Antibiotic resistance has spread to the deep, dark forest, bear teeth reveal

Staffan Widstrand/NPL/Minden Pictures

Sweden’s brown bears nearly disappeared in the early 1900s, the victims of enthusiastic trophy hunters and angry farmers. Conservation efforts have slowly brought them back—along with an unexpected bonus. DNA from the animals’ teeth now reveals that, almost immediately after antibiotics were introduced in the 1950s, the drugs had penetrated even the remotest Swedish forests. The new finding, out today, could help scientists better understand the spread of antibiotic resistance, a worldwide problem with major impacts on human health.

“[This study] is a really nice example of how one can use ancient DNA for real-world problems,” says James Fellows Yates, an archaeogeneticist at the Max Planck Institute for the Study of Human History who was not involved with the research.

To gather samples, microbiologist Jaelle Brealey spent long hours examining bear skulls from the collection of the Swedish National Museum dating back to 1842, looking for the telltale discolorations on their teeth that indicate the presence of dental calculus, or plaque. Such leavings have been studied for more than 10 years in people to better understand diet and health. “In humans, it looks like large clumps, but in bears it’s a light film across the teeth,” Brealey says.

Brealey and her co-authors collected material from 82 bears by scraping films of bear plaque onto sheets of aluminum foil. Genetic analysis revealed the diverse community of bacteria living in the animals’ mouths, known as the oral microbiome. The researchers also found genes for antibiotic resistance, which some bacteria evolve in response to antibiotics in the environment.

When researchers lined up their samples over time, the results were eye-opening: Antimicrobial resistance seemed to have exploded across Sweden after the introduction of antibiotics in 1951. Like most of the world, Sweden was caught up in a wave of enthusiasm for the drugs, using them everywhere—from hospitals to farms, where they were used to treat livestock disease and promote faster growth. By 1970, Sweden was producing more than 40,000 kilograms of antibiotics each year.

The bears’ teeth provide a record of what happened next: Widespread antibiotic use led to a rise in antibiotic-resistant bacteria. Between 1951 and 1970, bacteria in the calculus samples contained twice the number of antibiotic-resistance genes as calculus from before the antibiotic era, the researchers report today in Current Biology. “When humans start using antibiotics, antibiotics get into the environment,” says co-author Katerina Guschanski, a geneticist at Uppsala University

Guschanski says the bears serve as a sort of measuring stick for a larger problem: antibiotic resistance, which the World Health Organization calls “one of the biggest threats to global health, food security, and development.” The pervasive spread of antibiotic-resistant genes comes back to affect human health, creating reservoirs of bacteria able to survive even the strongest drugs.

To the scientists’ surprise, even bears living hundreds of kilometers from human settlements had nearly as many antibiotic-resistant bacteria in their dental calculus as bears living closer to humans. The research doesn’t show how this happened, but Guschanski and Brealey speculate that farm runoff may have contaminated water supplies—or hungry bears could have been feeding on antibiotic-laced prey. “Whatever happened,” Guschanski says, “it was spread all over the region.”

Using DNA from animal specimens to measure environmental change holds a lot of promise, says David Díez del Molino, a paleogeneticist at the Centre for Paleogenetics in Stockholm. “The use of historic samples to acquire information you couldn’t get in any other way is key. That makes this research really stand out.” Fellows Yates agrees, especially when it comes to understanding how organisms respond to environmental contamination over time. “There’s a lot of opportunity out there.”

The antibiotic-resistance story has a surprise happy ending, too. Sweden curbed the use of the drugs in livestock in 1986 and began to regulate antibiotic sales for humans and animals in 1995. Antibiotic production and use in the country then declined significantly. That trend is also seen in the bears: Animals living in the mid-2000s showed fewer markers for antibiotic resistance.

To Guschanski, that’s a sign that nature can heal. “We always think about humans screwing up everything,” he says. “But when humans do the right thing, there is a chance to turn back the effects—at least in this case.”