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Switching between two antibiotics in a well-designed sequence could prove to be a "surprising" new way to combat drug resistance, research suggests.

Scientists laboratory-tested several different sequences of low-dose antibiotics against a common bug.

They were baffled when some completely eliminated the bacteria, performing better than conventional combinations of drugs given at higher doses.

The study is published in the journal PLoS Biology.

Researchers from Exeter University believe the success of this strategy might be because one antibiotic sensitises the bug, making it more vulnerable to the effects of the other.

And if proven to work in humans it could provide a much needed way to use old antibiotics that are currently considered ineffective, they say.

Mutating rapidly

The rise in antibiotic resistance has been described by the World Health Organization as a "major global threat".

A core problem is that bacteria are capable of mutating rapidly, speedily evading the killing power of antibiotics.

To overcome this, doctors often prescribe a cocktail of two high-dose antibiotics together, hoping the combined chemical punch will defeat the bug before it has a chance to outwit the drugs.

But scientists working in Germany, England and Mexico took a different approach.

Using two commonly prescribed antibiotics - erythromycin and doxycycline - they pitted them against the bug E. coli in a laboratory.

They first deployed one drug for 12 hours, then removed it and switched to the other, repeating this process several times.

And to make it even harder, they introduced a gene into the bug, rendering it already partially resistant to the antibiotics.

Testing 160 different sequences, they found five that completely eliminated the bug within four days.

And the doses they used were much lower than conventional treatments, they say.

'Inform clinical trials'

Prof Robert Beardmore, of Exeter University, said: "We were very baffled when we discovered this. This research should really have been done 50 years ago.

"If it is proven to work in humans, perhaps we could take the arsenal of drugs we already have and use them more effectively with this switching strategy."

He added: "One outcome of this highly surprising result will be to set in motion a series of studies to determine ways of using antibiotics not only in combination but sequentially and with the potential for lower doses than is currently thought possible."

Commenting on the findings, Prof Nigel Brown, president of the Society for General Microbiology, said: "Given the challenges involved in the discovery and development of new antibiotics, potential methods that let us continue to effectively use the ones we already have are welcome.

"This laboratory study will inform clinical trials and may help in the fight against the rise of antimicrobial resistance."