The MRSA strain methicillin-resistant Staphylococcus aureus has emerged in livestock during the last 50 years in Europe, thought to be due to widespread antibiotic use in pig farming and farming intensification. It has led to particular concerns about livestock as reservoirs of antibiotic-resistant human infections.
MRSA was first identified in human patients in 1960 and the World Health Organisation now considers MRSA one of the world's greatest threats to human health.
"Cases of livestock-associated MRSA in humans are still only a small fraction of all MRSA cases in human populations, but the fact that they're increasing is a worrying sign," lead author of the study, Dr Lucy Weinert, said.
The University of Cambridge study found that the CC398 strain had maintained its antibiotic resistance over decades in pigs and other livestock, and was capable of rapidly adapting to human hosts while maintaining its antibiotic resistance.
The findings of the 'Stable antibiotic resistance and rapid human adaptation in livestock-associated MRSA' paper by the university's Department of Veterinary Medicine were published in the journal eLife today.
The results highlight the potential threat the MRSA strain poses to public health, with increasing numbers of infections in people who have not had direct contact with livestock, the report warns.
"Historically, high levels of antibiotic use may have led to the evolution of this highly antibiotic resistant strain of MRSA on pig farms, Dr Gemma Murray, another lead author of the study, said.
"We found that the antibiotic resistance in this livestock-associated MRSA is extremely stable - it has persisted over several decades, and also as the bacteria has spread across different livestock species."
MRSA doesn't cause disease in pigs. Its rise has been particularly evident in Danish pig farms where the proportion of MRSA-positive herds has increased from less than 5 percent in 2008 to 90 percent in 2018.
The study found the success of CC398 in livestock and its ability to infect humans was linked to three mobile genetic elements in the MRSA genome - chunks of genetic material that give the MRSA its characteristics, including resistance to antibiotics and its ability to evade the human immune system.
The researchers reconstructed the evolutionary history of two particular mobile genetic elements called Tn916 and SCCmec that conferred antibiotic resistance in MRSA.
They found these had persisted in a stable way in CC398 in pigs over decades and persist when CC398 jumped to humans, carrying high levels of resistance to antibiotics commonly used in farming.
A third mobile genetic element called φSa3, which enables the CC398 strain of MRSA to evade the human immune system, was found to have frequently disappeared and reappeared over time, in both human-associated and livestock-associated CC398. The researchers said this meant CC398 can rapidly adapt to human hosts.
Both researchers said ongoing reductions in antibiotic use on pig farms, due to recent policy changes, were likely to have a limited impact on the presence of this strain of MRSA in pigs because it is now so stable.
The European Union is banning the use of zinc oxide this month on farms, due to its environmental impact concerns and the potential promotion of antibiotic resistance in livestock. It is used on pig farms to prevent diarrhoea in piglets.
But the authors conclude the move may not help reduce the prevalence of CC398 because the genes conferring antibiotic resistance are not always linked to the genes that confer resistance to zinc treatment.