The largest bacterium in the world discovered in Guadeloupe reveals its secrets

It can be caught with tweezers: The largest bacterium in the world, 5,000 times larger than its peers and with a much more complex structure, was discovered in Guadeloupe, according to a study published Thursday in the journal Science.

“Thiomargarita magnifica” measures up to two centimeters, resembles an “eyelash” and shakes up the codes of microbiology, described to AFP Olivier Gros, professor of biology at the University of the West Indies, co-author of the study.

In his laboratory on the Fouillol campus in Pointe-à-Pitre, the researcher proudly displays a test tube with small white filaments. When the average size of a bacterium is two to five micrometers, “it can be seen with the naked eye, I can take it with tweezers!”, He wonders.

It was in the Guadeloupe mangrove that the researcher first observed the microbe, in 2009. “At first, I thought it was anything but a bacterium, because something two centimeters cannot be one”.

Pretty quickly, techniques for cellular description with electronic microscopy show that it is nevertheless actually a bacterial organism.

3D microscope image released June 23, 2022 by Lawrence Berkeley National Laboratory of the giant bacterium Thiomargarita magnifica

– Olivier GROS Lawrence Berkeley National Laboratory / AFP

But with this size, says Professor Gros, “we had no assurance that it was a single cell” – a bacterium is a single-celled microorganism.

A biologist from the same laboratory reveals that it belongs to the Thiomargarita family, an already known bacterial genus that uses sulfides to develop.

And work done in Paris by a CNRS researcher suggests that we are dealing with “one and the same cell”, explains Professor Gros.

“As high as Mount Everest”

Convinced of their discovery, the team attempts a first publication in a scientific journal, which fails. “We were told: it’s interesting, but we lack the information to believe you”, the evidence is not robust enough in terms of the image, the biologist remembers.

Enter Jean-Marie Volland, a young post-doc student from the University of the West Indies, who will be the first author of the study published in Science.

3D microscope image released June 23, 2022 by Lawrence Berkeley National Laboratory of the giant bacterium Thiomargarita magnifica

– Olivier Gros Lawrence Berkeley National Laboratory / AFP

After not getting a job as a teacher-researcher in Guadeloupe, the 30-year-old flew to the United States, where the University of Berkeley recruited him. When he got there, he had in mind to study “the incredible bacteria” he was already familiar with.

“It would be like meeting a man as high as Mount Everest,” he thought to himself. In the fall of 2018, he received a first package sent by Professor Gros to the Genome Sequencing Institute at Lawrence Berkeley’s National Laboratory, which is administered by the university.

The challenge was essentially technical: to succeed in reproducing an image of the bacterium as a whole, thanks to “three-dimensional microscopic analyzes, at higher magnification”.

In the US laboratory, the researcher had advanced techniques. Without forgetting significant financial support and “access to expert researchers in genome sequencing,” the scientist recognizes and qualifies this American-Guadeloupe collaboration as a “success story.”

3D microscope image released June 23, 2022 by Lawrence Berkeley National Laboratory of the giant bacterium Thiomargarita magnifica

– Jean-Marie VOLLAND Lawrence Berkeley National Laboratory / AFP

Its 3D images finally make it possible to prove that the entire filament is actually a single cell.

In addition to its “gigantism”, the bacterium also turns out to be “more complex” than its peers: A “totally unexpected” discovery that “shakes a lot of knowledge in microbiology”, the researcher testifies.

“While the DNA is usually in bacteria, it flows freely in the cell, in these it is compressed into small structures called pips, a kind of small bags surrounded by a membrane which isolates the DNA from the rest of the cell”, Jean develops – Marie Volland.

This division of DNA – the carrier molecule for genetic information – is “a hallmark of human, animal, plant cells … not at all for bacteria”.

Future research will have to say whether these properties are specific for Thiomargarita magnifica, or whether they are found on other species of bacteria, according to Olivier Gros.

“This bacterial giant questions many established rules in microbiology” and “gives us the opportunity to observe and understand how complexity arises in a living bacterium”, says Jean-Marie Volland enthusiastically.

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