Seen to the bare eye. The biggest bacterium on this planet found in Guadeloupe.

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, a professor of biology at the University of the West Indies and co-author of the study, told AFP.

In his laboratory on the Fouillol campus in Pointe-à-Pitre, the researcher proudly displays a test tube containing tiny 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 out with tweezers!”, he marvels.

It was in the mangroves 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 two centimeters cannot be one”.

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

But at that 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 lab reveals that it belongs to the Thiomargarita family, a well-known bacterial genus that uses sulfides to thrive.

And the work carried out in Paris by a CNRS researcher suggests that it is “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 imaging, recalls the biologist.

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

Not having obtained a professorship-researcher in Guadeloupe, the 30-year-old flew to the United States, where UC Berkeley recruited him. Going there, he had in mind to study the “incredible bacteria” he was already familiar with.

“It would be like meeting a human as high as Mount Everest,” he thought to himself. In the fall of 2018, he received a first packet sent by Professor Gros to the university-run Lawrence Berkeley National Laboratory’s genome sequencing institute.

The challenge was essentially a technical one: to be able to obtain an image of the bacteria as a whole, thanks to “three-dimensional microscopy analyses, at higher magnification”.

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

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

In addition to its “gigantism”, the bacterium also proves to be “more complex” than its peers: a “totally unexpected” discovery, which “greatly shook up knowledge in microbiology”, attests to the researcher.

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

This compartmentalization of DNA – the carrier molecule of genetic information – is “a characteristic of human, animal, plant cells… and not of bacteria”.

Future research will have to say whether these features are specific to Thiomargarita magnifica or whether they are found in 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 arises in a living bacterium”, enthuses Jean-Marie Volland.


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