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General Information
Piedraia hortae is a superficial fungus that exists in the soils of tropical and subtropical environments and affects both sexes of all ages. The fungus grows very slowly forming dark hyphae, which contain chlamydoconidia cells and black colonies when grown on agar. Piedraia hortae is a dermatophyte and causes a superficial fungal infection known as black piedra, which causes the formation of black nodules on the hair shaft and leads to progressive weakening of the hair. The infection usually infects hairs on the scalp and beard, but other varieties tend to grown on pubic hairs. The infection is usually treated with cutting or shaving of the hair and followed by the application of anti-fungal and topical agents. The fungus is surprisingly used for cosmetic purposes to darken hair in some societies as a symbol of attractiveness.

Appearance
Microscopic Appearance

Piedraia hortae is a slow growing, dark and small fungus that is covered with short dark hyphae containing chlamydoconidia cells. The ascostroma are irregular in shape and are black in colour, each of these structures contains one ascus, which hold eight ascospores. The ascospores, which are dark, curved and become very narrow at the ends to form whip like structures and they are only produced during the sexual phase of parasitic growth. The infected hairs also have black nodules, which are attached to the hair shaft and are known to cause weakness of the hair. When the infected hairs are removed and added to a KOH solution the hair gives off a fluorescent glow under UV light, but the the fungus does not. If the piedra does start to glow it indicates that there is bacteria on the hair and fungus.

Colonial Appearance

When grown on agar at 25°C Piedraia hortae grows very slowly to form black-greenish, limited and pointed colonies. Piedraia hortae taken from infected hairs have dark brown nodules ,which are made up of ascostroma. The nodules have a gritty feel, organized in a stromatic fashion and have a high concentration of chitin and melanoid pigments. The colonies produce a red pigment and remain smooth and covered with short aerial hyphae.

Genetic identification

By using PCR techniques with SYBR green dye, thermal cycling and fungal primers are all involved in fungal identification process. The process involves marking the fungus with the green dye and using thermal cycling to denature the double stranded DNA and anneal the primers to these strands. The primers that are involved are the ITS1 and ITS4 primers and they are used to identify the ITS region. The amplified product is purified and is sent through further sequencing and are finally analyzed using a gene database. The PCR process can also be performed without the instrument and can be sequenced by using additional primers. The application of these techniques allows for the identification of fungi such as Piedraia hortae and other fungal organisms.

Pathology
Piedraia hortae are commonly classified by the formation of nodules on the hair shaft, which is commonly known as black piedra. Black piedra is usually seen in tropical regions and it usually targets humans  of all ages and targets the scalp, moustache and occasionally pubic hair. The source of the infection is usually in soils, poor hygiene, long hair, cultural use of veils and the application of plant oils to wet hair favours the growth of the infection. Black piedra is a superficial fungal infection, which means that it is restricted to the stratum corneum and causes no inflammation. The infection of the hair shaft results in the formation of nodules on the scalp, moustache and pubic hair. The nodules are hard and gritty, which produce a metallic sound when the hair is combed.The nodules colonize the hair shaft,which causes progressive weakness of the hair and leads to breakage of the hair in severe cases, which can lead to hair loss and baldness. The fungus also has the potential to destroy the cuticular layers of the hair and move into the cortex. Piedraia hortae survives in the scalp is due to the slow rate of the keratin degradation near the cortex and the compact formation of the nodules and the hyphae are tightly packed in black piedra cases. The initial invasion of human hair by piedraia hortae is achieved by using an eroding hyphae, which force their way beneath or between the cuticular layer. The pressure that is applied between or beneath the hair cuticle is produced by the growth of the fungus itself. The breakdown of keratin is mainly due to enzymatic processes and can be identified by the presence of many fungal mitochondria. The breakdown of keratin begins with the cementing material, cuticular layer and the plasmalemma. As the fungus starts to break down the keratin, it works its way to the cortex and more enzymatic breakdown occurs. In the cortex two types of disruption patterns are produced, which are either parallel or vertical to the axis hair. The parallel pattern is produced by by fungal cells that grow by separating the external layers of the outer cortex. The vertical pattern is produced by hyphae that penetrate, which creates channels that increase in size as the cortex is broken down.

Treatment

The infection cannot be removed by a simple touch and is treated first by avoiding moisture. The next steps are more common, like cutting or shaving the hair, but other chemical agents can be used as well. For women some individuals use a fine comb to remove as much of the infection as possible and then they cut or shave their hair. This is then followed by the application of a sublimate solution in 60% alcohol solution on to the scalp. Some treatments also involve cologne, which uses mercury bichloride and is proportioned accordingly to the amount of alcohol that is used. The application of antifungal shampoos such as pyrithione zinc, formaldehyde and salicylic acid is also very effective against black piedra. The oral application of itraconazole and the newly discovered use of terbinafine causes the nodules to become defective and the fungus to breakdown. After hair is removed and topical agents are applied recurrences are very rare, but spontaneous remission might occur.

Cosmetic uses
Black piedra is sometimes cultivated for cosmetic purposes due to social factors that favour a specific hair colour, which makes them more attractive in their society. Several Indian tribes located from Panama have been known to use several methods in order to darken the hair of albino individuals within their community. One of these methods is the cultivation of black piedra for an extensive period of time in the individuals hair. In Malaysia the nodules of black piedra are very attractive and women are encouraged to sleep with their hair buried in the soil to encourage growth of the black nodules.

Other Varieties
The genus Piedraia contains another species known as Piedraia quintanilhae, which is more common in chimpanzees than humans. It differs from Piedraia hortae  in terms of the ascospores do not have any attachments. Another species known as Trichosporon biegelii is commonly known to cause white piedra. White piedra is more common in temperate and semitropical climates, such as South America, Asia, Europe, Japan, and parts of the southern United States. Black piedra usually affects scalp hair, whereas white piedra more commonly targets pubic hair, axillary hair, beards, moustaches, and eyelashes. White piedra affects horses and monkeys, in addition to humans and the nodules are white and brown in colour and can be easily detached from the hair shaft. White piedra is treated by using topical and antifungal agents, but a more effective approach is to use itraconazole therapy. Recent studies have shown that a species of fungus called xanthoriicola physciae has been proven to be part of the piedraia family. This was decided based on microscopic observation of the fungi and sequencing of the xanthoriicola genome.