The genus Microsporum comprises fungi classified as dermatophytes, a group that infects the skin, hair, and nails. These organisms utilize keratin, the tough protein forming the structural foundation of these tissues, as their sole source of nutrients and energy. Microsporum species cause common superficial fungal infections in humans and animals, often called “ringworm.” Understanding this group involves examining their biological placement, habitats, physical characteristics, and how they cause disease.
Taxonomic Classification and Ecological Groups
Microsporum belongs to the Kingdom Fungi, Phylum Ascomycota, and Order Onygenales, reflecting its position among keratinophilic fungi. It is part of the Family Arthrodermataceae, defined by its ability to cause superficial infections by degrading keratinized material. This classification places Microsporum alongside other major dermatophyte genera, such as Trichophyton and Epidermophyton.
Dermatophyte species are ecologically defined by their preferred habitat, which directly influences transmission and infection severity in humans. Anthropophilic species are adapted primarily to humans, causing infections that are typically chronic and less inflammatory. Examples include Microsporum audouinii and Microsporum ferrugineum.
Zoophilic species, such as Microsporum canis, are found on animals, particularly cats and dogs, and are the most common source of human infections, especially in children. Transmission from an animal reservoir often provokes a more pronounced inflammatory reaction due to the host’s less-adapted immune response. Geophilic species thrive in soil environments. Human infections are acquired through contact with contaminated soil and often present as highly inflammatory but self-limiting.
Modern molecular studies have revised the Microsporum genus, reclassifying former members, like M. gypseum, into new genera such as Nannizzia. This reclassification is based on genetic analysis rather than morphology alone. However, the ecological terms—anthropophilic, zoophilic, and geophilic—remain crucial for understanding the origin and risk associated with these infections.
Microscopic and Macroscopic Structure
Laboratory identification of Microsporum relies on observing macroscopic growth patterns and microscopic spore structures. When grown on Sabouraud dextrose agar, colonies exhibit textures ranging from powdery or granular to woolly or cottony, such as with M. canis. Surface color varies, presenting as white, cinnamon, or yellowish hues depending on the species.
A distinctive feature is the pigmentation on the reverse side of the colony. This can range from deep yellow or yellow-orange in M. canis to reddish-brown in M. ferrugineum. Colony growth is moderate, typically reaching 3 to 9 centimeters after one week of incubation. These physical appearances guide initial identification and help distinguish Microsporum from other fungal pathogens.
Under the microscope, the most characteristic structures are the large, multicellular asexual spores known as macroconidia. These are spindle-shaped, possessing thick, rough outer walls and multiple internal cross-walls, or septa. For example, M. canis macroconidia can contain between 5 and 15 internal cells and are essential for species identification.
The smaller, single-celled asexual spores, or microconidia, are generally sparse or rare in many Microsporum species. When present, microconidia are small, tear-drop or club-shaped, and are borne singly along the hyphal filaments. The presence and morphology of these distinct macroconidia are the defining microscopic feature separating Microsporum from other dermatophyte genera.
Mechanisms of Infection and Clinical Relevance
Microsporum species cause infection by producing specialized enzymes called keratinases. These proteases act as a virulence factor, breaking down the durable keratin protein found in the outer layer of the skin, hair shafts, and nails. Digesting this protein unlocks the necessary nutrients for fungal survival and proliferation within the host tissues.
This mechanism restricts the infection to non-living, keratinized tissues. The fungus colonizes the outermost layer of the skin and the exterior of the hair shaft in an arrangement called an ectothrix invasion. While the fungi do not typically invade deeper, living tissues in a healthy host, the immune response to fungal byproducts causes the inflammation and scaling associated with the resulting diseases, collectively called dermatophytosis, or tinea.
Clinical manifestations are named by the affected body site. The most significant infection caused by this genus is Tinea capitis, an infection of the scalp and hair. Microsporum canis frequently causes this globally, resulting in circular patches of hair loss and scaling. Severe inflammation can lead to a large, boggy lesion known as a kerion.
Diagnosis often begins with a simple microscopic examination of skin or hair samples treated with potassium hydroxide (KOH), which dissolves keratin to make fungal structures visible. A tool called a Wood’s lamp, which emits long-wave ultraviolet light, is also useful for some species. Hair infected with M. canis and M. audouinii shows a characteristic bright yellow-green fluorescence under this light, providing a rapid diagnostic clue.