A new finding about brown bears in the remote forests of Sweden is a sad indicator of just how far humanity's mess can extend – but a positive example of how we can turn things around with simple actions.

The growing problem of antibiotic resistance after industrial-scale production of antimicrobial products started in the 1940s can be traced in the teeth of Swedish brown bears (Ursus arctos). From the 1950s, the animals show an increase in antibiotic resistance.

However, after the 1990s, when Sweden introduced regulations controlling the use of antibiotics, the bears' antibiotic resistance decreased.

This suggests that regulation is a powerful tool for mitigating anthropogenic environmental harm.

Antibiotic resistance has been a growing problem for decades. Antimicrobial drugs harness the defense systems of molds like Penicillium. In nature, these molds produce antibiotic molecules to defend against their bacterial competitors; we've been able to hijack these molecules for our own benefit.

In nature, however, the bacteria evolve in response to the molds' defences (and the molds evolve new defences); that happens in response to antibiotic drugs, too. Bacteria have grown increasingly resistant to the antibiotic drugs we've developed, which means problems for us. According to a 2019 CDC report, over 35,000 people die just in the US every year because they're infected with an antibiotic-resistant bacterium.

Exactly how deeply antibiotics penetrate the environment isn't clear. We know that antibiotics and antibiotic-resistant bacteria can enter the environment via wastewater (that's a broader problem that is far from confined to antibiotics), and studies have examined the impact on aquatic wildlife, but understanding the broader impact over time has been difficult.

A team of researchers led by microbiologist Jaelle Brealey of the Norwegian University of Science and Technology solved this difficulty by hitting up museums. Their target: the skulls of brown bears. They were able to obtain samples of dental calculus from 82 brown bear teeth, dated between 1842 and 2016.

Calculus is a form of plaque buildup that preserves samples of the bacterial communities that live in the mouth. The team sequenced these bacterial genomes, looking for the genes for antibiotic resistance.

They found rising levels of these bacteria in the bears from the 1950s. This was followed by a decline after the Swedish government banned the agricultural use of antibiotics in the 1980s, and then implemented a program to combat antibiotic resistance in 1995.

"[The bacterial] abundance closely follows human antibiotic use in Sweden, increasing in the 20th century and then decreasing in the last 20 years," Brealey said.

"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."

The bears hailed from different regions in Sweden, which the team thought would reveal a variable spread of antibiotic resistance, based on their proximity to humans. Although Swedish brown bears tend to stay away from people, some bears live closer to humans than others, and these bears were expected to have a higher load of antibiotic-resistant bacteria.

Surprisingly, this turned out not to be the case.

"We found similar levels of antibiotic resistance in bears from remote areas and those found near human habitation," said geneticist Katerina Guschanski of Uppsala University in Sweden and the University of Edinburgh in Scotland.

"This suggests that the contamination of the environment with resistant bacteria and antibiotics is really widespread."

The bears born after 1995 show low levels of antibiotic resistant bacteria, which is encouraging. Those levels are not as low as they were before the introduction of industrial-scale antibiotic production, but they do show that we can do something about the environmental problems we cause.

"Our case study suggests that human actions, both negative and positive, can directly impact diverse microbial communities, including those associated with wild animals," the researchers wrote, "and provides evidence that large-scale policies limiting the use of antimicrobials in humans and livestock may be effective in curbing the dissemination of antimicrobial resistance through environmentally mediated pathways."

The research has been published in Current Biology.