Bacteria thought to be resistant to an antibiotic may be susceptible to treatment after all, Edinburgh researchers have found.
The food-poisoning bug Listeria was shown to respond to an antibiotic even though the bacteria carry genes that should make them highly resistant.
Scientists said the antibiotic, called fosfomycin, should be reconsidered as a treatment for Listeria infections.
Early lab tests had indicated fosfomycin failed to kill Listeria.
This was because the bacteria carry a gene that enables them to break down the drug.
Further studies, however, found the drug was effective at killing Listeria in infected cells in the lab and in mice.
Genes that are only activated when the bacteria infect the body cancel out the effects of the drug-destroying gene, Edinburgh University researchers found.
The findings suggest fosfomycin could prove to be a useful treatment for life-threatening Listeria case , the researchers said.
Listeria infection - also known as listeriosis - is the most lethal food-borne disease known and is often fatal.
It is caused by eating contaminated foods such as soft cheeses, smoked salmon, pates, meats and salads.
The infection is particularly deadly for those with weak immune systems, such as older people and newborns. It can also cause miscarriage.
The bacteria reproduce within the cells of the body and frequently affect the brain, which only certain medicines are able to treat.
This limits the treatment options for serious infections, and so fosfomycin may prove highly beneficial.
The study, published in the journal PLOS Genetics, was funded by Wellcome.
Prof Jose Vazquez-Boland, who led the research at the University of Edinburgh's division of infection medicine, said: "Our study focused on Listeria, but this important discovery may be relevant for other species of bacteria too.
"It is encouraging that we may be able to repurpose existing drugs in the race against antibiotic resistance."