Dr William Cookson from Imperial College London and colleagues from across Europe conducted two studies looking at children living in rural and suburban areas. Previous observational research has suggested that exposure to micro-organisms is linked to a lower risk of developing asthma.
Studies have measured levels of compounds produced by microbes as a guide to exposure to micro-organisms.
To produce a more accurate measure, Dr Cookson and his colleagues used DNA analysis to assess children's exposure to particular micro-organisms.
In both studies, children who lived on farms had a lower prevalence of asthma and were exposed to a greater variety of environmental micro-organisms than other children.
The researchers commented: 'These data support the idea that the greater diversity of microbial exposure among children who live on farms is associated with protection from the development of asthma.'
Dr Cookson and his colleagues also found that the diversity of microbial exposure, and exposure to particular bacteria or fungi, was related to a lower risk of developing asthma.
There was a 63 per cent reduction in the odds of developing asthma among children exposed to fungi in the genus eurotium, which are similar to certain aspergillus species.
In addition, children exposed to a variety of bacteria, including Listeria, bacillus and corynebacteria had 43 per cent lower odds of developing asthma.
Although the researchers' findings allowed them to identify these groups of microbes, they could not identify any individual species to which exposure may convey protection.
They said: 'The challenge will be to identify these species with the precision needed to allow specific tests of the relationship between microbial exposure and protection against asthma.'