For more than twenty years, researchers have been breeding E. coli bacteria in the laboratory. Image: Greg Kohuth, Michigan State University
Read aloud For more than twenty years, US researchers have observed evolution live in the lab? the example of Escherichia coli bacteria. Now they have presented an interim report of this unique experiment. Their conclusion after the analysis of 40, 000 generations: The evolution of the microbes follows in many exacts the rules already discovered by Charles Darwin, but is altogether much more complicated than previously assumed. In the end, the researchers are far from doing their job: the bacteria should continue to grow and in the future not only help answer questions about the theory of evolution, but also show which factors play a role in the targeted modification of bacteria, for example in biotechnology, In February 1988, Richard Lenski and his colleagues started the long-term experiment by growing twelve cultures of the predominantly E. coli bacteria. Since then, every day 0.1 milliliter of the bacterial suspension is placed in a new culture flask containing just under ten milliliters of nutrient fluid, so that the microbes always have enough food to grow. The scientists regularly take samples that they freeze for later experiments and analyzes. In the meantime, the bacteria have surpassed the 40, 000-plus mark, making it possible to compare the lifestyle and genome of the generations 2, 000, 5, 000, 10, 000, 15, 000, 20, 000 and 40, 000.

Until generation 20, 000, the development of the bacteria was very even, the researchers discovered: The speed with which changes in the genome? so-called mutations? accumulated, remained virtually constant during this time. However, as expected, this was not accompanied by a constant improvement in the fitness of the bacteria: at first, a large part of the mutations seems to have had a positive effect on the survival of the microbes, because they grew measurably faster than the original population. Then, however, this rate of adaptation subsided, although the mutation rate remained the same.

After about 26, 000 generations, there must have been a mutation that affected the DNA metabolism. As a consequence, the mutation rate increased rapidly. For example, while there were 45 detectable mutations after 20, 000 generations, it was already 653 after 40, 000 generations, and the genome at that time was 1.2 percent shorter than that of the original bacteria. However, most of these changes do not seem to have had a very positive or very negative impact on the viability of the bacteria, the researchers write. Some of the mutational variants that occurred in the laboratory cultures are found, or similar, in pathogenic bacteria. Laboratory evolution could therefore help to better understand and effectively counter these potentially dangerous changes.

Richard Lenski (Michigan State University, East Lansing) et al .: Nature, online pre-release, doi: 10.1038 / nature08480 ddp / Ilka Lehnen-Beyel advertisement


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