Close contact between humans and their pet dogs could lead to the spread of a gene which makes bugs resistant to a powerful antibiotic used by doctors as a last resort.
Some have suggested that means people should stop sharing a bed with their dogs.
Studies have suggested disease-resistance can also be spread by everything from feeding your dog raw food to picking up poo.
Researchers this week presented evidence to a European conference suggesting the gene – mcr-1 – was being passed between dogs and humans.
The mcr-1 gene gives bacteria resistance to colistin – a powerful antibiotic doctors use to fight dangerous infections when everything else has failed.
700,000 people a year already die because of drug-resistant disease, according to the UN, and they say that could rise to 10million a year by 2050.
The concern is that colistin-resistance will now find its way into other superbugs to create infections that doctors cannot treat.
The nightmare scenario that could emerge is mcr-1 combining with already drug resistant bacteria to create a truly untreatable infection, say the University of Lisbon team.
It would raise the prospect of untreatable infections – what is known as the antibiotic apocalypse that threatens to plunge medicine back into the dark ages.
Co lead author Dr Juliana Menezes, of the Centre of Interdisciplinary Investigation of Animal Health of the Faculty of Veterinary Medicine, said studies are needed to determine the full epidemiology of colistin resistance genes in humans and companion animals.
Five years ago scientists warned the world was on the cusp of a post-antibiotic era when such resistance was discovered in China.
A month later the mcr-1 gene was found on three farms in the UK and 15 bacterial samples collected from Britons by Public Health England.
Since then the mcr-1 gene has been found in various people and animals across the globe.
It confers resistance to colistin, which is an antibiotic of last resort used to treat infections from bacteria resistant to all other antibiotics.
Other genes in the same group, mcr-2 to mcr-9, have also been identified which act in a similar manner.
Added Dr Menezes: “These humans and dogs, if in direct contact, may transmit bacteria containing the mcr-1 gene to other humans, dogs, other animals and the environment and potentially be a hazard for public health.
“The situation we all want to avoid at all costs is any infection totally resistant to all antibiotics, caused by bacteria already resistant to most other antibiotics also acquiring this colistin resistance gene.”
Her team collected stools from 106 healthy humans and 84 cats and dogs – 49 of which were healthy with the other 35 having skin, soft tissue or urinary tract infections.
Molecular analysis revealed E. coli bacteria in two healthy participants and a dog with skin infection carried the mcr-1 gene.
They were all from different households – and the bugs were resistant to colistin in each case.
The two other samples found to be resistant to colistin – one from a healthy human and the other from a dog with a skin infection – did not test positive for mcr-1 to 5 genes. They will now be screened for the other mcr-6 to 9 genetic variants.
The bacteria in the faecal samples were not causing infections in humans, hence no specific treatment was recommended.
Only the sick dog received an oral antibiotic for the treatment of the skin infection.
The Chinese resistance cases were down to overuse of antibiotics in agriculture.
Colistin, used to promote growth and protect animals from diseases, has since been banned from being used in animal feed in China.
The findings were presented at the European Congress on Clinical Microbiology and Infectious Diseases.
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