New MRSA strain not picked up by current genetic tests

A group of British and Danish scientists has identified a new strain of methicillin-resistant Staphylococcus aureus (MRSA), which has been found in cows and their milk. The new strain is also known to have infected 27 people in the UK and 24 in Denmark.

A team of scientists from the University of Cambridge found the new MRSA strain while researching Staphylococcus aureus — the bacterium known to cause bovine mastitis.

The new strain has a different genetic make-up to previous MRSA strains. MRSA is normally detected by looking for the presence of the critical gene, mecA, which makes it resistant to methicillin. The mecA gene in this new strain of MRSA is only 60 percent similar to that of 'normal' MRSA in humans, making it impossible to be identified using standard molecular tests.

'The molecular tests most often used to confirm MRSA status will be falsely negative if we do not take into account the new strain', said Dr Mark Holmes, a senior lecturer in preventative veterinary medicine and author of the study. 'Some MRSA that may be out there may not have been detected'.

However, scientists at UK's Health Protection Agency (HPA) who also worked on this study, said newer tests currently being trialled in Britain and Europe have proved able to detect it. At the moment the scientists estimate there are probably less than 100 cases - a small fraction of the total number of MRSA cases. However, the number of cases appears to be growing rapidly. In 2008 there were two confirmed cases in England and Scotland, growing to 12 by 2010.

To find the same new strain in both humans and cows is certainly worrying. However, pasteurisation of milk will prevent any risk of infection via the food chain', said Dr Laura Garcia-Álvarez, who worked on Dr Holmes' team.

It is important to remember MRSA is still treatable with a range of antibiotics and the risk of becoming infected with this new strain is very low', said Dr Angela Kearns, head of the HPA's Staphylococcus laboratory.

Dr Holmes told a press conference it was a 'credible hypothesis' that overuse of antibiotics had led to the strain's emergence. He said dairy farmers were 'under relentless financial pressure' from supermarkets to produce milk as cheaply as possible. 'When you drive your cows hard, you end up with more mastitis', he explained. 'You end up using more antibiotics'.

But Andrew Opie, food director at the British Retail Consortium, said: 'It is ludicrous to blame supermarkets…. retailers work directly with farmers in their dedicated supply chains and through the group Responsible Use of Medicines in Agriculture to ensure antibiotics are used appropriately'.

The study was published in Lancet Infectious Diseases.