Filamentous fungi (molds) grow as multicellular hyphae and are important causes of human disease, particularly in immunocompromised patients. The most clinically significant genera include Aspergillus, the dermatophytes, and the Mucorales.
Aspergillus species are ubiquitous environmental molds of significance in clinical microbiology. Aspergillus fumigatus is the most common cause of invasive aspergillosis, a life-threatening infection in neutropenic patients, transplant recipients, and those receiving high-dose corticosteroids. Aspergillus flavus produces aflatoxins and is a common cause of fungal sinusitis. Aspergillus niger causes otomycosis and is used industrially for citric acid production.
Aspergillus lung disease presents in three forms depending on host immune status. Allergic bronchopulmonary aspergillosis occurs in asthmatics and cystic fibrosis patients. Aspergilloma is a fungal ball that develops in pre-existing lung cavities. Invasive pulmonary aspergillosis involves tissue invasion and angioinvasion, causing infarction, hemorrhage, and dissemination to other organs. Diagnosis combines chest CT showing the halo sign, detection of galactomannan antigen in serum or bronchoalveolar lavage, and culture.
Dermatophytes are molds that infect keratinized tissues including skin, hair, and nails. Trichophyton rubrum is the most common cause of dermatophytosis worldwide. Microsporum canis is a zoophilic species that causes tinea capitis in children. Dermatophytes produce proteases that digest keratin, causing superficial infection limited to the stratum corneum. Laboratory diagnosis involves potassium hydroxide preparation of skin scrapings for direct microscopy and fungal culture on Sabouraud dextrose agar with cycloheximide. Treatment uses topical or systemic antifungal agents such as azoles or terbinafine.
The Mucorales order includes Rhizopus, Mucor, and Lichtheimia species. Mucormycosis is a rapidly progressive, angioinvasive infection occurring in patients with diabetic ketoacidosis, hematological malignancy, or iron overload. Rhinocerebral mucormycosis presents with facial pain, necrosis, and cranial nerve palsies. Diagnosis requires tissue biopsy showing broad, ribbon-like, non-septate hyphae with right-angle branching.
