Bacteria Evolution on a MEGA Petri Plate – Michael Baym – Harvard Medical School (2016)

The Microbial Evolution and Growth Arena plate (MEGA-plate) experiment designed by Harvard Medical School researcher Michael Baym and his coworkers is an absolutely fascinating demonstration of evolution in action. They were able to visualize with a stunning didacticity (is that a word?) how bacteria evolves resistance to the antibiotic trimethoprim surprisingly fast for the Human time scale.

The experiment is reminiscent of a highly simplified 2-D version of a long-term evolution experiment by Richard Lenski of the Michigan State University. Dr. Lenski’s experiment has been running since 1988 and has reached a record breaking >65,000 generation of bacteria. However here Dr. Baym and his co-workers achieved something different. Evolution takes place in many different environments. It could take place in the liquid environment of bodily fluids such as blood or saliva or on a solid surface like the tiles of a bathroom, a kitchen or more frighteningly highly disinfected surfaces of a hospital operating room. Microbes have to grow over spatial obstacles, compete with their neighbors, expand into new territory where there’s more abundant food. Being able to see all of these happening on a 2-D surface is absolutely stunning.

Just like the Lenski’s long-term evolution experiment, Baym and his colleagues can freeze and store microbes from different locations (or branching points in the evolutionary tree generated in the end) on the plate and sequence their DNA. Sequencing of a bacterial genome can pinpoint the gradual accumulation of mutations that allowed some bacteria to survive from the non-toxic initial habitat to the lethal area in the middle. They can work out which mutations matter.

In 1969 Nobel prize was awarded to two scientists Max Delbrück and Salvador Luria for their 1943 experiment known as Luria–Delbrück Fluctuation Test which showed that mutations do not arise due to selection but they appear randomly in a pre-adaptive fashion. Pre-existing mutations gets selected if the conditions favor.

The following video shows the time lapse of evolution of resistance against the antibiotic ciproflaxin in E. coli taken from the supplementary of the Science article. The MEGA-plate with a ciproflaxin gradient (0-20-200-2000-20000-2000-200-20-0). Movie was compiled from time-lapse imagery every 10 minutes for 14.2 days, and played at 30fps (18000X speed).

There’s an article explaining behind the scenes details on the experiment in The Atlantic.


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