Newly discovered 'giant' bacterium visible to the naked eye

Bacteria are commonly thought of as microscopic single cells with DNA free-floating in their cytoplasm; as a group, however, they often show a surprising range of diversity. Jean-Marie Volland from the Lawrence Berkeley National Laboratory and colleagues have now added to this diversity by reporting the discovery of a sulfur-oxidising bacterium visibly growing as thin white filaments on the surfaces of decaying mangrove leaves in shallow tropical marine mangrove swamps in Guadeloupe, an archipelago in the Caribbean.

Though bacteria can typically only be viewed with a compound microscope capable of magnifying 100 to 1000 times, this one — named Thiomargarita magnifica and reaching one centimetre in length — is larger than all other known giant bacteria by ~50-fold, and is thus visible without a microscope. Volland and colleagues recorded their discovery in the journal Science.

“Cells of most bacterial species are around 2 micrometers in length, with some of the largest specimens reaching 750 micrometers,” the study authors wrote. “Using fluorescence, x-ray, and electron microscopy in conjunction with genome sequencing, we characterized Candidatus (Ca.) Thiomargarita magnifica, a bacterium that has an average cell length greater than 9000 micrometers and is visible to the naked eye.”

A single filament of Thiomargarita magnifica. Image credit: Jean-Marie Volland.

The bacterium is also quite complex in its structure, further challenging traditional concepts of bacterial cells. Instead of its DNA floating freely inside the cell as happens in other bacteria, the DNA is compartmentalised within membrane-bound structures, an innovation characteristic of more complex cells. These membrane-bound compartments are metabolically active, the researchers’ analyses show, with activity occurring throughout the bacterium cell length, as opposed to just at its growing tip.

“These cells grow orders of magnitude over theoretical limits for bacterial cell size, display unprecedented polyploidy of more than half a million copies of a very large genome, and undergo a dimorphic life cycle with asymmetric segregation of chromosomes into daughter cells,” the study authors continued.

It is possible that its unique spatial organisation and bioenergetic membrane system, which indicate a gain of complexity in the Thiomargarita lineage, may have allowed T. magnifica to overcome size- and volume-related limitations typically associated with bacteria. Exactly why these organisms need to be so large remains an open question — but their discovery suggests that other large and complex bacteria may still be hiding in plain sight, the authors concluded.

Top image caption: Thiomargarita magnifica shown next to an American dime coin.

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