A 'harmless' bacteria found in milk and products like cheese may carry a new antibiotic resistance gene which could jeopardise the use of last resort drugs to treat superbug infections in humans, scientists say.

Researchers of the University of Bern in Switzerland identified the antibiotic resistance gene in bacteria from dairy cows.

This gene confers resistance to all beta-lactam antibiotics including the last generation of cephalosporins used against methicillin-resistant Staphylococcus aureus.

A transfer to S aureus, which is likely according to the researchers, would jeopardise the use of reserve antibiotics to treat human infections caused by multidrug-resistant bacteria in hospitals.

Macrococcus caseolyticus is a harmless bacterium naturally found on the skin of dairy cows which can spread to milk during the milking process.

It can also be present in dairy products made from raw milk like cheese.

Over the last years, researchers investigated M caseolyticus present in milk of dairy cows suffering from mastitis.

Mastitis is an infection of the udder which is frequently treated with penicillins and cephalosporins, which are antibiotics of the beta-lactam class like methicillin.

These bacteria isolated from milk showed an unusual resistance pattern to antibiotics, but the known genes responsible for resistance were missing.

“We were intrigued by this novel resistance in M caseolyticus and wanted to know what was behind this resistance,” says Vincent Perreten from University of Bern.

Bacteria have the extraordinary ability to acquire novel genetic information such as antibiotic resistance genes.

Using Next Generation Sequencing (NGS), the researchers rapidly found that the M caseolyticus isolates acquired a novel antibiotic resistance island which contains a new resistance gene called mecD.

M caseolyticus containing the novel mecD gene has been so far mainly found in cattle but in one case it has been isolated from skin infection in a dog indicating that this bacteria has the potential to colonise different animal species.

“So far, we do not have any indication of the presence of mecD in humans, but its transfer from M caseolyticus to S aureus would further limit therapeutic options of this pathogen,” said Perreten.

“It is imperative to keep an eye on the evolution and spread of this novel resistance gene in both human and animal bacteria,” he said.