The world's largest bacterium discovered – and you can see it with your own eyes - Digital Journal

Mycobacterium tuberculosis (Public Health Image Library, NIAID, Image ID: 18139)

A remarkable discovery has made within the field of microbiology – the largest bacterium ever recorded, in terms of overall size. The organism is the approximate size of a fly. That is whether the term ‘microbiology’ remains appropriate, given that the organism is visible with the naked eye.

The giant string-like bacterium has been isolated from Caribbean mangroves and the cell can grow up to 2 centimetres in length. This places the organism around 5,000 times larger than most bacteria. The discovery challenges the accepted understanding of bacteriology in that the organism goes beyond what was hitherto considered biologically possible for a single-celled organism.

The bacterium has been named Thiomargarita magnifica by the consortium of scientists who have discovered it, based at the Lawrence Berkeley National Laboratory in the U.S. and from CNRS in France.

The discovery came about when Olivier Gros, a marine biologist at the University of the French Antilles, Pointe-à-Pitre, first came across the atypical organisms appearing as thin filaments on the surfaces of decaying mangrove leaves in a local swamp five years ago. Subsequent isolation and research established the organism as bacterial.

While other large bacteria have been isolated in the past, T. magnifica exceeds these other classified organisms by around 50-fold.

Stained microphotograph of Thiomargarita bacteria. Image: NASA, via Wikipedia CC 2.0 licence

Physical size is not the only feature in relation to size, as labelling the organism’s DNA with fluorescent tags has revealed. Whereas the typical bacterial genomes are about 4 million bases and about 3900 genes, T. magnifica boasts 11 million bases harbouring some 11,000 distinguishable genes.

The genus Thiomargarita includes vacuolate sulphur bacteria species (the cell cytoplasm is surrounded a central vacuole). The genus is only found in extreme environments on Earth, including methane seeps, mud volcanoes, brine pools, and organic-rich sediments. These niches are essential for energy production: the bacteria reduce inorganic species of sulphur to produce energy for the fixation of carbon.

Deeper research will be required to ascertain how the organisms are so large. It is established that the genome possesses several unusual features for a bacterium, such as the largest number of metacaspases (cysteine proteases) and introns (pathways used to assemble new genes) ever reported. There are also a large number of mobile genetic elements, which are essential to the core metabolism.

There is also a taxonomical conundrum, since the newly discovered microbe blurs the established line between prokaryotes and eukaryotes. This is because it is the first and only bacterium found to unambiguously segregate its genetic material in membrane-bound organelles in the manner of eukaryotes.

The bacterium contains two membrane sacs, one of which contains all the cell’s DNA. The other sac helps to keep the bacterium’s cellular contents pressed up against the outer cell wall in order for the molecules it needs can diffuse in and out. These sacs have been dubbed these sacs ‘pepins’, according to Nature, a name inspired by the pips in fruit.

The research paper describing T. magnifica can be accessed here, titled in somewhat understated fashion: “A centimeter-long bacterium with DNA compartmentalized in membrane-bound organelles.”

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2022-02-26 14:22:54Z
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