Medical School · Year 2 · Microbiology · includes a discussion video

Lecture 13: Fungi

Unit 2.8: Microbiology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the general characteristics and classification of fungi
2. Explain superficial and cutaneous fungal infections
3. Describe endemic dimorphic fungi
4. Explain opportunistic fungal infections
5. Describe systemic yeast infections (Candida, Cryptococcus)
6. Explain antifungal agents and mechanisms

Lecture Outline

I. Fungal Overview

Fungi represent a distinct kingdom of eukaryotic organisms that possess several unique characteristics distinguishing them from bacteria, protozoa, and other microorganisms. Unlike prokaryotic bacteria, fungal cells contain membrane-bound organelles including a true nucleus, mitochondria, and endoplasmic reticulum. The fungal cell wall is composed primarily of chitin, a polymer of N-acetylglucosamine, along with glucans and mannans that provide structural rigidity and represent important targets for antifungal therapy. The fungal cell membrane contains ergosterol rather than cholesterol, a critical difference exploited by multiple classes of antifungal agents including polyenes and azoles.

Fungi exist in several distinct morphologic forms that have important clinical implications. Yeasts are unicellular organisms that reproduce by budding, producing daughter cells that eventually separate from the parent cell. Molds are multicellular organisms composed of tubular filaments called hyphae, which may be septate (containing cross-walls) or non-septate (coenocytic). Dimorphic fungi can exist in either form depending on environmental conditions, typically growing as molds at environmental temperature (25 degrees Celsius) and converting to yeast form at body temperature (37 degrees Celsius), a phenomenon remembered by the mnemonic "mold in the cold, yeast in the heat." Pseudohyphae represent elongated yeast cells that remain attached in chains, characteristic of certain Candida species.

The classification of pathogenic fungi is often organized by the depth of tissue involvement and typical clinical presentation. Superficial mycoses, such as those caused by Malassezia, affect only the outermost layers of skin and hair. Cutaneous mycoses caused by dermatophytes involve the keratinized layers including epidermis, hair, and nails. Subcutaneous mycoses, exemplified by Sporothrix, are typically introduced through traumatic inoculation and involve deeper skin layers and lymphatics. Systemic mycoses include both endemic dimorphic fungi (Histoplasma, Blastomyces, Coccidioides) that can cause disease in healthy individuals, and opportunistic pathogens (Candida, Aspergillus, Cryptococcus, Mucor) that primarily affect immunocompromised hosts.

Laboratory diagnosis of fungal infections employs multiple techniques depending on the clinical scenario. The potassium hydroxide (KOH) preparation dissolves keratinized cells while preserving fungal elements, allowing direct visualization of hyphae or yeast forms. Culture on Sabouraud dextrose agar, which is acidic and typically contains antibiotics to inhibit bacterial growth, allows definitive identification though growth may require days to weeks. India ink creates negative staining to visualize the capsule of Cryptococcus neoformans, while calcofluor white binds to chitin and fluoresces under ultraviolet light. Antigen detection tests are available for Cryptococcus, Histoplasma, and Aspergillus, providing rapid diagnosis. The 1,3-beta-D-glucan assay serves as a panfungal marker, detecting cell wall components of most pathogenic fungi except Cryptococcus and Mucorales.

<image> Panel A: Comparative illustration showing yeast cells with budding and mold hyphae (both septate and non-septate) demonstrating the morphologic diversity of fungi.

Panel B: Cross-sectional diagram of the fungal cell wall highlighting the layers of chitin, glucan, and mannan, alongside the cell membrane with embedded ergosterol molecules.

Panel C: Microscopic view of a KOH preparation showing branching fungal hyphae against a cleared background with dissolved epithelial cells.

Panel D: Laboratory bench setup displaying Sabouraud agar plates with fungal colonies, calcofluor white fluorescent staining, and India ink preparation of Cryptococcus. </image>

II. Superficial and Cutaneous Mycoses

Malassezia species are lipophilic yeasts that constitute part of the normal skin flora, particularly in sebum-rich areas such as the scalp, face, and upper trunk. These organisms require lipids for growth, distinguishing them from other pathogenic fungi. Malassezia is responsible for pityriasis versicolor, a common superficial infection characterized by hypopigmented or hyperpigmented macules and patches that become more noticeable after sun exposure due to the organism's interference with melanin production. The condition is often called tinea versicolor, though this is technically a misnomer since it is not caused by dermatophytes. Additional conditions associated with Malassezia include seborrheic dermatitis, Malassezia folliculitis, and its role in atopic dermatitis exacerbation.

