**2.2.2 Fungi**

*Malassezia* species are fungal species that usually exist as yeasts in the healthy human skin. The major fungal members of human skin are *M. restricta* and *M. globosa* (Sugita et al., 2004). These fungi are aerobic and live only within the superficial portion of the hair follicle and the surface of the stratum corneum. The yeast forms are believed to be harmless to humans, but can change their shapes to form filaments if an unknown shift in the host-parasite relationship occurs. This shape change is associated with a disease known as tinea versicolor, a common infectious disease of the skin.

In addition, *Candida* species are detected in the skin less frequently. *Candida* species are wellknown members of microbiota of the gut, but also colonise on the skin.

#### **2.2.3 Viruses**

It remains controversial whether viruses exist on the skin as members of the microbiota. The classical idea is that viruses do not exist on the healthy human skin. However, in a small percentage of healthy humans, there exists a phage PA6, which infects *P. acnes* cells. It is classified as a member of *Siphoviridae* family (Farrar et al., 2007). It is thought that the presence of this virus may explain why the resident bacteria cannot be cultured from certain skin samples.

In addition, there are some conditions that viruses lurk below the skin. Herpes simplex virus (HSV), which causes herpes simplex, becomes latent deep within the sensory ganglions after the first infection. Nearly 100% of adult humans are believed to have this virus somewhere in the ganglions. Certain healthy adults develop recurrent herpes simplex around the mouth or the genitalia, when exhausted. Under such conditions, HSV invades the skin from the ganglions via sensory nerves. Varicella-zoster virus (VZV) first invades humans from the throat when varicella, a febrile condition with small pustules also called chickenpox, appears. After relief from this condition, VZV remains in some of the sensory ganglions in 70% of the population, and after decades, it may appear via sensory nerves as herpes zoster. Both are categorised as members of the *Herpesviridae* (herpesvirus) family.

#### **2.2.4 Arthropods**

Surprisingly, mites also live within the hair follicle structures. *Demodex folliculorum* and *Demodex brevis* are such arthropods. Nearly 100% of the humans have these mites as a part of their microbiota. *D. folliculorum*, which has a long body, is believed to live in the follicles. *D. brevis* has a shorter body and lives in the sebaceous glands. These mites usually live in some but not all of the follicles of the facial skin without causing any harm. However, in some cases, the mites cause severe acne (demodex folliculitis), rosacea (rosacea-like demodicidosis [demodicosis]), or perioral dermatitis (Burns et al., 2010).

**Known species Unknown (uncultured) species** 

Table 1. Example of novel bacterial species identified in facial skin by molecular methods

*Malassezia* species are fungal species that usually exist as yeasts in the healthy human skin. The major fungal members of human skin are *M. restricta* and *M. globosa* (Sugita et al., 2004). These fungi are aerobic and live only within the superficial portion of the hair follicle and the surface of the stratum corneum. The yeast forms are believed to be harmless to humans, but can change their shapes to form filaments if an unknown shift in the host-parasite relationship occurs. This shape change is associated with a disease known as tinea

In addition, *Candida* species are detected in the skin less frequently. *Candida* species are well-

It remains controversial whether viruses exist on the skin as members of the microbiota. The classical idea is that viruses do not exist on the healthy human skin. However, in a small percentage of healthy humans, there exists a phage PA6, which infects *P. acnes* cells. It is classified as a member of *Siphoviridae* family (Farrar et al., 2007). It is thought that the presence of this virus may explain why the resident bacteria cannot be cultured from

In addition, there are some conditions that viruses lurk below the skin. Herpes simplex virus (HSV), which causes herpes simplex, becomes latent deep within the sensory ganglions after the first infection. Nearly 100% of adult humans are believed to have this virus somewhere in the ganglions. Certain healthy adults develop recurrent herpes simplex around the mouth or the genitalia, when exhausted. Under such conditions, HSV invades the skin from the ganglions via sensory nerves. Varicella-zoster virus (VZV) first invades humans from the throat when varicella, a febrile condition with small pustules also called chickenpox, appears. After relief from this condition, VZV remains in some of the sensory ganglions in 70% of the population, and after decades, it may appear via sensory nerves as herpes zoster. Both are categorised as members of the *Herpesviridae* (herpesvirus) family.

Surprisingly, mites also live within the hair follicle structures. *Demodex folliculorum* and *Demodex brevis* are such arthropods. Nearly 100% of the humans have these mites as a part of their microbiota. *D. folliculorum*, which has a long body, is believed to live in the follicles. *D. brevis* has a shorter body and lives in the sebaceous glands. These mites usually live in some but not all of the follicles of the facial skin without causing any harm. However, in some cases, the mites cause severe acne (demodex folliculitis), rosacea (rosacea-like

demodicidosis [demodicosis]), or perioral dermatitis (Burns et al., 2010).

Species close to *Methylophilus methylotrophus* 

Species close to *Aquaspirillum autotrophicum*

Species close to *Ideonella dechloratans* Species close to *Ammoniphilus oxalaticus* Species close to *Anabaena cylindrica*

*Stenotrophomonas maltophilia Acidovorax temperans Corynebacterium* species

versicolor, a common infectious disease of the skin.

known members of microbiota of the gut, but also colonise on the skin.

*Dietzia maris Bacillus* species

(Dekio et al., 2005)

**2.2.2 Fungi** 

**2.2.3 Viruses** 

certain skin samples.

**2.2.4 Arthropods** 
