Researchers froze E. coli cultures for later study. ((Greg Kohuth, Michigan State University))

A 21-year study of the evolution of E. coli bacteria has revealed details on how genetic mutations emerge and spread over 40,000 generations.

Richard Lenski, a professor of microbial ecology at Michigan State University, began the study in 1988 to document natural selection in the intestinal bacteria.

The research team periodically froze some of the bacteria to provide a genetic snapshot for later study. The scientists were able to sequence the entire DNA genome of the frozen bacteria, something that wasn't possible at the beginning of the study.

"It's extra nice now to be able to show precisely how selection has changed the genomes of these bacteria, step by step over tens of thousands of generations," Lenski said in a release.

Halfway through the study, at 20,000 generations, the researchers found 45 genetic mutations in the surviving cells that gave a competitive advantage to the bacteria, as Darwin's theory of natural selection predicts.

A mutation arose at 26,000 generations that altered DNA metabolism, and caused the overall rate of mutation in the genome to increase. By 40,000 generations, the number of genetic mutations jumped to 653.

Study co-author Dominique Schneider of the Université Joseph Fourier in Grenoble, France, said the adaptive mutations "beautifully emphasize the succession of mutational events that allowed these organisms to climb toward higher and higher efficiency in their environment."

The later mutations appeared to be more random, and neither helped nor harmed the bacteria in which they occurred.

Lenski said they found the interactions between changes in the bacteria's genome and changes in their adaptation to their surroundings is "complex and can be counterintuitive."

"The genome was evolving along at a surprisingly constant rate, even as the adaptation of the bacteria slowed down a lot. But then suddenly the mutation rate jumped way up, and a new dynamic relationship was established," said Lenski.

The authors of the study, published this week in Nature, said their result could help scientists understand mutations in human diseases, such as cancer. Mutations in DNA metabolism, such as the one that occurred at generation 26,000 in the E. coli experiment, are involved in some cancers.

"Cancer progression is a fundamentally similar evolutionary process, so what we learn here can help us better understand the course of these diseases," said study lead author Jeffery Barrick.