The largest bacteria in the world discovered in Guadeloupe

It can be caught with tweezers: the world’s largest bacterium, 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, looks like an “eyelash” and shakes the codes of microbiology, Olivier Gros, professor of biology at the University of the West Indies, co-author of the study, described to AFP.

In his laboratory on the Fouillol campus in Pointe-à-Pitre, the researcher proudly displays a test tube containing small white filaments. When the average size of a bacterium is two to five micrometers, “it’s visible to the naked eye, I can grab it with tweezers!” he marvels.

It was in the mangrove swamp of Guadalupe that the researcher observed the microbe for the first time, in 2009. “At first I thought it was anything but a bacterium because something two centimeters can’t be.”

Quickly, cell description techniques with electron microscopy show that, however, it is a bacterial organism.

But at this size, says Professor Gros, “we weren’t sure 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, a well-known bacterial genus that uses sulfides to develop.

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

– ‘As big as Mount Everest’ –

Convinced of their discovery, the team attempts a first publication in a scientific journal, which fails. “They told us: it is interesting but we lack information to believe it”, the proof is not solid enough in terms of image, the biologist recalls.

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

Not having obtained a teaching-research position in Guadeloupe, the 30-year-old flew to the United States, where he was recruited by the University of Berkeley. Going there, he had in mind to study “the incredible bacteria” with which he was already familiar.

It would be like meeting a human as tall 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 the university-run Lawrence Berkeley National Laboratory.

The challenge was fundamentally technical: managing to generate an image of the bacterium as a whole, thanks to “three-dimensional microscopy analysis, at higher magnification.”

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

Their 3D images finally make it possible to prove that the entire filament is, in fact, a single cell.

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

“While in bacteria the DNA normally floats freely in the cell, in these it is compacted into small structures called nuggets, a kind of bag surrounded by a membrane, which isolates the DNA from the rest of the cell”, Jean-María develops. Volland.

This compartmentalization of DNA -the molecule that carries genetic information- is “a characteristic of human, animal, plant cells… not bacteria at all”.

Future research will have to say whether these features are specific to Thiomargarita magnifica or are found in other species of bacteria, according to Olivier Gros.

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

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