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).