2.6.3 Fungi

Overview of Medical Mycology

Fungi are eukaryotic organisms that cause a wide range of human diseases called mycoses. For USMLE purposes, it is essential to recognize that fungal infections often affect immunocompromised hosts, can mimic other diseases such as cancer or tuberculosis, and typically require specific diagnostic methods and prolonged treatment.

Fungi have nuclei, organelles, and rigid cell walls that contain chitin and usually ergosterol in their cell membranes. These features make them distinct from bacteria and are the foundation for many antifungal drug targets. Fungal infections are usually acquired from the environment rather than transmitted person to person, with a few important exceptions such as Candida and dermatophytes.

Basic Fungal Structure and Forms

Fungi exist in two main structural forms, yeasts and molds, and many clinically important species can switch between them.

Yeasts are unicellular, round to oval organisms that reproduce primarily by budding. On microscopy, they appear as single cells or small clusters. Budding may give rise to pseudohyphae, which are elongated chains of yeast cells that remain attached after division. Candida species are the classic example of yeast that forms pseudohyphae and, in some settings, true hyphae.

Molds are multicellular filamentous fungi. They grow as hyphae, which are long, branching filaments. Hyphae can be septate, with cross walls, or nonseptate, also called coenocytic, with few or no internal divisions. A mass of intertwined hyphae is called a mycelium, which can be visible to the naked eye as fuzzy growth on culture media or environmental surfaces.

Some medically important fungi are dimorphic. Dimorphic fungi exist as molds in the environment at lower temperatures, usually room temperature, and as yeasts or yeast-like forms in human tissue or at body temperature. For USMLE, the classic teaching is that they are mold in the cold and yeast in the heat.

Fungal Cell Wall and Membrane Targets

The fungal cell wall and membrane are crucial not only for understanding fungal biology but also for anticipating antifungal drug mechanisms, which appear in pharmacology and clinical questions.

The fungal cell wall contains chitin, glucan, and other polysaccharides, which provide rigidity and protection. Humans do not possess cell walls, so these structures are attractive drug targets. The fungal cell membrane contains ergosterol instead of cholesterol. This difference is central to the mechanism of many antifungal drugs, which selectively target ergosterol or its synthesis.

A simplified comparison of membranes and walls is useful:

These features are exploited by various classes of antifungal medications, which are detailed separately in pharmacology chapters.

Fungal Reproduction and Spores

Fungi reproduce by producing spores. From a medical standpoint, the main interest is in the infectious propagules and how they are acquired, rather than the fine details of sexual and asexual cycles.

Asexual spores, such as conidia, are common infectious forms for molds. These spores are formed on specialized structures on hyphae and are easily aerosolized. Inhalation of airborne conidia from environmental molds and dimorphic fungi is a major route of infection in systemic mycoses.

Yeasts reproduce primarily by budding. Each bud eventually separates to become a new yeast cell. Pseudohyphae result when buds remain attached and elongate, giving a filamentous appearance without the full structure of mold hyphae.

In clinical practice and exam questions, explicit terminology such as conidia, sporangiospores, and arthroconidia may appear, especially when describing laboratory findings or histopathology in systemic fungal infections.

Classification of Mycoses by Site and Depth

For USMLE, fungal diseases are often classified based on the depth and location of infection. This classification helps organize clinical presentations and expected host responses.

Superficial mycoses are limited to the outermost layers of the skin and hair, such as the stratum corneum. They cause cosmetic changes, like discoloration of skin or affected hair, but no significant inflammation or systemic symptoms. These infections are generally mild and often asymptomatic.

Cutaneous mycoses involve the keratinized tissues, including the epidermis, hair, and nails. Dermatophytes are the classical agents of cutaneous mycoses and cause ringworm or tinea infections. These infections commonly produce pruritic, scaly lesions and can be transmitted from person to person or from animals. Nail involvement can cause thickened, discolored nails that are slow to treat.

Subcutaneous mycoses result from inoculation of fungi into the deeper layers of the skin, usually through trauma such as a thorn prick. These infections are localized but can extend along lymphatic channels, and may cause nodules or ulcers at the site of inoculation. They are typically associated with soil or plant matter and are more common in certain geographic regions or occupational exposures.

Systemic mycoses involve internal organs or deep tissues. They usually begin with inhalation of spores into the lungs and then disseminate via the bloodstream or lymphatics. Some systemic fungi are truly pathogenic and can infect immunocompetent hosts, while others are opportunistic and primarily affect individuals with impaired immunity, such as HIV infection, neutropenia, or those receiving immunosuppressive therapy.

Opportunistic mycoses are a special group of systemic infections caused by fungi that rarely cause disease in healthy hosts but can be severe or fatal in patients with weakened immune systems. Candida, Aspergillus, Mucor, and Pneumocystis are examples that frequently appear in questions involving immunocompromised patients.

The table below summarizes the broad categories:

Host Factors and Fungal Pathogenesis

Fungal pathogenesis is strongly influenced by the host immune status. Many fungi are environmental saprophytes that become pathogenic when host defenses fail. Recognizing which immune defects predispose to particular fungal infections helps in both diagnosis and exam reasoning.

Cell-mediated immunity, especially T cell mediated responses and macrophage activation, is crucial for controlling many systemic and opportunistic fungi. Defects in T cells, such as those seen in advanced HIV infection, predispose to infections like Pneumocystis pneumonia and certain disseminated mycoses.

