The largest bacterium in the world has been discovered

One has never been seen before. It can be caught using tongs: 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, described by AFP Olivier Gros, professor of biology at the University of the West Indies, co-author of the study .

Seen in 2009

In his laboratory on the Fouillole campus, in Pointe-à-Pitre, the researcher proudly displays a test tube with tiny white filaments. If the average size of a bacterium is two to five micrometers, it is “visible to the eye, I can pick it up with tweezers! “, He was surprised.

It was in the mangroves of Guadeloupe that the researcher observed the germ for the first time, in 2009. “At first I thought it was just a bacterium, because something two centimeters can’t be one.” Quite simply, cellular imaging techniques with an electronic microscope show that it is a bacterial organism. But at this size, says Professor Gros, “we’re not sure it’s a cell”-a bacterium that is a unicellular micro-organism.

A biologist from the same laboratory revealed that it belongs to the Thiomargarita family, an already known genus of bacteria that uses sulphides to thrive. And work done in Paris by a CNRS researcher suggests that we are dealing with “one and the same cell”, Professor Gros explains.

“Like the height of Mount Everest”

Convinced of their findings, the team attempted a first publication in a scientific journal, which failed. “We were told: it was interesting but we lacked the information to believe you”, the evidence was not strong enough in terms of images, the biologist recalled.

Enter Jean-Marie Volland, a young post-doctoral student from the University of the West Indies, who will be the first author of the study published in Science. Unable to secure a teaching-researcher position in Guadeloupe, the 30-year-old flew to the United States, where he was recruited by the University of Berkeley. When he left there, he thought of studying the “strange bacteria” he was already familiar with. “It’s like meeting someone the size of 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’s Lawrence Berkeley national laboratory, which is run by the university.

The challenge is very technical: succeed in interpreting the bacterial image as a whole, thanks to “three-dimensional microscope analysis, at higher magnification”. In the American laboratory, the researcher has advanced techniques. Not forgetting the important financial support and “access to expert genome sequencing researchers”, recognized the scientist, who qualified this American-Guadeloupian collaboration as a “success story”. The 3D images finally make it possible to prove that the entire filament is a single cell.

Disruption of microbiology

In addition to its “gigantism”, the bacterium has also become “more complex” than its peers: an “unexpected” discovery, which “repeats the knowledge of microbiology a little bit”, the researcher confirms. “While in bacteria, DNA usually floats freely in the cell, it is assembled into small structures called pips, a type of small bags surrounded by a membrane, that separates DNA from the rest of the body. cell. the cell “, progressed. Jean-Marie Volland.

This compartmentalization of DNA – the molecule that carries genetic information – is a “characteristic of human, animal, plant cells … not of bacteria”. Future research should tell if these traits are specific to Thiomargarita magnifica or if they can be found in other bacterial species, according to Olivier Gros.

“This giant bacterium questions many established rules of microbiology” and “offers us an opportunity to observe and understand how complex a living bacterium is”, enthusiastically Jean-Marie Volland.

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