The Microbiota and Allergies/Asthma

What Is the Evidence Linking Changes in the Microbiota to the Development of Allergic Disease?Two lines of evidence suggest that environmental changes are a major factor in the development of allergies: the increase in the incidence of allergic diseases over the past 20-30 years and the dichotomy in the rate of allergic disease between industrialized and developing countries.These observations have led researchers to propose the ''hygiene hypothesis'' for allergies and asthma.Simply stated, a lack of early microbial stimulation results in aberrant immune responses to innocuous antigens later in life [1].However, an alternative interpretation of the evidence supporting the hygiene hypothesis forms the foundation of the ''microbiota (microflora) hypothesis'' [2,3].The concept is that significant perturbations in gastrointestinal (GI) microbiota composition in westernized areas (due to antibiotic use, dietary changes, and other lifestyle differences) have disrupted the mechanisms of mucosal immunologic tolerance.Epidemiologic and clinical data supporting this interpretation include 1) a positive correlation between increasing risk for asthma/allergies and increasing use antibiotics in industrialized countries, 2) correlations between altered fecal microbiota composition and atopic disease, and 3) successful prevention/reduction of allergies in some individuals by oral probiotics or dietary changes.Experimental data in mice include the observations that 1) germ-free animals display numerous defects in immune response generation, 2) antibiotic treatment can augment Th2 type immune responses, and 3) probiotics can diminish airway allergic responses.Altogether, these experimental, epidemiologic, and clinical observations support the hypothesis that changes in the indigenous microbiota can be a predisposing factor for allergic disease.