Bacteria Visible with the Naked Eye Discovered

Scientists from the Max Planck Institute in Bremen, Germany, report in the April 16 issue of Science magazine the discovery of the largest bacteria known to date. Residing in the greenish ooze of ocean sediment off the coast of Namibia, the spherical bacteria have diameters ranging from 100 to 750 µm—roughly the size of the period at the end of this sentence. Since the bacteria often form strands of a dozen or so cells and glisten white from light reflecting off sulfur inside them, scientists named the microbe Thiomargarita namibiensis, or sulfur pearl of Namibia.

This bacterial behemoth was discovered in African sediment samples obtained in 1997 by Heide Schulz of the Max Planck Institute for Marine Microbiology. On a visit to the coast of South America, she and her colleagues had identified bacteria that oxidized sulfur for energy. The scientists investigated whether similarly sulfur-rich sediments off the Namibia coast might harbor the same microbes.

Analysis of the DNA of the new sample confirmed that the organisms are in fact bacteria, and showed that they are relatives of the bacteria that Schulz and her colleagues had found earlier. Confounding the analysis was the presence of other colonizing bacteria in the sheath enveloping the bacteria. Consequently, the team often detected other DNA along with that of T. namibiensis.

Much of the bacteria's large size can be attributed to fluid-filled sacs, where large quantities of nitrate, used to oxidize sulfur, are stored. Since the nitrate concentration in the surroundings fluctuates, the vacuoles enable the microbes to endure times when the concentration is low. "They can survive and just wait for new nitrate," says Schulz.

The rest of T. namibiensis' interior consists primarily of sulfur globules dispersed throughout a thin layer of cytoplasm that surrounds the vacuole. The overall amount of cytoplasm is roughly normal for a bacterium, notes Schulz. Researchers have previously suggested that the volume of cytoplasm through which a bacterium can efficiently move proteins and other molecules limits its size.

The researchers at Max Planck are trying to grow T. namibiensis in the laboratory, to learn some of its secrets to packing in so much nitrate. Schulz is also curious about whether a microbe of such size has multiple chromosomes, since proteins made by a single set of genes would have to travel a considerable distance across the cell. In addition, Schulz and her colleagues will be returning to Namibia next month, hoping to uncover new bacterial treasures.

For more information, contact Heide N. Schulz, Max Planck Institute for Marine Microbiology, Celsiusstrasse D-28359, Bremen, Germany.