Fungi, which cause skin, hair and nail infections in humans and animals, are called dermatophytes. Dermatophytes are found among three fungal genera, namely Epidermophyton, Microsporum, Nannizzia (was earlier called Microsporum) and Trichophyton. The dermatophytes get their nourishment by digesting keratin. Keratin is a group of fibrous structural proteins, which occurs in vertebrates and builds up, among other things hair, feathers, nails, claws, hooves and the outer skin layer (epidermis). Dermatophytes usually colonize only the outermost dead layer of the epidermis, because they cannot penetrate the living tissue of immune competent individuals. Dermatophytes have proteases, elastases (serine proteases) and keratinases, which are virulence factors and the degradation products are nutrients for the fungi.
These fungi are usually divided into zoophilic, anthropophilic and geophilic dermatophytes according to their normal habitat.
- Zoophilic dermatophytes are found mainly in animals, but can also occur in humans, who come into contact with infected animals.
- Anthropophilic dermatophytes are found mainly in humans.
- Geophilic dermatophytes can often be isolated from soil, but can infect animals and humans.