Scientists have identified a new strain of methicillin-resistant Staphylococcus aureus (MRSA) which occurs both in human and dairy cow populations.
The study, led by Dr Mark Holmes at the University of Cambridge, identified the new strain in milk from dairy cows while researching mastitis (a bacterial infection which occurs in the cows’ udders).
The new strain’s genetic makeup differs greatly from previous strains, which means that the ‘gold standard’ molecular tests currently used to identify MRSA – a polymerase chain reaction technique (PCR) and slide agglutination testing – do not detect this new strain. The research findings are published in the journal The Lancet Infectious Diseases.
Dr Laura García-Álvarez, first author of the paper, who discovered the new strain while a PhD student at the University of Cambridge’s Veterinary School, said: “To find the same new strain in both humans and cows is certainly worrying. However, pasteurization of milk will prevent any risk of infection via the food chain. Workers on dairy farms may be at higher risk of carrying MRSA, but we do not yet know if this translates into a higher risk of infection. In the wider UK community, less than 1% of individuals carry MRSA – typically in their noses – without becoming ill.”
The scientists discovered the antibiotic resistant strain while researching S. aureus, a bacterium known to cause bovine mastitis. Despite the strain being able to grow in the presence of antibiotics, when they attempted to use the standard molecular tests available – which work by identifying the presence of the gene responsible for methicillin resistance (the mecA gene) – the tests came back negative for MRSA.
When Dr Matt Holden and a research team at the Wellcome Trust Sanger Institute sequenced the entire genome (decoding all of the genes in the bacteria’s DNA) they realised that the new strain possessed unconventional DNA for MRSA. They found that the new strain does have a mecA gene but with only 60% similarity to the original mecA gene. Unfortunately, this results in molecular tests (which identify MRSA by the presence of the mecA gene) giving a false negative for this strain of MRSA.
Subsequent research revealed that the new strain was also present in humans. During the study, the new strain was found in samples from Scotland, England and Denmark (some from screening tests and others from people with MRSA disease). It has since been identified in Ireland and Germany. Additionally, by testing archived S. aureus samples, the researchers have also identified a recent upward trend in the prevalence of the antibiotic resistant bacteria.
Dr Mark Holmes said: “The majority of MRSA testing in British hospitals is performed by seeing if the bacteria will grow in the presence of antibiotics, typically oxacillin and cefoxitin, rather than methicillin – which is now no longer manufactured. This type of testing detects both the new MRSA and conventional MRSA.
“However, it is important that any of the MRSA testing that is based on detection of the mecA gene – i.e. PCR based testing, or slide agglutination testing – be upgraded to ensure that the tests detect the new mecA gene found in the new MRSA. We have already been working with public health colleagues in the UK and Denmark to ensure that testing in these countries now detects the new MRSA.”