Neutrophils and phagocytic function are essential for defense against fungi that invade blood vessels or cause angioinvasive disease. Patients with neutropenia, chronic granulomatous disease, or those receiving high dose corticosteroids are particularly susceptible to invasive mold infections such as Aspergillus and mucormycosis.

Intact skin and mucosal barriers, along with normal microbiota, help prevent colonization and invasion by yeasts like Candida. Disruption of these barriers through trauma, catheters, surgery, or antibiotic use increases the risk of candidemia and invasive candidiasis.

Underlying metabolic conditions also matter. Diabetic ketoacidosis, especially with acidotic states, is an important risk factor for certain molds. Broad spectrum antibiotics can suppress bacterial flora and promote fungal overgrowth in the gastrointestinal or genital tract.

In many exam questions, the combination of a specific immune defect, such as low CD4 count or prolonged neutropenia, and a typical clinical setting, such as prolonged hospitalization, mechanical ventilation, or organ transplantation, points toward particular fungal pathogens.

Diagnosis of Fungal Infections

Diagnosis of fungal infections relies heavily on direct visualization, culture, and occasionally antigen or molecular tests. The general approach is important to understand, even though specific methods for individual pathogens are discussed elsewhere.

Direct microscopy can reveal yeasts, hyphae, or pseudohyphae in clinical specimens. Potassium hydroxide preparation is commonly used for skin scrapings, hair, and nail samples. The KOH dissolves keratin and Background material and leaves behind fungal elements that are more easily visible. Special stains such as silver stains, periodic acid Schiff, and specific tissue stains are used in histopathology to highlight fungi in tissue biopsy specimens.

Culture on specialized media helps identify fungi. Sabouraud dextrose agar is a standard medium for fungal culture. Fungi generally grow more slowly than bacteria, which can delay diagnosis. Distinctive colony morphology, pigment production, and microscopic features such as arrangement of conidia can aid in identification.

Serologic and antigen detection tests are particularly useful in systemic fungal infections, especially when tissue culture is difficult or too slow. Examples include antigen assays for certain dimorphic fungi and for opportunistic infections. Molecular methods such as PCR are increasingly used for sensitive and rapid detection but are usually not emphasized in great detail at the beginner level.

Imaging plays an adjunct role. Chest radiographs and CT scans can reveal characteristic patterns in pulmonary fungal infections, such as nodules, cavitary lesions, or halo signs, which guide further testing.

General Principles of Treatment

Although specific drugs and regimens are covered in pharmacology chapters, it is useful to understand broad treatment principles for fungal infections as they relate to disease categories and host factors.

Mild superficial and cutaneous mycoses often respond to topical antifungals applied directly to the skin, scalp, or nails. Topical therapy limits systemic toxicity and is generally sufficient for localized disease.

More extensive cutaneous or nail involvement, or infections refractory to topical agents, may require systemic oral antifungals. Treatment courses are often prolonged because nails and keratinized tissues grow slowly and drugs must accumulate in these areas to eradicate the fungus.

Subcutaneous and systemic mycoses usually need systemic therapy, often with potent agents that target ergosterol or cell wall synthesis. In serious disseminated infections, initial treatment may involve intravenous formulations. Once infection is controlled, patients might switch to oral maintenance therapy for weeks or months.

Management of opportunistic mycoses almost always requires both antifungal therapy and correction of underlying immune or metabolic abnormalities whenever possible. This can include reducing immunosuppression, treating neutropenia, or optimizing control of diabetes and other conditions. Without addressing host factors, relapse or treatment failure is common.

Surgical intervention may be necessary in some cases, such as debridement of necrotic tissue in invasive mold infections or removal of infected devices like catheters in candidemia. The need for combined medical and surgical therapy is a recurring theme in severe invasive fungal infections.

Geographic and Environmental Associations

Many systemic fungi have characteristic geographic distributions, which are frequently tested. While specific regions are explored for each pathogen individually, it is important to recognize the general pattern that several dimorphic fungi are endemic in defined areas and are acquired from soil or environmental reservoirs.

Exposure often occurs through occupational or recreational activities that disturb soil or bird and bat droppings. Inhalation of airborne spores in endemic regions can cause asymptomatic infection, mild respiratory illness, or, in some cases, severe pneumonia and dissemination. Knowledge of travel or residence history is therefore crucial in clinical reasoning.

In contrast, opportunistic fungi such as Candida and Aspergillus are ubiquitous and not restricted to specific geographic regions. Instead, the key association is the host immune status and hospital or healthcare environment, including exposure to invasive procedures, catheters, and broad spectrum antibiotics.

Understanding these distinctions helps differentiate an endemic systemic mycosis in an otherwise healthy traveler from a healthcare associated opportunistic infection in an immunocompromised patient.

Summary of Core Ideas for USMLE

In medical mycology, the most important exam relevant concepts are the structural uniqueness of fungi, their classification by infection depth, the role of host immunity in pathogenesis, and the key diagnostic and therapeutic principles.

Fungi are eukaryotic organisms with chitin containing cell walls and ergosterol rich membranes. They appear as yeasts, molds, or dimorphic forms and reproduce by budding or spore formation. Clinical disease can be superficial, cutaneous, subcutaneous, systemic, or opportunistic, with each category associated with typical routes of acquisition and host factors.

Diagnosis relies on direct visualization, specialized culture, and sometimes antigen or serologic tests. Treatment requires topical or systemic antifungals guided by the site and severity of infection, and, in opportunistic disease, correction of underlying immune or metabolic problems is essential.

These foundational ideas form the backdrop for the more specific fungal pathogens and syndromes that are addressed in subsequent chapters.