The relationship between advances in understanding the microbiome and the maturing hygiene hypothesis.

Expanding knowledge about an interaction of the bacterial colonization with pathogenic and non-pathogenic bacteria and the human immune system leads to speculation on potential effects on health and disease. Recent advances in sequencing technologies and new bioinformatic possibilities now allow investigating the microbes that colonize the human gut, skin and airways in more detail. In light of the hygiene hypothesis, the impact of the microbial composition of individuals with allergic sensitization and/or atopic diseases, i.e., allergic asthma or atopic eczema, were investigated in several clinical trials. Altered diversity of gut microbiota during infancy as well as colonization with specific pathogenic and apathogenic bacteria has been linked with an elevated risk for allergy. There are ongoing attempts to establish intervention strategies aimed at modifying initial colonization patterns in early life. While results from animal models, in-vitro data and epidemiological studies encourage the concept of a relationship between the microbiome and the development of allergic diseases, the transfer of these findings to intervention strategies still seems to be a major challenge.