It has been well established that allergic sensitization and the development of allergic disease are associated, at least in some infants, with characteristic developmental patterns in fecal microbiota composition that are atypical to healthy infants. With relative consistency these characteristics include low numbers of bifidobacteria and anaerobes in total and high numbers of clostridia, S. aureus and certain coliforms such as Klebsiellae. Data on lactobacilli, Bacteroides and E. coli are somewhat variable. How this aberrancy in fecal microbiota depicts the situation in the intestine and how it is clinically significant, remains to be known. The possibility that the characteristics are secondary to the disease cannot be excluded, but it is also feasible that they reflect their significance in the aetiology of allergy. Extensive experimental data implies that the development of atopic type immunoreactivity could be promoted by the establishment of an early gut microbiota that (1) is incapable of directing the immune system towards tolerogenic responses to, what should be, harmless environmental antigens and/or (2) induces inflammatory responses against itself, thereby increasing mucosal permeability to potential allergens.

It has been convincingly demonstrated that microbial exposure is likely to be the primary exogenous stimulus directing the immunological maturation away from allergic type immunoresponsiveness early in life. However, it is still not clear what are the qualitative or quantitative characteristics of the indigenous microbiota or other sources of microbial exposure that could protect from, or conversely promote ("allow"), the expression of allergies. Future studies should assess whether specific microbial species have particular importance in this respect or whether the "adequate" stimulus is only a matter of quantitatively high enough exposure or strongly variable exposure. More efforts should be directed to characterizing microbial composition of nasal and oral cavities and different compartments in the intestinal tract of children as well as the gut of pregnant women and the gut and breast-milk of breast-feeding mothers.

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