Scientists in Galway have discovered that penicillin can be used to make other antibiotics or the body more effective at fighting MRSA.
The finding may open up new avenues for combating antimicrobial resistance, which is claiming an increasing number of lives around the world.
The microbiology team at NUI Galway, working in conjunction with the University of Liverpool, found that while penicillin cannot kill MRSA, it weakens its virulence.
The research, published in the Journal of Infectious Diseases, shows that the penicillin causes the bacteria to switch off its toxin genes and thicken its cell wall.
The body's own immune system and other antibiotics can then exploit this state of change to kill off the infection.
A recent report from the UK government found that antimicrobial resistance infections will cause more deaths than cancer by 2050 if not addressed urgently.
Caused by a form of the staphylococcal bacteria, MRSA is resistant to most antibiotics used to treat ordinary infections, and up to a fifth of patients who contract it die from the infection.
The research was funded by the Health Research Board and Medical Research Council.
"Our findings explain the anti-virulence mechanism of penicillin-type antibiotics and support the re-introduction of these drugs as an adjunct therapeutic for MRSA infections.
"MRSA can be extremely virulent, which is part of the challenge in treating it," said Professor James O'Gara from NUI Galway.