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Autumn 2016


antibiotics; bacteria

photo of mutating bacterial colonies
Bacterial colonies growing within antibiotic-dosed medium

In a creative stroke inspired by Hollywood marketing wizardry, scientists from HMS and Technion-Israel Institute of Technology have designed a simple way to observe the life and death struggles of bacteria as they encounter, and often adapt to, increasingly higher doses of antibiotics. The experiments, described in the September 9 issue of Science, are thought to provide the first large-scale glimpse of bacterial mutation.

To capture this, the team constructed a 2-by-4-foot petri dish and filled it with 14 liters of agar, a seaweed-derived jellylike substance commonly used in labs to nourish organisms as they grow. They called the monster plate the Microbial Evolution and Growth Arena (MEGA) plate.

To observe how the bacterium Escherichia coli adapts to increasingly higher doses of antibiotics, the researchers infused serial sections of the dish with various doses of an antibiotic, ranging from no dose to a dose just above what the bacteria could survive, to a tenfold increase, culminating in a dose 1,000 times higher than the lowest concentration.

Over two weeks, a camera mounted on the ceiling above the dish took periodic snapshots that the researchers spliced into a time-lapsed montage. The result? A powerful visualization of bacterial evolution at work.

“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,” says study first author Michael Baym, an HMS research fellow in systems biology.

The researchers caution that their giant petri dish is not intended to perfectly mirror how bacteria adapt and thrive in the real world and in hospital settings, but it does mimic more closely the real-world environments bacteria encounter than traditional lab cultures. This is because, the researchers say, in bacterial evolution, space, size, and geography matter.

The researchers adapted the idea for an outsized dish from, of all places, Hollywood. Senior study investigator Roy Kishony, of HMS and Technion, had seen a digital billboard advertising the 2011 film Contagion, the tale of a deadly viral pandemic. The billboard featured a giant lab dish with hordes of painted, glowing microbes creeping across a dark backdrop to spell out the movie’s title.

“This project was fun and joyful throughout,” Kishony says. “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.”

“What we saw suggests that evolution is not always led by the most resistant mutants,” Baym says. “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.”

Image: Michael Baym

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