The dermatophytes comprise three genera of fungi that have evolved to infect keratinized tissues: Trichophyton, Microsporum, and Epidermophyton. Trichophyton species are the most versatile, capable of infecting skin, hair, and nails, and represent the most common cause of dermatophyte infections in humans. Microsporum species primarily affect hair and skin, with some species producing fluorescence under Wood's lamp examination. Epidermophyton floccosum affects skin and nails but notably does not infect hair follicles. These organisms produce keratinases that allow them to break down keratin for nutrition, and infection remains confined to the non-living keratinized layers without systemic invasion in immunocompetent hosts.

Dermatophyte infections are clinically classified by the anatomic site of involvement, collectively termed tinea infections or ringworm due to the characteristic annular appearance of lesions. Tinea capitis affects the scalp and is most common in children, potentially causing kerion formation (an inflammatory, boggy mass representing a delayed hypersensitivity response). Tinea corporis presents as the classic ringworm with annular, erythematous plaques with raised, scaly borders and central clearing. Tinea cruris affects the groin region and is commonly known as "jock itch," typically sparing the scrotum which helps distinguish it from Candida intertrigo. Tinea pedis, or athlete's foot, is extremely common and presents in several forms including interdigital, moccasin, and vesicular patterns. Tinea unguium describes dermatophyte infection of the nails, causing onychomycosis with nail thickening, discoloration, and subungual debris.

Diagnosis of dermatophyte infections relies on clinical presentation combined with laboratory confirmation when needed. KOH preparation of skin scrapings reveals branching septate hyphae that may show arthroconidia. Wood's lamp examination can identify certain Microsporum species that produce fluorescence, though most Trichophyton species do not fluoresce. Culture on dermatophyte test medium or Sabouraud agar allows species identification but requires one to four weeks for growth. Treatment of superficial and localized infections typically involves topical antifungals including azoles (clotrimazole, ketoconazole) or allylamines (terbinafine). Systemic therapy with oral terbinafine, itraconazole, or griseofulvin is required for tinea capitis, extensive tinea corporis, and onychomycosis due to poor topical penetration into hair follicles and nail plate.

<image> Panel A: Clinical photograph demonstrating the classic appearance of tinea corporis with an annular, erythematous plaque showing raised scaly borders and central clearing on the trunk.

Panel B: Microscopic view of a KOH preparation showing septate, branching fungal hyphae obtained from a skin scraping.

Panel C: Side-by-side comparison of Wood's lamp examination showing a Microsporum-infected hair fluorescing blue-green versus a Trichophyton infection without fluorescence.

Panel D: Clinical images of various tinea presentations including tinea capitis with patchy alopecia, tinea pedis with interdigital maceration, and onychomycosis showing nail thickening and discoloration. </image>

III. Subcutaneous Mycoses

Sporothrix schenckii is a dimorphic fungus found worldwide in soil, decaying vegetation, and plant matter including rose thorns, sphagnum moss, and hay. The organism exists as a mold in the environment and converts to a yeast form at body temperature following inoculation into human tissue. Infection typically occurs through traumatic implantation of contaminated material through the skin, leading to the classic eponym "rose gardener's disease." At 37 degrees Celsius, the organism appears as characteristic small, cigar-shaped or oval yeast cells, though these may be difficult to visualize on histopathology and culture often remains the gold standard for diagnosis.

The clinical presentation of sporotrichosis most commonly manifests as the lymphocutaneous form, which accounts for approximately 75 percent of cases. The infection begins with a papule or nodule at the site of inoculation that may ulcerate over several weeks. Subsequently, additional nodules develop along the lymphatic channels draining the primary lesion, creating a characteristic pattern of nodular lymphangitis that ascends proximally. Fixed cutaneous sporotrichosis presents as a single lesion at the inoculation site without lymphatic spread, typically occurring on exposed areas of the face or extremities. Disseminated sporotrichosis occurs primarily in immunocompromised individuals, particularly those with AIDS, alcoholism, or those receiving immunosuppressive therapy, and can involve bones, joints, lungs, and meninges.

Diagnosis of sporotrichosis relies primarily on culture, which demonstrates mold growth at room temperature with characteristic pigmented colonies and distinctive microscopic features including thin septate hyphae with rosette arrangements of conidia. Histopathologic examination may reveal granulomatous inflammation with asteroid bodies (yeast cells surrounded by eosinophilic material), but organisms are often sparse and difficult to identify. Serologic testing can support the diagnosis but is not widely available and may cross-react with other fungal infections. Molecular techniques including PCR are increasingly used in reference laboratories.

Treatment of lymphocutaneous and fixed cutaneous sporotrichosis consists of oral itraconazole for three to six months, with response rates exceeding 90 percent. Localized heat therapy (thermotherapy) can be used as adjunctive treatment since the organism is temperature-sensitive. Disseminated or severe disease requires initial treatment with amphotericin B, typically the lipid formulation, followed by long-term itraconazole for suppressive therapy. Other subcutaneous mycoses include chromoblastomycosis, which produces verrucous, cauliflower-like lesions with characteristic "copper penny" or muriform cells on histopathology. Mycetoma presents with the classic triad of swelling, draining sinus tracts, and grains (colonies of organisms visible in discharge). Phaeohyphomycosis encompasses infections caused by dematiaceous (darkly pigmented) fungi, while lobomycosis produces keloid-like nodules and is limited primarily to South America.

