**2. The microbiota (microflora) and atopic dermatitis**

Human skin harbours many bacterial and fungal species, which are not apparently harmful. The total population of such residential microorganisms is known as the 'microbiota' (microflora). The microbiota is well balanced with the effect of natural human immunity, and the microorganisms have mechanisms to balance their population as a part of this community. Thus, it is appropriate to consider the microbiota as a 'microbial community'. It is generally accepted that the microbiota become stabilised at different points of balance in AD patients. This altered balance relates to the pathogenesis of AD as will be discussed below.

#### **2.1 Ecology of skin microbiota**

The microbiota of human skin differs largely according to skin sites. This is probably due to the fact that the structure and physiology of the surface of the skin differs at different sites of the body. This leads to the natural selection of certain species for particular sites. Thus, facial skin may be considered as a 'swamp' of sebum, soles could be considered as a 'pond' of sweat, axillae could be analogous to a 'rain forest', and forearms are 'deserts' which generally lack water and lipid.

As a reflection of this, for example, healthy facial skin harbours about 108 microbial cells per square centimetre, which comprises up to 2 g of microorganisms per face. On the other hand, healthy forearm skin harbours only 103 microbes per square centimetre. Therefore, in some body sites, the population is large enough to consider that the microbiota has a certain

Microorganisms and Atopic Dermatitis 109

Fig. 1. Distribution of well-known skin inhabitants. (A) Typical hair follicle of the face. (B) Typical hair follicle of the trunk. SC, stratum corneum (a part of epidermis); E, viable portion of epidermis; D, dermis; H, hair; HF, hair follicle; SG, sebaceous gland; SwD, sweat duct; SwG, sweat gland. Note that the sweat duct and the sweat gland are

After this report, additional reports employing similar methods were published and a more complete picture of skin microbiota is emerging (Bek-Thomsen et al., 2008, Costello et al.,

considered to be sterile.

2009, Grice et al., 2009).

physiological effects on the microecology of the skin. The scalp, face, neck, axilla, external genitalia, groin, and soles are examples of such sites. On the other hand, the microbiota seems to have little effect on the skin physiology at other sites with smaller populations, such as arms, hands, and legs.

#### **2.2 Members of the normal skin microbiota**

Because the skin microbiota differs considerably across different sites of the human body, it is not possible to describe the microbiota of the entire body in a single entity. In general terms, however, the major population of the normal microbiota consists of coagulasenegative *Staphylococcus* species, *Propionibacterium acnes*, and *Malassezia* species. Coagulasenegative *Staphylococcus* species are aerobic bacteria, *P. acnes* is a facultative anaerobic bacterium, and *Malassezia* species are yeasts (a single-cell form of fungus). All of these microorganisms live on the surface of the skin and in the hair follicles (Fig. 1). No other human organ has such a unique composition of microbiota. These three groups of microorganisms are retained in a balance between human immunity and each other.
