Etiology and Morphology
The causative agents include representatives of the genera Mucor, Rhizopus, Absidia, as well as Rhizomucor, Basidiobolus, Conidiobolus, Cunninghamella, and Saksenaea. All of them belong to lower fungi.
A fundamental feature of zygomycetes is the aseptate mycelium. This means their branching hyphae lack transverse partitions and form a single multinucleated structure.
Reproduction of these microorganisms occurs via two pathways:
- Asexual: sporangiospores form inside specialized spherical receptacles (sporangia) located on sporangiophores.
- Sexual: two undifferentiated cells fuse to form zygospores.
Different species possess unique morphological traits. For instance, the genus Rhizopus is characterized by rhizoids (specialized root-like branches for anchoring to the substrate) and stolons resembling arched hyphae of aerial mycelium. The species Mucor mucedo features large, yellow-brown sporangia up to 200 µm in size and oval spores. In turn, Rhizopus nigricans has dark brown mycelium and rough spores, whereas Absidia corymbifera has smaller sporangia (40–60 µm) containing colorless, smooth, ellipsoid spores.
Epidemiology and Transmission Routes
Zygomycetes are widespread in the environment. Their natural reservoirs include soil, air, decaying plants, fruits, and food products.
There are three main mechanisms of infection transmission to humans:
- Aerogenic — inhalation of fungal spores with air.
- Contact — direct entry of the pathogen onto damaged skin areas.
- Alimentary — penetration of spores through the gastrointestinal tract via contaminated food.
Pathogenesis and Clinical Forms
The most important pathogenicity factors of zygomycetes are aggressive enzymes — lipases and proteases. They destroy host tissues, facilitating deep fungal invasion.
A key link in pathogenesis is angioinvasion. The pathogen penetrates blood vessels, which inevitably leads to thrombosis. Due to impaired blood supply, ischemic tissue necrosis develops, accompanied by the formation of a pronounced polymorphonuclear infiltrate. The body's defense response is manifested by cell-mediated immunity and a delayed-type hypersensitivity (DTH) reaction.
The disease presents in several clinical forms:
- Invasive pulmonary.
- Gastrointestinal.
- Cutaneous.
- Rhinocerebral zygomycosis (characterized by an extremely severe, fulminant course).
- Disseminated form involving brain structures and other internal organs.
Diagnosis, Treatment, and Prevention
Microbiological diagnosis is based on several approaches. Microscopy of smears from affected tissues reveals characteristic wide hyphae of uneven thickness with a near-total absence of septa. The culture method involves inoculation onto simple nutrient media, such as Sabouraud agar. Fungi are cultured at 22–37 °C, yielding gray, brown, or black-gray colonies. Identification is based on microstructural features (such as rhizoids and stolons). Serological tests like ELISA and precipitation reactions are also used to detect antibodies.
Treatment of zygomycosis is always multimodal:
- Pharmacotherapy includes polyene antibiotics (amphotericin B) and modern triazoles (posaconazole).
- The surgical phase is mandatory and consists of radical excision of all necrotic areas.
Prevention relies on general sanitary and hygienic measures. In a hospital setting, strict control of medical device sterility and air purity is critical to prevent nosocomial infection in high-risk patients.