New MRSA strain discovered in milk

Scientists from the UK have identified a new strain of the MRSA 'superbug' in cows' milk and in humans which cannot be detected using standard tests.

It is the first time the antibiotic resistant bacterium has been detected in dairy herds in the UK, and suggests that cows may be able to transmit the disease to humans.

The new strain came to light when a team from Cambridge University were researching the bacterium Staphylococcus aureus, a known cause of bovine mastitis.

The 'gold standard' method for detecting MRSA - called PCR - failed to identify the new strain which could grow in the presence of antibiotics.

The test works by identifying the mecA gene responsible for methicillin resistance in the bacterium.

However, when another team analysed the bacterial DNA, they found the new strain possessed a different version of the mecA gene, not recognised by PCR.

If hospitals relied on PCR alone to identify MRSA, the new strain would be missed and patients could be treated with antibiotics which would be useless in fighting off an infection, the researchers said.

Although the new MRSA strain was found in milk samples taken from dairy herds across the UK, the researchers said that dairy produce was still safe to consume.

“Drinking milk or eating dairy products is not a public health concern," said Dr Mark Holmes, who led the study.

"The pasteurisation of milk kills bacteria including MRSA without any problems. There is no survival or MRSA in milk or dairy products as long as the milk is pasteurised.

"There is no evidence the new strain causes more severe illness, however research to date suggests cases are increasing.

“The main worry will be that these cows represent a pool of the bacteria, that this bacteria ends up colonising people who work on farms and they take it to the wider community.”

Further research identified the new strain in samples taken from people in Scotland, England and Denmark.

The findings also suggest that cows may be important carriers of the new strain of MRSA, as some bovine and human isolates were found in the same geographical location and none of the bovine isolates appeared to share a common lineage with MRSA strains usually found in humans.

Dr Holmes said: “Although there is circumstantial evidence that dairy cows are providing a reservoir of infection, it is still not known for certain if cows are infecting people, or people are infecting cows. This is one of the many things we will be looking into next.

“Although our research suggests that the new MRSA accounts for a small proportion of MRSA – probably less than 100 isolations per year in the UK, it does appear that the numbers are rising.

"The next step will be to explore how prevalent the new strain actually is and to track where it is coming from."

The study findings are published in the journal Lancet Infectious Diseases.

This article was published on Fri 3 June 2011