Comprehensive Notes on General and Veterinary Mycology

General Information and Academic Requirements

The course on General and Veterinary Mycology (Micologia Generale e Veterinaria) utilizes educational materials provided through slides and mini-handouts available on the Moodle platform. The evaluation process for students consists of a written exam, with the possibility of an exemption test (esonero). For students seeking more in-depth study, the recommended textbook is "Microbiologia e immunologia veterinaria" authored by G. Poli.

Principal Characteristics of the Kingdom Fungi

Fungi constitute a distinct kingdom of life characterized by a specific set of biological traits. They are eukaryotic organisms that can be either unicellular or organized into complex multicellular structures. One of their primary defining features is that they are immobile. Fungi exhibit both mitosis and meiosis during their life cycles. From a metabolic standpoint, they are heterotrophs, meaning they must absorb dissolved organic substances or minerals rather than producing their own food via photosynthesis. They can be aerobic or facultative anaerobes.

Fungi possess a distinct cell wall and are notably insensitive to antibiotics, which target prokaryotic structures. Reproduction primarily occurs through the production of spores. Ecologically, there are hundreds of thousands of fungal species, many of which are identified through modern molecular DNA analysis. The majority of these species are saprophytic or commensal and are ubiquitous in the environment without being harmful. Examples include common filamentous molds and food-altering organisms. However, a minority of species are pathogenic, affecting animals, plants, arthropods, and humans. Some of these pathogenic species produce toxins that can contaminate food supplies, leading to health issues for the final consumer.

Fungi also provide significant positive effects, particularly in the food industry. They are utilized in the preparation of fermented products such as blue cheeses, where fungi like those in the genus Penicillium are introduced to grant specific organoleptic characteristics (e.g., Gorgonzola). Furthermore, they are essential in the production of beer and bread.

The Evolution of Fungal Infections in Humans and Animals

Currently, approximately 600 species of fungi are documented as causes of infection in humans and animals, though only about 100 of these are involved on a regular basis. The number of pathogenic fungal species is steadily increasing due to three primary factors. The first is the emergence of new pathogenic species resulting from selective pressures. A notable example is Candida auris, a yeast first isolated in the 2000s from a human patient with otitis. It has since been identified as a cause of severe nosocomial septicemia. Its emergence occurred suddenly and simultaneously across all continents, often in hypersaline or high-temperature environments, leading to hypotheses that its rise is linked to climate change. Its pathogenic potential in animals is currently unknown.

The second factor is the increase in susceptible patients. This is largely due to immunosuppression of various origins, especially in human medicine. Advanced medical treatments for chronic diseases, organ transplants, and conditions like AIDS have increased life expectancy, but these patients remain highly susceptible to fungi that were previously considered minimally pathogenic. The third factor is the improved diagnostic and discriminatory capacity of modern medicine. Advanced molecular techniques now allow clinicians to identify specific species as etiological agents that were historically misclassified or grouped under broader categories. Additionally, travel and climate change have facilitated the appearance of fungal pathologies in zones traditionally considered free of such diseases, as "exotic" pathogens move into Europe and other regions.

The Anatomy and Structural Composition of the Fungal Cell

The fungal cell is defined by several key structures. The nucleus house the genetic material. Surround the cytoplasm is the cell membrane, which is phospholipid-based and regulates permeability for the diffusion of solutes and the active and passive transport of substances. A critical component of the fungal membrane is ergostercl, a sterol that serves as the primary target for many antifungal drugs which inhibit its synthesis.

Encasing the membrane is a rigid cell wall. This structure provides shape and structural stability, facilitates the exchange of metabolic substances with the environment, and is responsible for adhesiveness and antigenic power. The wall contains abundant polysaccharide polymers, including chitin, $\beta$-glucans, and mannans. These are often complexed with mannoproteins. These surface antigens are significant for diagnosis, as specialized tests can detect them. In certain yeasts, an external capsule is present. This capsule is a vital virulence factor because it inhibits phagocytosis. It is also a diagnostic marker, as tests can identify capsular antigens in the blood of infected animals, and the capsule can be visualized using specific cytological stains.

Morphological Classification: Filamentous Fungi and Yeasts

The vegetative body of a fungus is known as the thallus. Based on the morphology of the thallus, fungi are categorized into two main types: filamentous fungi (molds) and yeasts. Filamentous fungi or molds produce colonies that appear cottony or granular. They originate from a single reproductive cell, such as a spora or a conidium, which generates a germ tube (tubo germinativo). Through apical growth and centrifugal expansion, this tube develops into long, hollow filamentous structures called hyphae (ife). These hyphae represent incomplete introflections of the cell wall and membrane.

