane production rises. That indicates MT perceives the flagellin of PT to prepare for syntrophy. 

In conclusion, this situation between our bacterium and archaeon is a good example of what biologists call “co-evolution”: on one side, evolution “shaped” PT’s flagellin to specifically attach to MT; on the other side, evolution “shaped” MT’s genes to be triggered by the adherence of PT’s flagellin. If we wanted to illustrate this relationship between PT and MT, we could say that they work “hand in hand”. 

Note from MicroBites:

In this article, we’ve discussed the close relationship between bacteria and archaea. But what are archaea? Archaea are like bacteria single-cell microorganisms without a nucleus and therefore categorized as prokaryotes. So what is the difference between archaea and bacteria? 

Well, archaea are the third domain of life and as seen in the figure they are categorized between Bacteria and Eukaryotes (cells with a nucleus, such as plants or you). As already mentioned, like bacteria they are prokaryotes however their cell membrane is distinct from both bacteria and eukaryotes while their DNA replication and translation machinery are closely related to eukaryotes. For some features they are like bacteria and others like eukaryotes while also having unique features. Most archaea are found in extreme environments (high temperature or high salinity) but some are also found in our gut.

Tree of life with the three domains: WikiCommons

Link to the original post: Takefumi Shimoyama, Souichiro Kato, Shun’ichi Ishii, Kazuya Watanabe, Flagellum Mediates Symbiosis Science 20 Mar 2009

Featured image: Alice van Helden (personal work, 15-04-2021), with kind permission from the author.