Scientists unlock long-living bacteria's secrets
An international team of researchers has extracted DNA from living bacteria which are over half a million years old. The scientists believe their work has implications for our understanding of how cells age, as well as for how we might search for life on Mars. The work, which was partly funded by a Marie Curie grant from the EU, is published in the journal the Proceedings of the National Academy of Sciences (PNAS). For a long time, it was assumed that cells which survived for long periods of time in ice or sediments did so by switching off all metabolic activity and becoming dormant. However, dormancy also switches off DNA repair mechanisms, and over time, DNA in dormant cells becomes degraded by chemical reactions. Eventually, so much damage accumulates that the cell is unable to reproduce. In this latest study, Professor Eske Willerslev of the University of Copenhagen and colleagues looked at samples of bacteria which had been sealed inside the Canadian and Siberian permafrost for millennia. 'Our project is about examining how bacteria can live after having been frozen down for millions of years,' explained Professor Willerslev. 'Other researchers have tried to uncover the life of the past and the following evolutionary development by focusing on cells that are in a state of deadlike lethargy. We, on the other hand, have found a method that makes it possible to extract and isolate DNA traces from cells that are still active.' The researchers were able to successfully extract DNA from samples which were between 400,000 and 600,000 years old, indicating that DNA repair was taking place. Furthermore, they were able to detect evidence of respiration, further proof that the ancient bacteria were still metabolically active. This metabolic activity, which includes DNA repair mechanisms, is what has kept the bacteria alive for such a long time. 'Many studies have suggested that dormancy is the most effective survival strategy for bacteria over long time periods; our data indicate that despite short-term robustness, however, dormant bacteria are unlikely to be the most persistent cells over thousand-year timescales in the cold and desiccated conditions represented by our samples,' the scientists conclude. 'Instead, bacteria with an active DNA repair mechanism are most likely to persevere.' The results have a number of interesting implications. For example, the permafrost could contain a treasure trove of old yet viable bacteria which evolved in and are adapted to ancient environments. Furthermore, the discovery that some ancient bacteria are not dormant but metabolically active could influence the way we search for life in similar environments on Mars and on Jupiter's moon Europa.