Hyphae are often septate (settati), meaning they are divided by walls, and in pathogenic species, these are the elements that invade tissues. Conversely, non-septate or sparsely septate filaments are referred to as siphons or cenocytic hyphae (ife cenocitiche). The dense network of hyphae is called the mycelium. In a growth medium or the environment, fungi exhibit a vegetative mycelium that anchors the fungus and an aerial (reproductive) mycelium. The aerial portion contains reproductive forms like spores or conidia, a location that facilitates environmental dispersal. Macroscopically, mold colonies vary in pigmentation and texture, which may be woolly, cottony, velvety, or granular. While usually microscopic in tissue lesions, they form visible colonies in laboratory cultures. Identification often requires advanced tools such as PCR or MALDI-TOF mass spectrometry.

Yeasts are unicellular fungi that produce smooth, glabrous colonies with a creamy consistency, appearing similar to bacterial cultures in vitro. They typically reproduce by budding (gemmazione), where mitotic division creates daughter cells called blastoconidia. Examples include Cryptococcus neoformans and Malassezia pachydermatis.

Fungal Dimorphism and Reproduction

Some fungi exhibit dimorphism, a trait often dependent on temperature. These dimorphic fungi exist in different forms depending on their environment: in human or animal tissues (or in cultures at $37-38\,^{\circ}\text{C}$), they take on a yeast-like form. In the environment or in cultures at $25\,^{\circ}\text{C}$, they appear in a filamentous form with volatile spores. These are often primary pathogens causing severe deep or disseminated infections. Examples include Coccidioidomycosis (caused by Coccidioides spp.) and Histoplasmosis (caused by Histoplasma capsulatum). While rare in Europe, climate change may increase their prevalence.

Fungal reproduction is classified into two types. Sexual reproduction results in the teleomorph or "perfect" stage. This involves the fusion of two haploid cells and produces true spores. Taxonomic classification is often based on these structures (e.g., Ascomycetes are named for their ascopores). Many fungi are heterothallic, requiring two sexually compatible individuals (mating types $+$ and $-$) to reproduce. These mating types are identified via genomic analysis or contact testing. Sexual reproduction is rarely seen in clinical primary cultures because it requires specific controlled conditions (pH, temperature) and the presence of both mating types.

Asexual reproduction, or conidiogenesis, is the most common form observed in nature and laboratory settings. It results in the anamorph stage and produces conidia. This form is essential for initial diagnostic identification. Conidiation can be thallic, where an existing hypha fragments into conidia (e.g., arthroconidia in dermatophytes like Trichophyton verrucosum), or blastic, involving the ex novo production of cells from specialized hyphae called conidiophores (e.g., Aspergillus fumigatus) or through budding in yeasts. Chlamydoconidia are a form of resistant conidia, though they are less significant in veterinary medicine.

Diagnostics, Taxonomy, and Pathogenesis

Classical fungal classification was based on morphology and physiology, but this is now considered outdated due to subjective interpretation and the existence of morphologically similar but phylogenetically distinct species. Current classification relies on molecular analysis of DNA sequences. Common genetic targets include the Internal Transcribed Spacer (ITS), which is ideal for identifying dermatophytes but not aspergilli, as well as the Large Subunit (LSU) and Small Subunit (SSU) of ribosomal DNA, $\beta$-tubulin, and Calmodulin. Whole Genome Sequencing (WGS) is also frequently employed. From a "One Health" perspective, fungal pathogens often link humans, animals, and the environment. Animals can serve as direct sources of infection (zoonosis) or play roles in the environmental cycle of fungi.

Fungi are categorized as either primary pathogens or opportunists. Primary pathogens possess specific virulence factors that allow them to invade a healthy host. Opportunists require predisposing factors to cause disease. These factors include concurrent diseases (infections, allergies, endocrine dysfunction), physiological status (pregnancy, extreme age), mechanical factors (trauma, burns, penetrating wounds), and iatrogenic factors (drugs that alter microflora, immunomodulators, or surgical interventions like catheters). In veterinary medicine, individual sensitivity also depends on breed predisposition and immunosuppressive diseases like Feline Immunodeficiency Virus (FIV). The final outcome of an opportunistic infection depends on the interaction between fungal virulence, the host's immune response, and external environmental or medical factors.