63,000 Strong

Ever wonder about those big numbers posted in a window in that tall building on the east side of Farm Lane, across from the entrance to the MSU Dairy Store?

Right now, the digits read 63000. That’s the number of generations in an experiment that’s been running in my lab for over a quarter century.

We call it the LTEE, which stands for the Long-Term Evolution Experiment. There are 12 populations of E. coli bacteria in the experiment, and they all started from the same strain.

Every day—weekends and holidays included—a member of my team takes 1% of the cells in each population and puts them in a flask with fresh food. Over the next 24 hours, the population grows 100-fold and then runs out of food. These dilutions and renewals go on day after day, week after week, month after month, year after year, decade after decade. I hope the experiment will continue long after I’m gone, so that someday someone can write “and century after century.”

Bacteria grow by binary fission: 1 cell makes 2 cells, 2 cells make 4, 4 make 8, etc. So the 100-fold growth in the fresh medium represents about 6.6 doublings, or generations, every day. (There’ve been some interruptions since the LTEE began in 1988, but not many.)

Now consider a bacterial cell that gets a mutation in its DNA that lets it acquire more food and grow a little faster. That cell will leave more descendants than its competitors—that’s adaptation by natural selection. Over time, the bacteria are becoming stronger and fitter in their flask-worlds.

By watching the 12 populations evolve, we can answer questions about the dynamics and repeatability of evolution in a group of organisms—bacteria—that are essential for life on Earth as well as important players in health and disease. We measure the growth rates of the bacteria, we sequence their DNA, and we see just how much evolution can achieve even in short order.

Oh, about the sign. Zachary Blount is a talented postdoc who works on this project, and he likes to have fun with science. He put up the window display which, if you look closely, has a picture of Charles Darwin on the left, “The E. coli Long-Term Evolution Experiment” over the number, and “Generations and Counting” to the right. Every 1,000 generations or so, Zack updates the sign.

[Photo credit: Zachary D. Blount]

Note:   This piece first appeared at eastlansing.org after an invitation from Alice Dreger to explain the numbers in the window to our community.