New MRSA Strain Found

June 5, 2011

I’ve written here several times before about the development of antibiotic-resistant bacteria, and the possibility that the current routine  use of antibiotics in animal feed may be contributing to the phenomenon.   Now an article at the BBC News site is reporting that a team of scientists from the University of Cambridge, led by Dr. Mark Holmes, has discovered a new strain of methicillin-resistant Staphylococcus aureus [MRSA], which is not only resistant to many antibiotics, but is also not detected by some standard tests for MRSA, because it differs genetically from the strains previously known.

A new strain of the MRSA “superbug” has been found in British cows and is believed to be infecting humans.

There is also an article on this research in the “Science Now” section of the Science magazine site.

The researchers found the new bacterial strain while studying mastitis, an infection of the udders of dairy cows.  They found S. aureus organisms that grew in the presence of antibiotics, an obvious suggestion of antibiotic resistance; but a standard DNA test for MRSA was negative.

Some milk samples from sick cows contained S. aureus bacteria that grew in the presence of antibiotics, which is one test for MRSAs. Yet the same samples turned up negative for the drug-defying bacterium when the team used PCR, a DNA amplification technique, to detect a gene called mecA, which is found in all MRSA strains.

(PCR, a polymerase chain reaction, is a standard laboratory technique that can produce a very large number of copies of a DNA sequence from a small initial sample.)    A team from the Wellcome Trust Sanger Institute subsequently sequenced the entire genome, and found that the new strain did have a mecA gene, but one that differs enough from those seen before to produce a false negative test.

Subsequent investigation, described in a news release from the University of Cambridge, found evidence that the new strain was also present in humans, based on samples from Scotland, England, and Denmark.  In some cases these were from people with MRSA infections; in others, the new strain was found from routine screening tests.  (Many people may harbor MRSA organisms on their skin or in their noses without developing an infection.)  There appears to be very little chance of the bacterium being passed to humans through dairy products, since pasteurization kills the organism.   However, it could be passed to agricultural workers by contact, and then passed to other people.

But workers who come into contact with infected dairy cows could be carriers. Holmes’s team reports “circumstantial evidence” for this, such as the fact that genetic subtypes of the human and cow samples from the same geographical areas were nearly identical.

The research was published online in the most recent edition of Lancet Infectious Diseases.  The abstract is supposed to be available here, but the Lancet‘s site seems to be broken at this writing.  I’ll post an updated link if I can find one.*

This is another piece of admittedly circumstantial evidence that there may be a relationship between the routine use of antibiotics in industrial agriculture and the development of antibiotic-resistant bacteria.  Clearly, further investigation is needed, and is underway; if nothing else, these results seem to show that our understanding of how this microbial ecosystem works is far from complete.

Update Monday, 6 June, 11:40 EDT

* The link to the abstract is working now.  If you register (or have already done so at the site), you can get the complete article at no cost.

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