Link between MRSA and prescription drug debunked by Scots scientists

Prescriptions of the antibiotic methicillin were not responsible for the emergence of the MRSA superbug, a study has found.

MRSA (methicillin-resistant Staphylococcus aureus) emerged 14 years before the drug was first used in the 1950s, scientists have learned.

The S. aureus bug acquired the gene for methicillin resistance, mecA, as early as the mid-1940s.

Professor Matthew Holden, from the University of St Andrews, said: “Our study provides important lessons for future efforts to combat antibiotic resistance.

“It shows that new drugs which are introduced to circumvent known resistance mechanisms, as methicillin was in 1959, can be rendered ineffective by unrecognised, pre-existing adaptations in the bacterial population.

“These adaptations happen because, in response to exposure to earlier antibiotics, resistant bacterial strains are selected instead of non-resistant ones as bacteria evolve.''

The mecA gene confers resistance by producing a protein called PBP2a, which reduces the ability of anti-S. aureus antibiotics to target the bacterial cell wall.

It was the introduction of penicillin in the 1940s that led to the natural selection of S. aureus strains with the methicillin resistance gene, said the scientists.

Co-author Dr. Catriona Harkins, from the University of Dundee, said: “Within a year of methicillin being first introduced to circumvent penicillin resistance, strains of S. aureus were found that were already resistant to methicillin.

“In the years that followed, resistance spread rapidly in and outside of the UK. Five decades on from the appearance of the first MRSA, multiple MRSA lineages have emerged which have acquired different variants of the resistance gene.''

To uncover the origins of MRSA and trace its evolutionary history, the researchers mapped out the genomes, or genetic codes, of 209 historic S. aureus samples.

The oldest of the isolates were identified more than 50 years ago and have been freeze-dried and stored ever since. Genes were found in the bugs that confer resistance to numerous antibiotics and reduce susceptibility to disinfectants.

Professor Holden added: “S. aureus has proven to be particularly adept at developing resistance in the face of new antibiotic challenges, rendering many antibiotics ineffective.

“This remains one of the many challenges in tackling the growing problem of antimicrobial resistance.

“In order to ensure that future antibiotics retain their effectiveness for as long as possible, it is essential that effective surveillance mechanisms are combined with the use of genome sequencing to scan for the emergence and spread of resistance.''

The findings appear in the journal Genome Biology