Microbial exposure of rural school children, as assessed by levels of N-acetyl-muramic acid in mattress dust, and its association with respiratory health

Abstract

Background Endotoxin exposure has been shown to be associated with a decreased prevalence of atopic sensitization and symptoms. Yet endotoxin represents only a part of the indoor microbial exposure. Muramic acid, a constituent of peptidoglycan, is present in gram-negative and gram-positive bacteria in the environment and may therefore serve as an additional marker of microbial exposure. Objective To study the factors determining the level of indoor exposure to muramic acid/peptidoglycan, as well as its potential association with respiratory health. Methods In 553 farm and nonfarm school children from Austria, Switzerland, and Germany, mattress dust muramic acid concentrations were determined, and health was assessed by using IgE measurements and questionnaire information. Results The muramic acid concentration was found to be significantly higher in dust from farm children's mattresses than in dust from nonfarm children's mattresses (157 vs 131 ng/mg). Children with higher mattress dust muramic acid concentrations had a significantly lower prevalence of wheezing (odds ratio of highest vs lowest tertile of muramic acid concentration, 0.3; 95% CI, 0.1-0.9), regardless of farming status and endotoxin exposure. The association for asthma was similar, and no association was found with atopic sensitization. Conclusion Next to endotoxin, muramic acid provides us with an independent marker of microbial exposure. Unlike endotoxin, muramic acid was inversely associated with wheezing rather than with atopic sensitization.