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Aug. 25 (UPI) -- Increased use of antibiotics in medicine and agriculture from the 1950s through the 1990s led to a rise in resistance to the drugs among wild Swedish brown bears, a study published Wednesday by Current Biology found. However, there was a downward trend in antibiotic resistance following the implementation of national policies designed to limit use of the medications, the researchers said.
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"We found similar levels of antibiotic resistance in bears from remote areas and those found near human habitation," study co-author Katerina Guschanski said in a press release.
"This suggests that the contamination of the environment with resistant bacteria and antibiotics is really widespread," said Guschanski, a researcher in ecology and genetics at Uppsala University in Sweden and the University of Edinburgh in Scotland.
Pathogens that are resistant to currently available antibiotics pose a significant global health threat.
Hundreds of thousands of people die each year from infections caused by resistant bacteria, including roughly 35,000 in the United States alone, according to the Centers for Disease Control and Prevention.
Resistant bacteria can escape from hospitals and other settings into the environment through wastewater treatment plants and spread by water and wind over large distances, research suggests.
From there, they can be picked up by wild animals, which in turn can transmit resistant bacteria to humans during recreational activities or hunting, according to Guschanski and his colleagues.
Sweden was one of the first countries to implement strict control measures governing the use of antibiotics, introducing a ban on antibiotics in agriculture in the mid 1980s and a national strategic program against antibiotic resistance in medicine in 1995.
For this study, the researchers used historical specimens of bacterial communities that live in the mouths of wild animals and remain as solid calculus deposits -- calcified dental plagues -- on teeth from museum collections to study the effects of human-made antibiotics over the entire history of their application.
They focused specifically on the bacterial microbiomes, or communities of bacteria, from Swedish brown bears as old as 180 years.
Scandinavian brown bears usually live far away from humans but sometimes approach villages and cities, according to the researchers.
They expected to find more antibiotic resistance genes in bears that lived in more densely populated regions of Sweden, though this was not the case, they said.
However, 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.
The findings suggest that historical microbiomes such as those used in this research could be a tool to investigate to monitor environmental changes in response to new strategies for reduction of contamination and pollution, according to the researchers.
The study also shows how governmental policies can be effective in mitigating a major health threat on a national level, they said.
The abundance of "bacterial genes that provide resistance to antibiotics ... closely follows human antibiotic use in Sweden, increasing in the 20th century and then decreasing in the last 20 years," study co-author Jaelle Brealey said in a press release.
"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," said Brealey, a postdoctoral researcher at NTNU in Norway.
