Immortals they are but what could possibly live forever? Well its now common knowledge especially to biological scientists that endospores can survive for long periods in a dormant state. It is clear from experiments that endospores can remain alive for at least several decades, but is this period of longevity defined?
A bottle of Clostridium aceticum endospores was found in a storage room at the University of California at Berkeley and then revived. Originally, it was isolated by a Dutch scientist, K.T. Wieringa in 1940 but was thought to have been lost until 1947. This suspension of the endospores of the bacterium C. aceticum was later placed in a sterile growth medium 34 years after, 1981, and it began growth in less than 12 hours, leading to a flourishingly pure culture.
In the UK, a 2000-year-old Roman archaeological site was examined using microbiological techniques and it yielded a significant number of viable Thermoactinomyces endospores in several pieces of debris. Also, Thermoactinomyces endospores were recovered from lake sediments known to be over 9000 years old. Although contamination is always a possibility in such studies, samples in both of these cases were processed in such a way as to virtually rule out contamination with “recent” endospores.
Thus, endospores can last for several thousands of years, but is this the limit? apparently not.
What factors could limit the age of an Endospore?
Cosmic radiation has been considered a major factor because it can introduce mutations in DNA. It has been hypothesized that over thousands of years, the cumulative effects of cosmic radiation could introduce so many mutations into the genome of an organism that even highly radiation-resistant structures such as endospores would succumb to the genetic damage. However, if the endospores were partially shielded from cosmic radiation, for example, by being embedded in layers of organic matter (such as in the Roman archaeological dig or the lake sediments described above), they might well be able to survive several hundred thousand years.
Amazing, but is this the upper limit?
In 1995 a group of scientists reported the revival of bacterial endospores they claimed were 25–40 million years old. The endospores were allegedly preserved in the gut of an extinct bee trapped in amber of known geological age. The presence of endospore-forming bacteria in these bees was previously suspected because electron microscopic studies of the insect gut showed endospore-like structures and because Bacillus DNA was recovered from the insect. Incredibly, samples of bee tissue incubated in a sterile culture medium quickly yielded endospore-forming bacteria. Rigorous precautions were taken to demonstrate that the endospore-forming bacterium revived from the amber-encased bee was not a modern-day contaminant.
Subsequently, an even more spectacular claim was made that halophilic(salt-loving) endospore-forming bacteria had been isolated from fluid inclusions in salt crystals of Permian age, over 250 million years old. These cells were presumably trapped in brines within the crystal as it formed and then remained dormant for more than a quarter billion years.
Molecular experiments on even older material, 425-million-year-old halite, showed evidence for prokaryotic inhabitants as well.
If these astonishing claims are supported by repetition of the results in independent laboratories, then it appears that endospores stored under the proper conditions can remain viable indefinitely. This is remarkable testimony to a structure that undoubtedly evolved as a means of surviving relatively brief dormant periods or as a mechanism to withstand drying, but that turned out to be so well designed that survival for millions or even billions of years may be possible.