A genetically modified mouthwash has been developed which could effectively eliminate tooth decay, scientists heard yesterday.

The mouth rinse contains a friendlier GM version of the bug that rots the teeth, Streptococcus mutans. But, unlike their cousins, these bacteria do not produce enamel-eroding acid. When the solution is squirted into the mouth, the good bugs take over from the bad ones and prevent them returning.

According to the researchers, a single five-minute treatment costing less than £100 would last a lifetime.

Jeffrey Hillman, from the University of Florida, said: "If this approach works as well as we hope, it has the potential to eliminate the majority of tooth decay."

He hopes to start clinical trials this year, using a solution squirted on to the teeth of adult volunteers.

The mouthwash would be ideally suited to infants cutting their first teeth, he added.

Dennis Mangan, of the US national institutes of health, said the approach showed great promise.