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World’s largest bacterium discovered

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An unprecedented discovery. It is caught with tweezers: the largest bacterium in the world, 5,000 times larger than its peers and with a much more complex structure, has been discovered in Guadeloupe, according to a study published Thursday in the journal Science.

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

Spotted in 2009

In his laboratory on the Fouillole campus in Pointe-à-Pitre, the researcher proudly displays a test tube containing small white filaments. When the average size of a bacterium is between two and five micrometers, “you can see it with the naked eye, I can pick it up with tweezers! “, He marvels.

It was in the mangroves of Guadeloupe 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.”. Quite quickly, electron microscopy cell description techniques show that it is still a bacterial organism. But at this size, says Professor Gros, “we weren’t sure it was a single cell” – a bacterium being 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 the work done in Paris by a CNRS researcher suggests that we are dealing with “the same cell”, explains Professor Gros.

“As high as Everest”

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

Enter Jean-Marie Volland, a young post-doctoral student of the University of the West Indies, who will become the first author of the study published in Science. Having failed to get a teaching-research position in Guadeloupe, the 30-year-old flew to the United States, where the University of Berkeley recruited him. When he left, he planned to study “the amazing bacteria” he was already familiar with. It would be like meeting a human being the size of Everest, he thought to himself. In the fall of 2018, he received a first package sent by Professor Gros to the University-run Lawrence Berkeley National Laboratory Genome Sequencing Institute.

The challenge was essentially technical: to be able to make an image of the bacterium as a whole, thanks to “three-dimensional microscopic analyzes, at greater magnification”. In the American laboratory, the researcher had advanced techniques. Without forgetting an important financial support and “access to researchers expert in genome sequencing”, acknowledges the scientist, qualifying this American-Guadeloupe collaboration as a “success story”. The 3D images of him finally allow us to prove that the entire filament is indeed a single cell.

Disruption of microbiology

In addition to its “gigantism”, the bacterium turns out to be also “more complex” than its peers: a “totally unexpected” discovery, which “overturns a bit the microbiological knowledge”, testifies the researcher. “While in bacteria, DNA usually floats freely in the cell, in bacteria it is compacted into small structures called seeds, a kind of small sacs surrounded by a membrane, which isolate the DNA from the rest of the cell. The cell,” Jean develops. -Marie Volland.

This compartmentalization of DNA – the molecule carrying genetic information – is “a characteristic of human, animal, plant cells … not bacteria at all”. Future research will have to say whether these characteristics are specific to Thiomargarita magnifica or whether they are found on other species of bacteria, according to Olivier Gros.

“This bacterial giant challenges many established rules in microbiology” and “offers us the opportunity to observe and understand how complexity emerges in a living bacterium,” enthuses Jean-Marie Volland.

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