Seen in the video above, a tank (basically an oversize petri dish) was constructed in which the bacteria (in this case, Escherichia coli) were given a surface conducive to their growth, agar. The tank was sectioned into areas with graded amounts of drug. The outermost areas did not contain any drug, and the drug was introduced into the next areas, with levels increasing into the center of the dish. The center contained 1000 times the antibiotic as the low level sections.
Using a camera that filmed over a period of two weeks, the researchers created a movie of the microbes that showed evolution in progress as the bacteria moved over the surface. The researchers employed their device, called the Microbial Evolution, and Growth Arena (MEGA) plate, to investigate the relationship between physical space and evolutionary pressures that can induce organisms to change, or cause them to die.
“We know quite a bit about the internal defense mechanisms bacteria use to evade antibiotics, but we don’t really know much about their physical movements across space as they adapt to survive in different environments,” explained the first author of the study Michael Baym, a research fellow in systems biology at HMS.
This work was not intended to exactly replicate the real world environment encountered by bacteria but does provide a better sense of how actual environmental conditions can influence evolution in a better way than a bacterial culture setting. The researchers say that’s because space, geography and size are important factors in bacterial evolution; a giant surface with gradations of drug is an experimental improvement over small dishes with homogenous levels of antibiotic.
Some of the important insights gained from the work include these observations: bacteria spread until they reached a drug dose they could not overcome, but at each level, at least a small group of bacteria were able to adapt and live through genetic mutations. In those adapted bacteria, progeny migrated to areas of higher drug levels until reaching one in which they were not able to survive. As drug dose increased, resistance of the mutated microbes also increased, giving rise to highly resistant bacterial strains that could defeat even the highest doses of antibiotic.
In another dramatic observation, bacteria finally spread to the areas of highest drug levels (after ten days). While the first mutations occurred, the bacterial growth slowed, suggesting that the development of mutations interfered with growth speed. After resistance was obtained, normal growth rates resumed. However, the most resistant strains weren’t necessarily the fastest, sometimes taking cover behind weak strains as they encountered higher drug amounts.
The classical assumption is that mutants that live in the highest levels of drug are the most resistant, but these findings indicate the opposite. “What we saw suggests that evolution is not always led by the most resistant mutants,” Baym commented. “Sometimes it favors the first to get there. The strongest mutants are, in fact, often moving behind more vulnerable strains. Who gets there first may be predicated on proximity rather than mutation strength.”
“This project was fun and joyful throughout,” said the senior investigator, Roy Kishony, of HMS and Technion. “Seeing the bacteria spread for the first time was a thrill. Our MEGA plate takes complex, often obscure, concepts in evolution, such as mutation selection, lineages, parallel evolution, and clonal interference, and provides a visual, seeing-is-believing demonstration of these otherwise vague ideas. It’s also a powerful illustration of how easy it is for bacteria to become resistant to antibiotics.”
“This is a stunning demonstration of how quickly microbes evolve,” said Tami Lieberman, who worked as a graduate student in Kishony’s lab at the time of the research and is now a postdoctoral research fellow at MIT. “When shown the video, evolutionary biologists immediately recognize concepts they’ve thought about in the abstract, while nonspecialists immediately begin to ask really good questions.”
Sources: Harvard Gazette, Science