<image> Panel A: Clinical photograph showing the lymphocutaneous pattern of sporotrichosis with a series of nodules ascending along the lymphatic channels of the forearm from a primary lesion on the hand.

Panel B: Microscopic image of tissue section showing characteristic cigar-shaped Sporothrix yeast cells within macrophages, stained with periodic acid-Schiff (PAS) stain.

Panel C: Illustration depicting the mechanism of traumatic inoculation from a rose thorn puncture introducing fungal elements into the dermis and subcutaneous tissue.

Panel D: Culture plate showing Sporothrix schenckii mold colonies at room temperature with characteristic brown to black pigmentation and typical conidiophore morphology under microscopy. </image>

IV. Endemic (Dimorphic) Fungi

Endemic dimorphic fungi share several fundamental characteristics that distinguish them from other fungal pathogens. These organisms demonstrate temperature-dependent dimorphism, growing as molds with infectious conidia at environmental temperatures (25 degrees Celsius) and converting to yeast or other tissue forms at body temperature (37 degrees Celsius). Each species has a specific geographic distribution corresponding to particular soil and environmental conditions. Infection is acquired through inhalation of airborne conidia, and the spectrum of disease ranges from asymptomatic infection to severe disseminated disease. Immunocompromised individuals, particularly those with HIV/AIDS, organ transplant recipients, and patients receiving TNF-alpha inhibitors, face substantially higher risk of severe and disseminated disease.

Histoplasma capsulatum is endemic to the Ohio and Mississippi River valleys in the United States, with additional endemic areas in Central and South America, Africa, and Asia. The organism thrives in soil enriched with bird or bat droppings, making caves, chicken coops, old buildings with bird roosts, and areas under starling roosts high-risk environments. Histoplasma produces small yeasts (2-5 micrometers) that are among the smallest of pathogenic fungi, allowing them to reach the alveoli and be phagocytosed by alveolar macrophages. Unlike most other yeasts, Histoplasma is a facultative intracellular pathogen that survives and replicates within macrophage phagolysosomes, disseminating to the reticuloendothelial system. Healed infection often leaves characteristic calcified granulomas in the lungs, mediastinal lymph nodes, and spleen that may be discovered incidentally on chest imaging.

Histoplasmosis presents across a clinical spectrum depending on the inoculum size, host immune status, and duration of exposure. Acute pulmonary histoplasmosis following low-level exposure typically causes a mild, self-limited illness resembling influenza with fever, headache, myalgias, and nonproductive cough. Heavy exposure can cause severe pneumonitis with diffuse infiltrates, respiratory failure, and acute respiratory distress syndrome. Chronic pulmonary histoplasmosis occurs in patients with underlying emphysema, producing cavitary disease that mimics tuberculosis clinically and radiographically. Progressive disseminated histoplasmosis occurs primarily in immunocompromised patients, particularly those with AIDS and CD4 counts below 150 cells per microliter, presenting with fever, pancytopenia, hepatosplenomegaly, and mucosal ulcerations. Mediastinal complications include fibrosing mediastinitis (excessive fibrotic reaction to mediastinal granulomas causing compression of mediastinal structures) and mediastinal granuloma. Diagnosis relies on antigen detection in urine and serum (highly sensitive in disseminated disease), serologic testing, histopathology showing yeast within macrophages, and culture (which requires biosafety level 3 precautions). Treatment consists of itraconazole for mild to moderate disease and amphotericin B followed by itraconazole for severe or disseminated disease.

Blastomyces dermatitidis shares a similar geographic distribution with Histoplasma but extends further into the Great Lakes region, with additional endemic areas in Africa and India. The organism is found in moist soil near waterways, and infection is associated with outdoor activities in endemic areas. Blastomyces produces large (8-15 micrometers) yeasts with characteristic broad-based budding, where the bud connection is as wide as the parent cell, a distinctive feature helpful in diagnosis. Unlike Histoplasma, Blastomyces elicits a pyogranulomatous inflammatory response with both neutrophils and granulomatous inflammation. Clinical syndromes include acute or chronic pulmonary infection, skin involvement with verrucous or ulcerative lesions, bone involvement (particularly vertebrae and long bones), and genitourinary disease (prostatitis, epididymoorchitis). Central nervous system involvement can occur with disseminated disease. Diagnosis relies on visualization of the characteristic yeasts in clinical specimens, culture, and urine antigen testing (which cross-reacts with Histoplasma antigen). Treatment parallels that for histoplasmosis, with itraconazole for mild to moderate disease and amphotericin B for severe disease.

