A Giant Among Us

Bacteria are single-cell wonders. They live, grow, reproduce, and can perform all the functions of life as solitary organisms independent from their brethren. With rich and complex genetics, these microbes vary enormously in their properties and populate every possible niche from the frigid Arctic to the interior of our bodies. But one common feature is that they are small in size. Really, really small. So small that they are invisible to the naked eye and require a microscope to see them. Some are round (cocci), some are rod-shaped (bacilli), and some are spiral-shaped (spirilla), but all are too tiny to see unaided. There are even physiochemical constraints that predict an upper limit to their maximum cell volume. Or so we thought. A preprint published on bioRxiv destroys that dogma with the identification of a new, gigantic bacterium designated Thiomargarita magnifica.

Found on mangrove leaves from Guadeloupe, Lesser Antilles, Thiomargarita magnifica measures in at a whopping average length of 9.72 millimeters (roughly 3/8 of an inch) with some specimens reaching nearly 20 millimeters long (just over 3/4 of an inch). This size is unheard of for a bacterium and makes them easily visible without magnification. Other members of the genus Thiomargarita are also large, but the next largest relative (Thiomargarita namibiensis) is less than 1 millimeter in diameter (just barely visible), making it is 10 times smaller than this newly discovered species. Consistent with its huge bacterial size, magnifica has a genome with around 12 million base pairs compared to an average bacterium with 4 million base pairs per genome. The large magnifica genome encodes at least 11,000 genes which is 2-3 times the number found in typical bacteria. In addition to its huge genome size, even more surprising is the number of copies of the genome present per cell. Humans are diploid animals with 2 genome copies per cell (the maternal and the paternal) while all giant bacteria have multiple genome copies (called polyploidy). However, magnifica has an average of 37,000 DNA copies in each cell, giving it 10 times more copies than the next closest bacterium. Even more unique, the magnifica DNAs are enclosed in a membranous structure. Membrane-bound DNA (the nucleus of the cell) is a characteristic of eukaryotic cells and is unheard of for prokaryotic organisms (bacteria, archaea, and blue-green algae). Clearly, this new species defies conventional knowledge about bacteria and indicates that bacterial size and complexity are much greater than previously suspected. One wonders how many more huge and marvelously different bacteria lurk undiscovered around the planet. This work with Thiomargarita magnifica is sure to spur an intense hunt for additional giant bacteria whose biology may drastically change our perception of the microbial world.

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