What is already known on this topic
After the rise in antibiotic use in medicine and agriculture over the past few decades, antimicrobial resistance has become a major global health threat. Resistant bacteria and antibiotics can spread in the environment through wastewater treatment plants and be picked up by wild animals. However, little is known about how antimicrobial resistance has spread and evolved in wildlife over time.

What this research adds
By analyzing calcified dental plaque from museum specimens of wild brown bears, researchers traced the history of bacterial communities living in the mouth of Swedish bears. The team found that the levels of antibiotic resistance genes in bears increased from the 1950s through the 1990s, mirroring the rise in the use of antibiotics in Sweden, before decreasing after national policies to control antibiotic use were put in place in the 1980s and 1990s. Bears from remote areas had similar levels of antibiotic resistance genes than those found near human settlements, suggesting that environmental contamination with resistant bacteria and antibiotics is widespread.

Conclusions
The study highlights how microbiotas from the past could be used to monitor environmental changes. It also provides an example for how policies to control antibiotic use can help mitigate a major health threat.

The use of antibiotics in humans and livestock has skyrocketed in the past few decades. A new study found that this increase also led to heightened antimicrobial resistance in wild animals. But there’s reason for hope: the study, conducted in Sweden, also detected a downward trend in the spread of antimicrobial resistance after national policies to control antibiotic use were put in place.

The findings, published in Current Biology, highlight how microbiotas from the past could be used to monitor environmental changes. The study also provides an example for how policies to control antibiotic use can help mitigate a major health threat.

Hundreds of thousands of people die each year because of infections with pathogens that are resistant to currently available antibiotics. Both antimicrobial drugs and resistant microbes can spread in the environment through wastewater treatment plants and be picked up by wild animals. However, little is known about how antimicrobial resistance has spread and evolved in wildlife over time.

To address this question, researchers led by Jaelle Brealey and Katerina Guschanski at Uppsala University analyzed calcified dental plaque from 57 Swedish brown bear samples from the Swedish Museum of Natural History in Stockholm. Because bacterial DNA in calcified dental plaque can remain unchanged for centuries, the researchers could investigate the mouth microbiota of bears that lived up to 180 years ago.

Resistance spread

In their analysis, the researchers looked for bacterial genes that provide resistance to antibiotics. The team found that the levels of such genes increased from the 1950s through the 1990s, mirroring the rise in the use of antibiotics in Sweden. Antimicrobial resistance started to decrease after the country introduced a ban on antibiotics in agriculture in 1986 and a national program against antibiotic overuse in medicine in 1995.

Bacteria living in the mouth of bears born after 1995 show lower antimicrobial resistance than oral microbes from older animals. However, the levels of antibiotic resistance genes were not as low as in bears that lived before humans started to mass-produce antibiotics in the 1950s, the researchers found.

The abundance of antibiotic resistance genes follows human antibiotic use in Sweden, Brealey says. “We also find a greater diversity of antibiotic resistance genes in the recent past, likely as a result of different kinds of antibiotics being used by humans,” she says.

Mitigating policies

Scandinavian brown bears usually live far away from humans, but sometimes the animals approach villages and cities, the researchers say. Unexpectedly, bears from remote areas had similar levels of antibiotic resistance genes than those found near human settlements, they found. This suggests that environmental contamination with resistant bacteria and antibiotics is widespread, Guschanski says.

The findings suggest that the dissemination of antibiotic-resistant bacteria in the environment can be reduced if countries control and regulate antibiotic use. “Our case study suggests that human actions, both negative and positive, can directly impact diverse microbial communities, including those associated with wild animals,” Guschanski says.

The study also suggests that national policies limiting the use of antibiotics in medicine and agriculture can help mitigate the spread of antimicrobial resistance, the researchers say.