Lenski's long-term evolution experiment: the evolution of bacteria that can use citrate as a carbon source

Richard Lenski set up twelve flasks of E. coli B back in 1988. They were allowed to grow overnight in minimal medium containing 139μM glucose and 1,700 μM citrate. The glucose was the only carbon source that the parent strain could use and it limited growth of the cultures. The citrate was present as a standard chelating agent. The bacteria could not take up citrate and use it as an additional carbon source.

Every day the culture was diluted by transferring one ml to 99 ml of fresh medium (1/100). There were 6.64 generations per day or 2,423 generations per year (slightly more in a leap year).

These cultures were under strong selective pressure. Individual bacteria that could grow faster would out-compete other bacteria in the culture and take over. Lenski expected that each culture would show a variety of different solutions to the selective pressure with many common mutations and many that could be unusual.

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