Every so often, research laboratories and hospitals testing patients for the superbug methicillin-resistant Staphylococcus aureus (MRSA) have come across an oddity: a strain that appeared to be MRSA because it was resistant to antibiotics but one that tested negative with the ‘gold standard’ molecular test. The quirky cases were so infrequent that they were usually filed away for future analysis or disregarded. Until, that is, PhD student Laura Garcia-Alvarez from Cambridge’s Department of Veterinary Medicine had the tenacity to look a little further at a bacterial strain she had spotted in cows’ milk.
MRSA first appeared in 1961 and epidemic strains of this difficult-to-treat bacterium have since spread worldwide in hospitals and the community. In the farming world, MRSA causes bovine mastitis – an infection of cows’ udders – affecting both animal welfare and milk yields.
Garcia-Alvarez was working with Dr Mark Holmes on bovine mastitis when she came across one of the ‘curious anomalies’. The strain was resistant to antibiotics but in the standard molecular test was negative for the presence of mecA – the gene responsible for methicillin resistance. She had the isolates retested and then sequenced at the Wellcome Trust Sanger Institute.
It turned out that she had discovered a new strain of MRSA. Its antibiotic resistance is carried not by mecA but by mecC, a gene that is so genetically dissimilar to mecA that it can’t be picked up by the standard molecular test used to define MRSA but only by DNA sequencing.
As Holmes and Garcia-Alvarez began to spread the information to colleagues around Europe, it soon became clear that this phenomenon was not confined to cows: others had found the unusual samples in humans. “We started to get calls from hospitals and research groups who had come across a small number of human MRSA strains that behaved differently,” said Holmes. “Within a few weeks, we had a further 50 isolates. This meant that what we were looking at was a human problem too.”
Garcia-Alvarez, who at the time was a student on the Department’s postgraduate training in infectious disease dynamics programme, described how finding the same new strain in both humans and cows was worrying, although no cause for immediate alarm: “Pasteurisation of milk will prevent any risk of infection via the food chain. In the wider UK community, less than 3% of individuals carry MRSA – typically in their noses – without becoming ill.”
“Nonetheless,” added Holmes, “MRSA presents a major challenge to the control of infectious diseases. Finding a new strain – studying its prevalence, how it confers antibiotic resistance and how it’s transmitted – can tell us enormous amounts about the origins and evolution of antibiotic resistance.”