What do you get when you stick E. coli in a test tube and don't feed it for three years? According to a new report in mBio, bacterial evolution at its finest.

In this study, scientists grew a single tube of E. coli in the laboratory for 1,200 days (over three years!). They wanted to explore how bacteria adapt to nutrient-limited conditions for an extended period of time, as microbes often face periods of starvation and stress in the natural world. 

The growth medium in the tube was never replenished. The only nutrients available were those the bacteria recycled from waste accumulating in the tube. This approach was different from other long-term E. coli evolution experiments (some of which have gone on for over 25 years), in which bacteria are transferred to fresh media everyday. 

When bacteria divide, they sometimes acquire mutations in their DNA. If those mutations are beneficial to the organism, their frequency within the population increases. By analyzing these mutations and their frequency, we can learn how bacteria evolve and adapt to their surroundings.

In this study, the researchers sequenced DNA collected from the E.coli collected at regular intervals over the course of the experiment. Based on the type and frequency of mutations they found, the researchers identified two main genetic groups, or lineages, that had diverged from the parent cells. While cells within each lineage coexisted with each other in the tube, one generally outnumbered those from the other, with the dominant lineage changing from one time point to the next. This indicates that the lineage best equipped to handle life in the tube varied according to the challenges faced by the cells at any given time. 

In addition to being the first to explore evolution of a single bacterial population under nutrient starvation conditions for this length of time, this study illustrates how complex and dynamic evolving bacterial communities can be. It also highlights specific mutations that may help bacteria thrive in diverse, often stressful, environments in the world beyond the test tube.