The Army announced Wednesday that investigators found 9,220 undocumented samples of dangerous pathogens at a Maryland military research facility. Some of the materials had been frozen for decades. How long can a pathogen be frozen and still come back to life?
Probably hundreds of thousands of years, but no one knows for sure. Under laboratory conditions, 10 years of storage will have absolutely no impact on the viability of a sample. If you store a pathogen in the proper way—that means drying it out and freezing it at a very low temperature—it should be resilient to all natural decay except that caused by cosmic radiation, which is constantly striking all terrestrial objects and has the capacity to kill cells. But that process takes such a long time that a frozen sample of some 10 billion organisms (a typical size for laboratory cultures) would likely outlast human civilization as we know it.
Viruses are relatively easy to put into long-term storage, because they contain little more than RNA or DNA sequences in a protein shell. To freeze them, researchers place the isolated virus in a solution that mimics chemical conditions inside a living cell—where a virus feels most at home—and adds dimethyl sulfoxide or glycerol to prevent freezing damage. The mixture is loaded into an ampoule from which any air and water has been sucked out, and the whole thing is sealed with a blowtorch. The ampoule is stored in a freezer at around minus-80 degrees Celsius.
The process for bacteria is largely the same, although they are often kept in liquid nitrogen vapor at even lower temperatures. Unlike viruses, bacterial cells contain a significant amount of water, which forms potentially damaging ice crystals at subzero temperatures. Fortunately, most bacteria have natural cryoprotectants that make those crystals smaller and less destructive to the cell membrane.
This storage technique is being used around the world. There are overlapping "libraries" of microorganisms in Virginia, Paris, Germany, and Tokyo, in case the storage systems at one or more institutes become compromised. When scientists discover a new microbiological life form, they send a sample to each library for authentication and storage.
Some bacteria may be capable of surviving for tens of thousands of years even without human help. Last year, a team of Penn State researchers announced the discovery of live, 120,000-year-old bacteria in the Greenland ice sheet, topping the previously agreed-upon record of 32,000 years. It is not entirely clear, however, whether these cells are the originals from thousands of years ago or their descendants. There is some evidence that these organisms secrete enzymes that melt a small amount of the surrounding ice and free up the nutrients and minerals stored in it. (Certain algae species do something similar.) The cells might therefore be eating and reproducing in slow motion, unlike their laboratory-based cousins, which remain in suspended animation.
Curiously, freezing may not be the best tactic for extreme long-term storage of bacteria. One group of scientists has said it revived microbes more than 250 million years old from salt deposits in New Mexico. This claim, however, is hotly disputed.
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Explainer thanks William M. Gelbart of UCLA and Robert B. Hoover of NASA.