Commonly used anti-malaria drugs may have an unwanted side-effect -- decreasing the power of many antibiotics.
Remote South American rainforest villagers were found to carry bacteria resistant to antibiotics that had never been used there.
Canadian scientists believe the malaria drug chloroquine -- chemically similar to the antibiotics -- may have caused the problem.
The study was published in the Public Library of Science journal. Antibiotic resistance is a major problem in Western countries, where strains of disease-causing bacteria such as Staphylococcus beat some of the most commonly-used drugs.
However, for a resistant strain to develop, bacteria usually need to be exposed to the drug involved, or a drug similar to it.
The researchers from the Lakeridge Health Centre in Oshawa, Ontario, traveled to the remote rainforest regions of Guyana, in the north of the South American continent.
They took rectal swabs from more than 500 villagers, and checked the bacteria found, including E.coli and Salmonella, for their resistance to ciprofloxacin, one of the most commonly used antibiotics in the world.
They found more than five per cent of the swabs had ciprofloxacin-resistant E.coli, which compares to the four per cent found in US intensive care units where the drug is used intensively.
The degree of resistance found was so high that it was likely that other antibiotics in the same 'family' as ciprofloxacin would also have been affected, the BBC said.
Although antibiotics had not been used in these areas, anti-malarial drugs, particularly chloroquine, had been.
