Antibiotic resistance has been found in wild brown bears in Sweden by researchers using museum collections to study the effects of the drugs over time.
Uppsala University scientists analysed teeth from the brown bears and found that the increased use of antibiotics in medicine and agriculture from the 1950s to the 1990s led to increases in antibiotic resistance in wild animals.
The researchers also detected a clear downward trend in antibiotic resistance after national policies to control antibiotic use were implemented.
Lead author Katerina Guschanski said: "We found similar levels of antibiotic resistance in bears from remote areas and those found near human habitation.
Watch: What is antibiotic resistance?
"This suggests that the contamination of the environment with resistant bacteria and antibiotics is really widespread."
The scientists used specimens from museum collections to analyse changes in the bacterial communities that live in the mouth of wild animals.
These are preserved as solid calculus deposits on teeth.
This dental calculus can remain unchanged for millennia, which allowed the study of historical bacterial communities, the microbiomes, from Swedish brown bears that lived as long as 180 years ago.
Jaelle Brealey, a postdoctoral researcher at the Norwegian University of Science and Technology (NTNU) in Norway and lead author of the study, said: "We specifically looked for bacterial genes that provide resistance to antibiotics.
"Their abundance closely follows human antibiotic use in Sweden, increasing in the 20th century and then decreasing in the last 20 years.
"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."
Sweden was one of the first countries to implement strict control measures for the use of antibiotics, introducing a ban on antibiotics in agriculture in the mid-1980s and a national strategic programme against antibiotic resistance in medicine in 1995.
These measures seem to have taken effect. Oral bacteria of bears that were born after 1995 show low antibiotic resistance, albeit not as low as in bears that lived before humans started antibiotic mass-production.
Historical microbiomes could be used to not only investigate the past but also to monitor environmental changes in response to new strategies for reduction of contamination and pollution.
The researchers said their study provided an encouraging example for how governmental policies could be effective in mitigating a major health threat on a national level.
It showcases that human actions, both negative and positive, have a profound effect on the environment, they said.
Watch: Rescue Bears Have a Blast Playing With Cardboard Box at New York Sanctuary