A potentially untreatable superbug is being harboured by dogs and passed on to their owners, new research has revealed.
Scientists are warning of a "nightmare scenario" after discovering transmission of a gene known to prompt resistance to a powerful antibiotic used by doctors as a last resort to save lives.
Sharing beds with dogs is just one of the ways they believe the mcr-1 gene is being passed on.
It is harboured in the gut and transported via microscopic fecal particles, also making dog baskets an area of increased risk.
First reported in China in 2015, the mcr-1 gene is resistant to colistin, an antibiotic used to defeat bacterial infections which cannot be managed by any other drugs.
For years, experts have warned that over-use of colistin, particularly on meat-producing animals, risks the rise of mutant genes that could render the drug useless in humans.
It is part of a growing crisis of antimicrobial resistance which is already estimated to kill 700,000 people a year globally and which is forecast to kill 10 million a year by 2050 if left unchecked.
Although the gene is believed to be carried by multiple species, scientists are particularly worried about its presence in domestic dogs because of their proximity to humans.
A team at the University of Lisbon took fecal samples from 126 healthy people living with 102 cats and dogs in 80 households over the two years up to February 2020.
Eight of the dogs and four humans were found to be harbouring bacteria including mcr-1.
Three of the dogs appeared to be healthy, the others had tissue or urinary tract infections.
In two of the households where dogs had tissue infections, the mcr-1 gene was found in both the dog and owner.
Analysis suggests the gene was passed from dog to owner, although it is possible to transmit from human to animal, too.
Dr Juliana Menezes, who led the research, said: "Colistin is used when all other antibiotics have failed, it is a crucial treatment of last resort.
"If bacteria resistant to all drugs acquire this resistance gene, they would become untreatable, and that's a scenario we must avoid at all costs.
"We know that the overuse of antibiotics drives resistance and it is vital that they are used responsibly, not just in medicine but also in veterinary medicine and in farming."
Presented at the European Congress of Clinical Microbiology and Infectious Diseases conference this weekend, the study is one of the first reports of mcr-1 found in a human.
It is believed to be particularly prevalent in regions with high agricultural use of colistin, such as southern European countries.
Even before the discovery of the mcr-1 gene, the EU had set a limit on the amount of colistin countries could use in farming.
The new research comes alongside a separate study revealing the extent to which increasingly trendy raw dog food is a major source of antibiotic-resistant bacteria.
Researchers at the University of Porto analysed samples from a range of types of dog food, finding that 54 per cent contained the bacteria Enterococci, of which 40 per cent were resistant to a range of strong antibiotics.
Meanwhile, a quarter of the samples with Enterococci were resistant to linezolid, another last-resort antibiotic.
All of the raw dog food samples contained multi-drug resistant Enterococci, including bacteria resistant to linezolid, compared to only a handful of the non-raw samples.
Dr Ana Freitas, who conducted the research, said: "The close contact of humans with dogs and the commercialisation of the studied brands in different countries poses an international public health risk.
"European authorities must raise awareness about the potential health risks when feeding raw diets to pets and the manufacture of dog food, including ingredient selection and hygiene practices, must be reviewed.
"Dog owners should always wash their hands with soap and water right after handling pet food after picking up faeces."