Bacteria that cause gum disease and tooth decay swap genetic material that helps them build up drug resistance while they breed in the mouth, scientists have discovered.

A team led by Dr Peter Mullany, of the Eastman Dental Institute, London, has found that bacteria in the mouth and gut have identical antibiotic resistance genes.

This may prolong the useful lifetime of antibiotics

Dr Peter Mullany

They also found that many of these genes are found on short, mobile pieces of DNA called transposons which theoretically can be transferred between different types of bacteria.

In order to test the theory, Dr Mullany's group produced artificial dental plaque in the laboratory, using bacteria from a sample of human saliva.

They added a donor bacterium, Streptococcus salivarius, to the plaque, which contained a transposon with a gene for resistance to the antibiotic tetracylcine.

After a while other Streptococcal species in the plaque became resistant to tetracycline.

Dr Mullany said: "We also found that when a soil bacterium was used as a donor it could pass on its transposon, even though it couldn't grow in plaque.

"In the long term, by understanding how antibiotic resistance genes are transferred, we can work out how to stop or slow down gene transfer.

"This may prolong the useful lifetime of antibiotics."

The research was presented at a meeting of the Society for General Microbiology at Loughborough University.

Natural defences

The conference also heard that good bacteria growing in dental plaque could help fight off bugs that cause gum disease and tooth decay if they are given a competitive edge.

Under normal circumstances, the plaque that grows on teeth contains a mix of good and harmful bacteria.

However, harmful bacteria tend to thrive in the acidic environment that is created when we eat sugary foods to the detriment of beneficial bacteria, which increases the risk of tooth decay.

Similar disruptions in the delicate bacterial balance can lead to gum disease.

Antibiotics are available to treat gum disease, but they work by wiping out all bacteria, good and bad.

This gives good bacteria no opportunity to fight back, which they can do if the environment in the mouth is less made less acidic.

Professor Philip Marsh, of the Centre for Applied Microbiology and Research, Salisbury, said it should be possible to control the environment in the mouth more effectively to stop the good bacteria being killed off, and to modify the conditions that favour the growth of the harmful organisms.

"Bacteria involved in gum disease feed on host proteins and cannot grow in air.

"They could be controlled using anti-inflammatory drugs (to reduce their food supply) or redox agents (to make the gum environment more oxygen-rich).

"And by using alternative sweeteners, or stimulating saliva production we can reduce the growth of bacteria that cause tooth decay."

"We think that these and other novel approaches may be developed to control or prevent dental diseases."

A British Dental Health Foundation spokesperson said, "The research sounds very promising, although it is likely to be a considerable time before the public can gain the benefits.

"But gum disease is the most common disease in the world, and poor oral health is generally recognised as an indicator of social deprivation.

"So potentially it could have a significant impact."