• Scientists have discovered the world’s largest bacterium in a Caribbean mangrove swamp.

• The bacterium, called Thiomargarita magnifica, is noteworthy not merely for its size — colossal for a single-celled organism at up to about 2 cm long — but also because its internal architecture is unlike other bacteria.

• The DNA, an organism’s blueprint, is not free-floating inside the cell like in most bacteria but contained within numerous small membrane-bound sacs. Membrane-bound structures in cells are called organelles.

• Bacteria are single-celled organisms that reside nearly everywhere on the planet, vital to its ecosystems and most living things. Bacteria are thought to have been the first organisms to inhabit Earth and remain quite simple in structure billions of years later. The bodies of people are teeming with bacteria, only a relatively small number of which cause disease.

• Thiomargarita magnifica is not the largest-known single-celled organism. That honour goes to the aquatic alga Caulerpa taxifolia, reaching 6-12 inches (15-30 cm) long.

• Caribbean mangrove swamps are packed with organic matter, with microbes in the sediment degrading this matter and producing high concentrations of sulphur. The sulphur-rich environment offers an energy source for bacteria like Thiomargarita magnifica.

• The bacterium has been found in several locations in Guadeloupe, a French archipelago in the Caribbean. It was first spotted in the sulphur-rich seawater of a Guadeloupe swamp by Université des Antilles microbiologist and study co-leader Olivier Gros in 2009.

• Techniques including electronic microscopy revealed it was a bacterial organism. Specialist 3D microscope images finally made it possible to prove that the entire filament was indeed a single cell.

The “Mount Everest” of bacteria

• A normal bacterial species measures 1-5 micrometers long. This species averages 10,000 micrometers (four-tenths of an inch/1 cm) long, with some Thiomargarita magnifica twice that length.

• The largest-known bacterium until now had a maximum length around 750 micrometers.

• The unusual size is notable because bacteria aren’t usually visible without the assistance of microscope. It’s 5,000 times bigger than most bacteria. To put it into context, it would be like a human encountering another human as tall as Mount Everest, said marine biologist Jean-Marie Volland, a co-leader of the study published in the journal Science.

• For most bacteria, their DNA floats freely within the cytoplasm of their cells. This newly discovered species of bacteria keeps its DNA more organised. The big surprise of the project was to realise that these genome copies that are spread throughout the whole cell are actually contained within a structure that has a membrane.

• Mapping its genome showed that Thiomargarita magnifica has lost some genes essential for cell division and has more than the usual number of copies of genes responsible for cell elongation. This may explain, in part, why the cell grows into such an elongated filament. The genome is also very big and contains three times the average number of genes that are usually found in bacteria. 

• It is named Ca. Thiomargarita magnifica because magnus in Latin means big.

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