A simple experiment conducted by scientists from Harvard Medical School and Technion-Israel Institute of Technology demonstrates how bacteria are able to evolve as they encounter larger doses of antibiotics. What’s even more alarming is the fact that these bacteria also learn how to thrive in these environments, making them all the more dangerous.
The purpose of the experiment was to study how the bacterium Escherichia coli (E. coli) is able to adapt as it encounters higher doses of antibiotics. In order to conduct their experiment, the scientists began by building a large 2-by-4-foot petri dish and filled it with 14 liters of agar, a seaweed-derived gelatin commonly used as nourishment for organisms in labs. They then proceeded to divide the dish into sections with varying saturations of medication. The outermost sections of the dish contained no antibiotics, while the next section contained just above the minimum required to kill the bacteria. Each of the following sections contained a 10-fold increase in dosage, with the middle section containing 1,000 times as much as the section with the lowest antibiotic.
The scientists monitored the evolution over a period of two weeks by mounting a camera on the ceiling above the dish. The snapshots taken were then combined into a time-lapse video creating a startling visualization of bacterial evolution.
The researchers describe the giant petri dish, named the Microbial Evolution and Growth Arena (MEGA) plate, as a simple platform to examine how challenging forces can cause an organize to adapt or 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,” said study first author and research fellow in systems biology at HMS Michael Baym.
The MEGA is not meant to perfectly represent bacterial evolution in the real world or hospital settings, but it is a closer simulation to the real-world environments bacteria face than traditional lab situations. This is due to the fact that bacterial evolution is determined by space, size, and geography. The process of moving through varying antibiotic concentrations over a large space presents more of a challenge than in the tiny plates filled with uniform doses in a traditional lab experiment.
Senior study investigator Roy Kishony of HMS and Technion explained the need to conduct the experiment:
“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.”
Tami Lieberman, a co-investigator and graduate student in the Kishony lab at the time of the experiment, says the images have been a catalyst for conversation for professionals and laymen alike. “This is a stunning demonstration of how quickly microbes evolve,” Lieberman said. “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.” Lieberman has since gone on to become a postdoctoral research fellow at MIT.
Since concluding the experiment, the scientists were able to ascertain some key insights:
- Bacteria spread until they reached a concentration (antibiotic dose) in which they could no longer grow.
- At each concentration level, a small group of bacteria adapted and survive. Resistance occurred through the successive accumulation of genetic changes. As drug-resistant mutants arose, their descendants migrated to areas of higher antibiotic concentration. Multiple lineages of mutants competed for the same space. The winning strains progressed to the area with the higher drug dose, until they reached a drug concentration at which they could not survive.
- Progressing sequentially through increasingly higher doses of antibiotic, low-resistance mutants gave rise to moderately resistant mutants, eventually spawning highly resistant strains able to fend off the highest doses of antibiotic.
- Ultimately, in a dramatic demonstration of acquired drug resistance, bacteria spread to the highest drug concentration. In the span of 10 days, bacteria produced mutant strains capable of surviving a dose of the antibiotic trimethoprim 1,000 times higher than the one that killed their progenitors. When researchers used another antibiotic—ciprofloxacin—bacteria developed 100,000 fold resistance to the initial dose
- Initial mutations led to slow growth—a finding that suggests bacteria adapting to the antibiotic aren’t able to grow at optimal speed while developing mutations. Once fully resistant, such bacteria regained normal growth rates.
- The fittest, most resistant mutants were not always the fastest. They sometimes stayed behind weaker strains that braves the frontlines of higher antibiotic doses.
This information revealed that the mutants that were able to survive in the highest concentration of antibiotics weren’t necessarily the most resistant.
“What we saw suggests that evolution is not always led by the most resistant mutants,” said Baym. “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.”
You can read all about the research here.
[Via: Harvard Medical School]
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