MRSA found in dairy cows

“Scientists in the UK have discovered a new strain of MRSA that appears to spread to humans from cattle and can cause life-threatening illness,” reported The Guardian . It said that a study of dairy herds had found the drug-resistant strain in cows' milk.

MRSA (methicillin-resistant Staphylococcus aureus) is usually detected using a technique called antibiotic-susceptibility testing. Borderline cases of MRSA are confirmed with molecular testing, which detects the presence of a gene that is common to these “superbugs”.

This study looked at strains of MRSA from cattle and humans to see whether they possessed any new genetic features that affect the reliability of these tests.

The study found a new type of gene in many of the cattle samples. This gene makes the bacteria resistant to a range of antibiotics. While the bacteria with this gene showed up in antibiotic-susceptibility testing, molecular testing could not recognise the gene and failed to identify the bacteria as MRSA.

Therefore, if molecular testing is used to detect MRSA or to confirm borderline cases, it will not identify bacteria with the new gene.

The researchers say that only a small proportion of MRSA bacteria possess this gene. However, as it has been detected in MRSA samples from dairy cows, these animals might form a “reservoir of infection”. They warn that close links with farms or contact with dairy cattle could increase the risk of MRSA being transmitted to humans. Further studies are needed to inform tests for diagnosing MRSA.

Experts highlight that the main worry is that bacteria may colonise people who work on farms, and not that people may be at risk from drinking milk. As almost all milk sold in the UK is pasteurised, drinking or eating dairy products is reportedly “not a health concern”.

Where did the story come from?

The study was carried out by researchers from the Department of Veterinary Medicine at the University of Cambridge and other health and academic institutions in Cambridge and the UK.

Funding was provided by the Department for Environment, Food and Rural Affairs, the Higher Education Funding Council for England, the Isaac Newton Trust (University of Cambridge) and the Wellcome Trust.

The study was published in the peer-reviewed medical journal The Lancet.

The news headlines have oversimplified this complex research and may imply that people are at risk from drinking milk, which is not the case. The main implications of these findings are in the field of laboratory and diagnostic testing.

What kind of research was this?

This laboratory study looked at strains of MRSA from samples taken from cattle and humans. The researchers wanted to see if they possessed any new genetic features that meant that they could not be detected by standard tests for diagnosing MRSA.

The researchers explained that animals are known to act as a “reservoir” for new strains of bacteria, so could be a source of new strains of the superbug MRSA (methicillin-resistant Staphylococcus aureus) in humans. Staphylococcus aureus causes a wide variety of infections in humans, from skin infections to pneumonia and blood poisoning. However, many people carry the bacteria harmlessly on their skin.

MRSA has developed resistance to methicillin and other penicillin antibiotics that would normally kill Staphylococcus aureus. This means that MRSA can cause disease that is harder to treat. It is believed that the Staphylococcus aureus bacteria evolved to develop this resistance by acquiring a certain chromosome element (called SCCmec) which contain a gene called mecA. This gene encodes a protein that binds to penicillin.

The researchers describe how MRSA is usually identified in the laboratory using “antimicrobial susceptibility testing”. In this test, the bacteria are incubated with discs that are impregnated with antibiotics. The zone around the disc where bacterial growth has been prevented is measured. There are standard zones around the disc that confirm the presence of MRSA. If the results are borderline, molecular testing (called PCR testing) is used to detect the mecA gene or penicillin-binding protein in the bacteria.

Before 2003, most cases of MRSA were associated with human transmission and infection, but after this time it was found in livestock. Evidence was also found that some strains may not be restricted to a single species but can cross between humans and farm animals. There is concern that farm animals could act as a reservoir for MRSA and that close human-animal contact could increase the risk of transmission.

What did the research involve?

The researchers took isolates (a pure strain that has been separated from a mixed bacterial culture) of MRSA bacteria from both humans and cows and determined whether antimicrobial susceptibility testing could detect the bacteria.

In 2007, the researchers obtained 24 isolates of bovine MRSA from the Veterinary Laboratories Agency in the UK. These came from a collection of 940 Staphylococcus aureus isolates obtained from the milk of 465 different herds of cows with mastitis, which had been submitted to the agency for testing.

Isolates of human MRSA were obtained from the Health Protection Agency and the Scottish MRSA Reference Laboratory in the UK, and the National MRSA Reference Laboratory in Denmark. The human bacteria had been cultured from blood samples or infected wound swabs.

The researchers carried out antimicrobial susceptibility testing on these bovine and human isolates, and used PCR testing to see whether the mecA gene could be detected.

What were the basic results?

A new mecA gene (called mecALGA251) was discovered in 15 of 24 Staphylococcus aureus isolates from dairy cattle in England. These isolates were from three different strains of MRSA. The new mecALGA251 gene was also identified in 12 of 16 isolates from human samples from Scotland, 15 of 26 isolates from England, and 24 of 32 isolates from Denmark.

Antibiotic-susceptibility testing identified that these isolates were resistant to a wide range of antibiotics. However, PCR testing showed negative results for the mecA gene and penicillin-binding protein. This suggests that if PCR testing is used on its own or to confirm the results of antibiotic-susceptibility testing, it may fail to identify the infection as being due to MRSA.

How did the researchers interpret the results?

The researchers concluded that routine culture and antimicrobial susceptibility testing will identify Staphylococcus aureus bacteria with the new mecA gene as being resistant to methicillin and related antibiotics. However, PCR testing to confirm the results will not detect this gene and will fail to identify the bacteria as MRSA. The researchers concluded that new guidelines for the detection of MRSA should consider including tests for mecALGA251.

Conclusion

MRSA is usually detected by using antibiotic-susceptibility testing. Results are confirmed using molecular testing (PCR), which detects the presence of the mecA gene that is common to these bacteria. This laboratory research tested MRSA obtained from cattle and milk samples, which were stored at veterinary agencies in the UK, and MRSA samples from humans, which were stored at reference laboratories in the UK. In many of the cattle samples tested, the researchers detected a new type of mecA gene. Antibiotic-susceptibility testing showed that MRSA bacteria carrying this gene were resistant to a range of penicillin-related antibiotics, but further PCR testing could not identify these bacteria as MRSA.

The most important finding from this research is that if molecular testing techniques are used to detect or confirm the presence of MRSA, they will not correctly identify the new type of MRSA bacteria.

The researchers noted that only tentative interpretations can be made from these results, and more samples need to be studied. Some points of note include:

The strains containing this new gene were only obtained from existing MRSA collections. Researchers will need to carry out the same tests in samples obtained from other populations.
It is not known whether disease caused by MRSA with the new mecA gene is any different to that caused by conventional MRSA.
According to the researchers, their data suggest that the MRSA infections with the new gene are likely to account for 1 in 100 to 1 in 500 of total MRSA in the UK and Denmark. This is a small proportion of MRSA infections.
As this gene has been detected in MRSA samples from dairy cows, it suggests that these animals might form a reservoir of infection. Close links with farms or contact with dairy cattle could increase the risk of this type of MRSA being transmitted to humans. As the study did not look at the spread of resistance from cattle to humans, this will need to be investigated in further research.

The discovery of this previously undetected MRSA, which carries the new mecA gene, is potentially important to public health. Further quality evidence is required from observational and experimental studies to inform tests for diagnosing MRSA.