The Trail of Antibiotic-Resistant Superbugs Leads Back to the First Land Animals

Researchers say powerful bacteria that are resistant to common antibiotics acquired their hard-to-kill traits 450 million years ago.

One of today’s biggest biohazards may have its roots in the days before the dinosaurs, when animals first started crawling onto land.

Enterococcus bacteria are hard-to-kill bugs that flourish in the digestive systems of nearly all land animals. They’re also able to survive long periods without food or water and withstand many of the disinfectants used in hospitals — traits they started to acquire shortly after emerging more than 425 million years ago, researchers at Harvard and the Massachusetts Institute of Technology reported today.

Knowing how they got that tough can help scientists figure out new ways to protect people from the harmful infections they produce, said Ashley Earl, a microbiologist and head of the bacterial genomics group at the Broad Institute, a joint venture of Harvard and MIT.

The fact that enterococcus species are so widespread “suggests an early origin,” Earl said. The researchers then used what geneticists call a molecular clock, a technique used to estimate when an evolutionary change occurred.

“Using genomic data, we can make best guesses for the timing at which different species likely went their separate ways from their last common ancestor and became something new,” she said. And when she and her partners compared those estimates to the fossil record, which shows when other species emerged, “We start to see this really interesting pattern that kind of fits beautifully for the expansion and the reductions of terrestrial animal life.”

“It fits really nicely with the dating we get from the genomic information,” she said. “That sort of lands the timing for the very first emergence of enterococcus lands at right around the time that terrestrialization first began,” Earl said.

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The findings were published in the scientific journal Cell. The research team included scientist from Massachusetts Eye and Ear and the Harvard-wide Program on Antibiotic Resistance.

The rise of drug-resistant bacteria has become a leading worry for doctors and scientists, raising fears that old killers like pneumonia or tuberculosis could make a comeback. Modern enterococci are a common source of hospital illnesses; they can take root in the blood, urinary tract, and the lining of the heart and fight off several common antibiotics.

Bacteria excreted from the guts of marine life find themselves in a “comparatively hospitable” water, where those excreted from land animals “would experience comparative isolation, starvation, desiccation, and possibly extinction.” Those excreted by land animals risk drying out or starving to death — but the ones that could survive those threats passed those traits on to their descendants, producing the hardy microbes that concern doctors today.

Over the course of the study, the scientists compiled what Earl called a “parts list” of genes that might reveal the bacteria’s vulnerabilities.

“We want to take those strengths and turn them into their weaknesses,” she said. She compared the bacteria to a tank — a dangerous, durable piece of military hardware.

“We can begin to basically take piece by piece of that tank away with modern genetic approaches … I think that’s really an important outcome of the work.”

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