Bacteria can mutate quickly into rantibiotic esistant strains

The researchers found that the small number of strains considered to be the most dangerous to patients also tended to have the biological features which made them highly efficient at "colonising" humans.

They believe that the worst outbreaks happen when a strain of MRSA with the natural ability to quickly infect humans goes on to develop drug-resistant abilities.

This happens because naturally less resistant strains tend to be killed off by antibiotics, while those which are genetically tougher persist.

Places where antibiotics are frequently used, such as hospitals, will accelerate the development of drug-resistant strains, which is why such strains are rarely found in the community at large.

'Tough nut'

Dr Nick Day, from Oxford's Department of Tropical Medicine, said: "We think that our findings will have a significant impact on future research directions, as they demonstrate what a tough nut MRSA transmission will be to crack.

We think that our findings will have a significant impact on future research directions, as they demonstrate what a tough nut MRSA transmission will be to crack

Dr Nick Day, Oxford University

"Epidemic MRSA strains already clearly possess the biological recipe for success in colonising humans, even without antibiotic resistance.

"By comparison of the virulent strains with their less successful or aggressive relatives, genes critical for success in humans could be found and targeted."

He said that while many dismissed MRSA as an opportunistic infection which only preyed on the weak, the results of the tests suggested that it was capable of becoming a major health threat, "highly specialised in gaining a foothold" in the human body.