Both protective and adverse effects of indoor microbial exposure on asthma have been reported, but mostly in children. To date, no study in adults has used non-targeted methods for detection of indoor bacteria followed by quantitative confirmation.

A cross-sectional study of 198 asthmatic and 199 controls was conducted within the European Community Respiratory Health Survey (ECRHS) II. DNA was extracted from mattress dust for bacterial analysis using denaturing gradient gel electrophoresis (DGGE). Selected bands were sequenced and associations with asthma confirmed with four quantitative PCR (qPCR) assays.

15 out of 37 bands detected with DGGE, which had at least a suggestive association (p<0.25) with asthma, were sequenced. Of the four targeted qPCRs, Clostridium cluster XI confirmed the protective association with asthma. The association was dose dependent (aOR 0.43 (95% CI 0.22–0.84) for the fourth versus first quartile, p for trend 0.009) and independent of other microbial markers. Few significant associations were observed for the three other qPCRs used.

In this large international study, the level of Clostridium cluster XI was independently associated with a lower risk of prevalent asthma. Results suggest the importance of environmental bacteria also in adult asthma, but need to be confirmed in future studies.