Researchers have discovered a new strain of methicillin-resistant Staphylococcus aureus, a mecA homologue that is genetically different compared with existing strains, in both human and bovine populations in the UK and Denmark. Standard molecular tests failed to identify the strain, therefore new diagnostic guidelines for the detection of MRSA should consider tests for the mecALGA251 gene, according to data presented at a press conference in advance of publication in The Lancet Infectious Diseases.“The test that we use at the moment to confirm MRSA will be falsely negative if we don’t take into account the new mecA gene,” Mark A. Holmes, PhD, of the department of veterinary medicine at the University of Cambridge, said during the press conference. “We are concerned that there is circumstantial evidence that MRSA is moving between cattle and people. The new MRSA is found in strains or family types of S. aureus that previously were only thought to exist within cows.”
In 2007, Holmes and colleagues identified a pair of isolates (strains LGA251 and LGA254) resistant to methicillin-like drugs in milk from dairy cows and set out to determine the cause of resistance in the bacteria. The researchers tested for mecA, the methicillin-resistant gene, using the standard polymerase chain reaction (PCR).
Compared with PCR test results, which yielded negative results for methicillin resistance, whole genome sequencing revealed that there was, in fact, a mecA gene, a homologue with only 60% similarity to the conventional mecA gene.
Subsequently, the researchers searched for human MRSA isolates, and similar families in both cows and people in different parts of the countries were found with this strain, suggesting geographical clustering.
Approximately 51 isolates tested positive for mecA; 15 of 26 from England; 12 of 16 from Scotland; and 24 of 32 from Denmark. Further, after assessing trends in annual incidence of mecA detection, the researchers found that rates of these MRSA increased substantially between 2007 and 2010.
“The discovery of this new mecA homologue raises issues about the detection and confirmation of MRSA,” according to the researchers. “Irrespective of whether an infection is caused by mecALGA251 MRSA or conventional MRSA, after culture and antimicrobial susceptibility testing, appropriate decisions about care of patients can be made. However, when existing PCR or monoclonal antibody methods are used as the only method to detect MRSA, or when these methods are used to confirm provisional detection of MRSA, then mecALGA251 S. aureus will be wrongly diagnosed as methicillin-susceptible.”
Although the discovery of this previously undetected mecA homologue is of public health importance, Holmes said drinking milk and consuming dairy products is not a public health concern.
“There is no survival of MRSA in milk or dairy products, as long as the milk is pasteurized,” he said. “The main worry would be that these cows represent a pool of the bacteria, that these bacteria end up colonizing people who work or live on farms and they take it out to the wider community.”
The researchers said the ramifications of not detecting S. aureus strains that carry this new strain should be considered in diagnostic protocols.
Disclosure: This research was funded by the Department for Environment, Food and Rural Affairs, Higher Education Funding Council for England, Isaac Newton Trust (University of Cambridge), and the Wellcome Trust.