The survey in February 2015 looked at 103 (52 pork and 51 chicken) pre-packaged fresh meat products, labelled as being of UK farm origin, purchased from supermarkets in five locations.
Two of the pork samples – one from sausages, one from minced pork – tested positive for MRSA; the sausage sample contained two strains of the bacteria.
First evidence of MRSA
An analysis of the genetic make-up of the bacteria showed it belonged to a type of MRSA known as LA-MRSA CC398, which has emerged over the last few years in continental Europe, particularly in pigs and poultry, but was not believed to be widely distributed in the UK.
Presence of LA-MRSA CC398 in the human food chain demonstrates a possible further pathway for transmission of antimicrobial resistance from livestock to the human population, and not just via those with direct contact with farm animals, according to the study.
Adequate cooking (heating above 71°C) and hygienic precautions should minimise the likelihood of human colonisation via contaminated pork. However, finding LA-MRSA CC398 identifies a potential pathway from farms to the wider population, said the researchers.
Dr Mark Holmes from the Department of Veterinary Medicine at the University of Cambridge said it is the first time that MRSA has been detected in retail meat products in the UK.
“The public should not be overly worried by this as sensible food precautions and good hygiene should prevent its spread. It’s also usually pretty harmless and only causes health problems if it infects someone in poor health or gets into a wound," he said.
“However, this does suggest that MRSA is established in our pig farms and provides a possible route of transmission from livestock, through those in direct contact with pigs, into the wider population.”
MRSA origin and numbers
In many countries LA-MRSA CC398 represents an occupational risk for those in close contact with livestock, particularly pigs and veal calves.
While human contamination of carcasses or meat products in the abattoir or at the meat packing plant may occur, there is evidence that ST398 isolates are of animal origin.
Testing used a sensitive method of detection of bacterial contamination so the numbers of MRSA may be low.
It cannot be ruled out the meat packing plants from which MRSA originated also handle imported meat so cross-contamination might have occurred between non-UK to UK sourced meat.
The study did not examine thaw water separately or did it find ST398 in poultry samples which researchers suggested points to the lineage being present in the UK at lower rates than in continental Europe but further studies are required to establish this.
Preparation of meat samples followed ISO 6887–2:2003. After thawing, the exterior packaging was disinfected before the meat was removed. A 10g sample was excised, mixed with 225ml of 6% w/v NaCl Nutrient Broth (P and O laboratories) and homogenised using a Stomacher (Stomcher80 Laboratory System) for two minutes.
Multilocus sequence typing using the assembly sequences found all three isolates belonged to sequence type ST398.
Sample collection and financial support was provided by the Alliance to Save our Antibiotics.
Dr Des Walsh, head of Infections and Immunity at the Medical Research Council (MRC), said such studies are crucial to reveal concerns to human health through contaminated livestock and show resistance to antibiotics is a growing problem.
“To win the fight against antimicrobial resistance, we need an all hands on deck approach, and that’s why we’ve teamed up with leading experts in biological, social and others sciences in a joint initiative designed to find new solutions, fast.”
“Detection of livestock-associated methicillin-resistant staphylococcus aureus CC398 in retail pork, United Kingdom, February 2015”
Authors: N F Hadjirin, E M Lay, G K Paterson, E M Harrison, S J Peacock, J Parkhill, R N Zadoks, M A Holmes