Zygomycosis

Etiology and morphology

The causative agents of the disease are 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 non-septate mycelium. This means that their branching hyphae lack transverse septa and form a single multinucleated structure.

These microorganisms reproduce in two ways:

• Asexual: sporangiospores are formed inside special rounded receptacles (sporangia) located on sporangiophores.
• Sexual: two cells not differentiated into gametes fuse, resulting in the formation of zygospores.

Various species have their own unique morphological features. For example, the genus Rhizopus is characterized by the presence of rhizoids (specialized branches for attachment to the substrate) and stolons resembling "tendrils" — arching hyphae of the aerial mycelium. The species Mucor mucedo is distinguished by large yellowish-brown sporangia up to 200 µm in size and oval spores. In turn, Rhizopus nigricans has a dark brown mycelium and rough spores, while Absidia corymbifera has smaller sporangia (40–60 µm) containing colorless, smooth, ellipsoid spores.

Epidemiology and routes of transmission

Zygomycetes are widely distributed in the environment. Their natural reservoir includes soil, air, as well as decaying plants, fruits, and food products.

There are three main mechanisms of transmission of infection to humans:

1. Airborne — inhalation of fungal spores with air.
2. Contact — direct entry of the pathogen onto damaged areas of the skin.
3. Alimentary — penetration of spores through the gastrointestinal tract upon consumption of 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 invasion by the fungus.

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 occurs, accompanied by the formation of a pronounced polymorphonuclear infiltrate. The body's defense reaction is manifested by the development of cellular immunity and a delayed-type hypersensitivity (DTH) reaction.

The disease occurs 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.

Diagnostics, treatment, and prevention

Microbiological diagnostics is based on several approaches. Microscopy of smears from affected tissues reveals characteristic wide hyphae of uneven thickness with a complete or near-complete absence of septa. The culture method involves inoculation onto simple nutrient media, such as Sabouraud agar. The fungi are cultured at a temperature of 22–37 °C, resulting in the growth of gray, brown, or brownish-black colonies. Identification is based on microstructural features (presence of the same rhizoids and stolons). Serological tests — ELISA and precipitation reaction — are also used to search for antibodies.

Treatment of zygomycosis is always comprehensive:

• Drug therapy includes polyene antibiotics (amphotericin B) and modern triazoles (posaconazole).
• The surgical stage is mandatory and consists of the radical removal of all areas of necrosis.

Prevention boils down to compliance with general sanitary and hygienic measures. In a hospital setting, strict control of the sterility of medical devices and air cleanliness is critical to prevent nosocomial infection in patients at risk.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed study of morphological differences between the species Rhizopus nigricans and Absidia corymbifera.
• Mechanisms of angioinvasion and the cascade of thrombosis formation in fungal infections.
• The role of the cellular immune response and delayed-type hypersensitivity (DTH) in zygomycoses.
• Growth features and visual characteristics of zygomycete colonies on Sabouraud agar.
• Pharmacological aspects of the use of amphotericin B and posaconazole in mycology.

Other topics in the section "Opportunistic mycoses (candidiasis, aspergillosis, pneumocystosis, etc.)"

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