B.O.U.S. ( Bacteria of Unusual Size)

Immense, colossal, massive – these are words you rarely hear in microbiology. However, they all apply to Thiomargarita magnifica, the most recent bacteria to make headlines. In fact, such adjectives barely do justice to the size of this species.

Members of T. magnifica are over 50 times larger than the previous world’s biggest bacteria (Thiomargarita namibiensis) and over 1,000 times larger than more familiar bacteria like Escherichia coli. Jean-Marie Volland, one of the scientists who discovered the species, vividly describes it this way “It would be like meeting another human the size of Mount Everest.” In terms of quantitative numbers, T. magnifica measures in at lengths around 1 centimeter (0.4 inches). This means that these bacteria can be observed by the naked human eye which makes them macro microbes!

Given this size, it’s unsurprising that the species was initially misidentified as a eukaryote when it was first discovered in 2009. Biologist Olivier Gros first collected the species in shallow salt waters in a mangrove forest on a Caribbean island. He says “When I saw them, I thought, ‘strange.’… I thought it was just something curious, some white filaments that needed to be attached to something in the sediment like a leaf.” Fellow discoverer Silvina Gozalez-Rizzo adds “I thought they were eukaryotes; I didn’t think they were bacteria because they were so big with seemingly a lot of filaments.” 

However, when they took a closer look, they discovered that the specimens were single, super-sized, and semi-compartmentalized cells. The images produced showed that each cell had a large central vacuole that runs the entire length of the organisms as well as several membrane-bound sub-sections. The latter are novel structures that the scientists named “pepins”. These discoveries were enabled by new microscope technologies including x-ray tomography, confocal laser scanning microscopy, and transmission electron microscopy.

Next-generation sequencing shows that T. magnifica is also a genomic giant. The species is a prolific polyploid with hundreds of thousands of whole-genome copies throughout the cell. The species also has around three times more genes than most bacteria. All this DNA is arranged into clusters and each pepin contains at least one cluster. This last fact has researchers excited. It’s the first known bacteria to segregate its DNA into membrane-bound organelles. The higher level of complexity means that T. magnifica not only challenges the concept of a bacteria cell but may also be a missing link between procaryote and eukaryote.  

The researchers published their discovery at the end of June 2022 in Science. You can read the free preprint version here.

Maulucioni y Doridí, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
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