The discovery of a bacterium that is 10,000 times larger than normal surprises microbiologists

The discovery of a bacterium that is 10,000 times larger than normal surprises microbiologists

The living organisms that we see around us are made up of billions or even trillions of cells. Multicellular organisms are often considered more complex than smaller unicellular organisms. Eukaryotes, a group that includes humans, have complex cells that can form large multicellular organisms. Bacteria and Archaea, on the other hand, were considered simple cells that never grow large enough to see with the naked eye. However, new techniques have made it possible to identify microbes that can grow to a size of several hundred micrometers, about 100 times larger than the average size of known bacteria.

A group of researchers from France and the USA investigated giant microorganisms living in the seawater of mangrove forests, where they found strings of whitish bacteria over a centimeter long growing on sunken leaves. While several species of bacteria are known to form filaments by chaining multiple cells together, each strand was made of just one long organism. The group named this organism Candidate Thiomargarita magnifica, means magnificent (magnifica) sulfur pearl (Thiomargarita) that has not yet been grown in a lab (Candidate).

To understand how Candidate Thiomargarita magnifica can grow so large that the researchers used several different dyes to mark where compartments can form in the cell. Using a technique called confocal laser scanning microscopy, which uses a focused beam of light and a special pinhole that blocks out-of-focus objects to produce images of everything in a sample at a certain depth, the team found many different compartments. One such compartment was a single giant vacuole, essentially a fluid-filled sac, in the center of the cell.

Many bacteria lack active ways to move nutrients around. Because of this, they have to rely on chemical diffusion, the same process that causes milk to slowly diffuse into the coffee. On a scale of a few micrometers, diffusion is an efficient way to move nutrients around. On the other hand, larger cells cannot receive essential nutrients fast enough by diffusion and are forced to remain small or die.

But the research team suggests that by having a large vacuole at its center, which takes up much of the internal space, Candidate Thiomargarita magnifica can minimize the distance that nutrients need to move within the cell while supporting its gigantic size.

While the large vacuole compartment was striking, the researchers noted many smaller membrane-bound compartments in the cell as well. The team was curious if these compartments could hold DNA, which could make them similar to the nucleus of eukaryotes. When the team used a DNA-specific dye called DAPI, they found that thousands of these compartments contained DNA. The team named these compartments pepins.

By then counting all the DAPI-stained pepins, the research team found that Candidate Thiomargarita magnifica had over 36,000 copies of its genome. In comparison, our human cells contain two copies of our genome and are called diploid cells. With so many copies of its genome, this giant is a polyploid cell and can use these copies individually to produce RNA.

Most of the pepins also had ribosomes that use RNA to produce proteins. This means that throughout its long, stem-like structure, Candidate Thiomargarita magnifica can create new proteins rather than waiting for them to be transported over thousands of micrometers to reach the place where they are needed.

The researchers were curious as to how such a large bacterium could reproduce. By collecting many different samples, they found that Candidate Thiomargarita magnifica produces small buds from the end of the cell which can then slowly grow into a fully mature, centimeter-long giant. During most cell divisions, proteins that pinch the center of the cell are produced to pinch the parent cell in half, producing two new cells. However, this organism appears to lack many of these proteins. Instead, it has many copies of proteins that elongate and support the cell.

Finding a bacterium that we can see with the naked eye challenges many of the assumptions we make about life. The team notes that further research is needed to understand how this newly discovered organism regulates its metabolism, size and many other functions. Although microbiologists have learned a lot, there is always something new to discover when it comes to the micro and macro scales of life.

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