We have witnessed the birth of a strain of MRSA

Dr Nick Day, John Radcliffe Hospital

However, until now, no researcher had been able to pinpoint the exact moment when this happened.

However, the case of a baby who fell ill with a previously unknown drug-resistant strain has apparently provided scientists with a snapshot of the process.

The baby was hospitalised from birth because of respiratory problems and quickly picked up a respiratory tract infection.

This was identified as a standard, non-resistant Staphylococcus bacteria, and standard antibiotics were given.

The infection cleared up, but then recurred some days later - and this time a swab revealed a different strain of Staphylococcus was to blame.

This was MRSA, and doctors were forced to switch antibiotics completely in order to beat it.

No contact

Doctors were confused because they felt that the baby had had no contact with anyone carrying MRSA.

Genetic analysis of the two strains involved revealed that they were identical in many ways - strongly suggesting that one had evolved from the other.

The new, antibiotic-resistant strain was compared to hundreds of other resistant strain, but differed from all of them.

The evolution had taken place because the original strain had acquired a slice of DNA called MecA, which gave it the ability to fight off various antibiotics.

Dr Nick Day, an MRSA expert from the John Radcliffe Hospital in Oxford, said: "It's an interesting observation - we have witnessed the birth of a strain of MRSA.

"I suspect this only happens quite rarely."

Although the new strain emerged, it is likely its arrival was closely followed by its annihilation, as the baby's infection was dealt with swiftly by other antibiotic drugs.

Other research backs up the theory that MRSA beats antibiotics by grabbing bits of DNA from other strains to augment its defences.

Those strains which are most resistant tend to become prevalent because the use of antibiotics kills off weaker competitors.

Doctors have been urged to cut down on their antibiotic use to try to stop this happening.

The research was published in the Lancet medical journal.