III.2. Internal exposome
Animal and plant microbiota contain a fungal component, the mycobiota; conversely, fungi possess their own microbiota (57). Crosstalk between fungal and bacterial components of the microbiota and between each of them and the host are essential for sustained commensalism (24,57). The identification of fungal species associated with human mucosae and skin needs to be complemented by demonstrating their transient or resident status, the latter allowing recognition as genuine members of the mycobiota (24). The most prevalent fungal genera in the healthy gut areSaccharomyces , Malassezia , Candida , andCyberlindnera (24). In fact, the question of a gut mycobiota, defined as persistent commensal fungal species detected in stools but not in oral or food samples, is still open. Indeed, all gut fungal species were found to be transient in experiments performed with healthy Western adults, raising the hypothesis that, at least in this population, fungal colonization might be lacking (58). Strikingly, frequent fungal taxa associated with oral, pulmonary, intestinal, or cutaneous locations, such as Aspergillus, Cladosporium, Alternaria , or Penicillium (24) overlap with environmental counterparts described in the previous section. On the other hand, even if fungi do not colonize the healthy human host, their ubiquitous presence results in sustained contact and, therefore, the need for an adaptive immune response, often a Th17-oriented one (59). A special case could be represented by breast milk mycobiota, which comprisesMalassezia, Penicillium, Davidiella, and Sistotremagenera, possibly explaining the abundance of Malassezia in the neonatal and young infant gut mycobiota (24). It was suggested that the establishment of gut mycobiota could begin prior to birth, that fungal species in infant gut exhibit high variability during the first year of life, and that a higher abundance of gut fungi in infants was predictive of later development of allergic diseases (31,60).
Among fungi associated with human skin, Malassezia , mainlyM. sympodialis , is probably the most important in terms of relationship to allergy and atopy especially in the development and progression of atopic dermatitis, explained by cross-reactivity between conserved eukaryotic proteins, such as thioredoxins (e.g. Mala s 13), manganese superoxide dismutase (Mala s 11) and cyclophilin (Mala s 6) being potential panallergens, found in fungi and humans (10,24).