The dangerous mcr-1 gene, which provides resistance to the last-resort antibiotic colistin, has been found in four healthy humans and two pet dogs, according to new research, which found that the gene can be transmitted between dogs and their owners.

The study raises concerns that pets can act as reservoirs of the gene and so aid the spread of resistance to precious last-line antibiotics, said researchers from the University of Lisbon.

They found two cases, in which both dog and owner were harbouring the gene.

If mcr-1 combines with already drug-resistant bacteria, it can create a truly untreatable infection, the researchers said.

"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," Menezes added.

For the study, the team looked for resistance to colistin in bacteria in faecal samples from people and pets.

Samples were taken from 126 healthy people living with 102 cats and dogs in 80 households in Lisbon between February 2018 and February 2020. All of the humans and 61 of the pets were healthy. A total of 23 pets had skin and soft tissue infections (SSTI) and 18 had urinary tract infections (UTI).

Eight dogs out of the 102 pets (7.8 percent) and four humans out of 126 (3.2 percent) harboured bacteria with the mcr-1 gene. Three of the dogs were healthy, four had SSTIs and one had a UTI. None of the cats were carrying the gene. Further analysis showed that the bacteria isolated from all 12 samples that were mcr-1 positive were resistant to multiple antibiotics.

While transmission in both directions is possible, it is thought that in this case, the gene passed from dog to human, Menezes said.

The owners did not have infections and so did not need treatment. The sick dogs were successfully treated.

The study was presented at the ongoing European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) taking place online between July 9 and 12.

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