The researchers, from the University of Iowa, reckon such a product might be available within five years - even though they have only so far tested their theory on mice.

They found that mice who sniffed DNA from bacteria were less likely to develop asthma and other allergies.

Another study - conducted in the UK - has also shown promise for a vaccine using bacteria found in soil.

Different reactions

The Iowa researchers think exposure to the genetic material caused the mice to develop what are know as Th1 immune responses as opposed to Th2 responses - and Th2 responses are thought to trigger allergies.

Bacteria have more CpG DNA

The researchers believe the key ingredient is a type of DNA called CpG DNA, which is made up of substances called cytosine and guanine and is more plentiful in bacteria's genetic make-up than in humans'.

Dr Joel Kline, a lung specialist at the University of Iowa, led the research and believes early exposure to bacterial DNA pushes the body's immune system to favour a Th1 rsponse.

"It makes sense," he told the magazine. "Your body can't wait weeks for a specific antibody to be produced to begin attacking."

Immune response

Dr Kline's team used parasite eggs to sensitise the immune systems of 12 mice - when the animals were exposed to a protein from the eggs later on they developed many symptoms of asthma.

Some of the mice were then given eggs mixed with CpG DNA - when those mice next sniffed the protein their symptoms were far less severe than the others.

An inhaler strategy was then tested by placing a protein that causes allergies mixed with the DNA on the nostrils of the mice.

They also failed to develop asthma-like symptoms.

The research was presented at a workshop on bacterial DNA in Germany and was reported in New Scientist magazine.

Dirty solution

Scientists are also investigating the possibility of using DNA from bacteria in soil as a vaccine for asthma following early results from a British trial.

The drug, called SRL172, produced a 30% reduction in the narrowing of the patients' airways when they were exposed to the allergen, compared to their response before they received the drug.

However, it is not yet known if the effect lasts longer than the six weeks of the trial. Furthermore, the trial involved just 24 adult patients, a random half of whom received placebos.

Professor Stephen Holgate, at Southampton University where the trial was conducted, said that modern life was just too clean in developed countries.

"Infants need to get infections to develop the immune responses which protect them against allergies," he said.

"For example, Eastern European children have far lower rates of allergies than in the UK and have a completely different gut flora."