Milk Contains a new MRSA Strain

A new superbug strain has been discovered by researchers in milk. The previously undiscovered strain also causes human infections.

The bacterium, a strain of methicillin-resistant Staphylococcus aureus (MRSA), appears to be relatively rare. The bacterium is found in approximately 1% of MRSA cultures from human subjects in the U.K.

It is not a threat to those who consume milk products or dairy products such as cheese. Pasteurization and digestion have killed bacteria including MRSA.

Researchers believe that any risk to humans would be from coming in contact with the affected cows.

The question of whether or not this new strain might be found in American milk or cattle remains open.

"The main worry would be that these cows represent a pool of the bacteria and these bacteria end up colonizing people that work or live on farms and they take it out to the wider community," says study researcher Mark A. Holmes, VetMB, a senior lecturer in the department of veterinary science at the University of Cambridge, England.

The fact that MRSA is resistant to current "gold-standard" polymerase chain react (PCR) tests, which are preferred by hospitals and laboratories for their speed, is even more concerning.

Holmes says that if you have a severe infection due to this bacteria, your specimen is sent to a lab to be tested. The only way they can test it is the PCR. If you are falsely negative, you might be prescribed methicillin-like medications to cure the problem.

MRSA may be missed in some tests

The study researchers identified the new strain because they tested for bacteria using two methods: one in which scientists swab bacteria on a gel food called agar that's dotted with antibiotic-impregnated discs to see if the drugs can kill the bacteria; and the second, PCR testing, which involves looking for a gene called mecA that makes the pathogen resistant to a host of antibiotics, including methicillin.

The agar plate test showed that new strain was antibiotic-resistant, but curiously, the PCR test did not find the mecA gene.

Scientists sequenced the complete genome of the bacteria to discover the cause.

Although the copycat gene (called a homologue) conferred some antibiotic resistance in this new strain, it could not be detected using PCR since it isn't identical to the reference.

It's similar to trying to find a document online but mispelling the search terms, according to experts.

Holmes points out that it is possible to search for the Staphylococcus word in long documents if the user typed it correctly. But if the spelling of the word is wrong, the search will not find it.

Hospitals and laboratories are increasingly moving away from the use of the agar-plate sensitivity test. It is slower, taking up to three days, and can cause delays in getting results. The rapid PCR test, however, takes only 30 minutes.

Experts believe that MRSA cases are being overlooked because of greater reliance upon PCR.

When researchers tested bacteria cultures from people, the results were surprising. The new strain was found in Britain and Denmark.

Many of those samples were unexplained cases of antibiotic resistance, some from patients with serious infections and others that were found through routine screening, which had been kept in freezers for further study.

Experts say that only around half the unsolved cases were solved by this new strain.

"There are more things out there than what we know," states Professor Ruth N. Zadoks DVM, PhD (Moredun Chair of Veterinary Epidemiology) at the University of Edinburgh in Scotland.

Zadoks called the new study "fantastic detective work" that will have "considerable public health impact."

Are there MRSA cases in the U.S. Milk?

Zadoks had previously tested U.S. dairy samples for MRSA. However, he was not involved with the current research.

A 2009 paper in Journal of Dairy Science reported that she found no MRSA from testing milk from 542 U.S. farms.

However, nine of her specimens tested positive for mecA, which indicates antibiotic resistance.

Researchers from the U.K. claim that they didn't test U.S. milk samples and bacterial cultures so they don't know if this new strain has reached America.

Holmes states, "It's possible." Holmes says, "I don't doubt that they will be busy in the coming months looking for it on their farms."