<image> Panel A: Map of the United States showing the overlapping endemic regions for Histoplasma capsulatum (Ohio-Mississippi River valleys) and Blastomyces dermatitidis (extending into the Great Lakes region) with color-coded prevalence zones.

Panel B: Microscopic image showing small Histoplasma yeasts within macrophage cytoplasm on a Giemsa-stained bone marrow aspirate from a patient with disseminated histoplasmosis.

Panel C: Microscopic image demonstrating the characteristic large Blastomyces yeasts with thick refractile cell walls and broad-based budding on a wet mount preparation.

Panel D: Chest CT scan showing calcified mediastinal lymph nodes and pulmonary granulomas in a patient with prior histoplasmosis infection, alongside a chest radiograph demonstrating cavitary pulmonary blastomycosis. </image>

V. More Endemic Fungi

Coccidioides immitis and Coccidioides posadasii cause coccidioidomycosis, commonly known as Valley fever, named for the San Joaquin Valley of California where the disease was first recognized. These species are endemic to the arid and semiarid regions of the southwestern United States (California, Arizona, Nevada, New Mexico, Utah, and Texas), northern Mexico, and parts of Central and South America. The organism exists in the soil as a mold producing arthroconidia, which become airborne during soil disruption from dust storms, construction activities, earthquakes, or archaeological excavations. Upon inhalation and exposure to body temperature, Coccidioides undergoes a unique transformation into spherules (large, round structures 20-200 micrometers in diameter) that contain numerous endospores; when the spherule ruptures, these endospores disseminate and each can form a new spherule, perpetuating the infection.

Coccidioidomycosis presents with diverse clinical manifestations reflecting the interaction between fungal burden and host immunity. Acute pulmonary coccidioidomycosis occurs in approximately 40 percent of infected individuals, presenting as a flu-like illness with fever, cough, chest pain, and fatigue; the remaining 60 percent have asymptomatic infection. A distinctive feature is the occurrence of immune-mediated phenomena including erythema nodosum (painful red nodules on the shins), erythema multiforme, arthralgias, and toxic erythema, collectively termed "desert rheumatism" - paradoxically, these manifestations correlate with a robust immune response and good prognosis. Chronic pulmonary coccidioidomycosis may develop, typically producing thin-walled cavities that can cause hemoptysis or spontaneous pneumothorax. Disseminated coccidioidomycosis occurs in less than 1 percent of infections but carries significant morbidity and mortality, with particular tropism for skin, bones, joints, and meninges. Risk factors for dissemination include immunosuppression (HIV/AIDS, organ transplant, TNF-alpha inhibitor therapy), pregnancy (especially third trimester), and certain ethnic backgrounds (Filipino, African American ancestry).

Coccidioidal meningitis represents the most serious complication of disseminated disease and is uniformly fatal without treatment. Patients present with chronic meningitis featuring headache, altered mental status, and cranial nerve palsies. Cerebrospinal fluid analysis reveals lymphocytic pleocytosis, elevated protein, and hypoglycorrhachia; notably, CSF eosinophilia is a distinctive feature highly suggestive of coccidioidal meningitis. Treatment requires lifelong suppressive therapy with fluconazole, as relapse is common upon discontinuation. Hydrocephalus is a frequent complication requiring ventriculoperitoneal shunting. Diagnosis of coccidioidomycosis utilizes serologic testing with IgM antibodies appearing early in infection and IgG antibodies developing subsequently; rising complement fixation titers correlate with dissemination and can be used to monitor treatment response. Culture grows readily but poses significant laboratory hazard (biosafety level 3), and laboratory personnel must be notified of suspected cases. Histopathology demonstrating spherules with endospores is diagnostic.

Several other endemic dimorphic fungi cause disease in specific geographic regions. Paracoccidioides brasiliensis is endemic to rural areas of Central and South America, particularly Brazil, and causes paracoccidioidomycosis, which produces characteristic "pilot's wheel" or "ship's wheel" yeasts with multiple narrow-based buds radiating from the parent cell. Talaromyces (formerly Penicillium) marneffei is endemic to Southeast Asia and southern China, associated with bamboo rat exposure, and causes severe disseminated disease in HIV/AIDS patients; the yeast form demonstrates a distinctive central transverse septum not seen in other pathogenic fungi. Emergomyces species have been recently described, particularly from South Africa, causing disseminated disease in immunocompromised patients with cutaneous lesions, and represent an emerging threat. Treatment approaches for these infections generally parallel those for other endemic mycoses, with amphotericin B for severe disease and long-term azole therapy for consolidation and maintenance.

<image> Panel A: Geographic map highlighting the endemic region of Coccidioides in the southwestern United States and northern Mexico, with shaded areas indicating higher incidence zones and notation of dust storm patterns.

Panel B: Histopathologic image showing a large Coccidioides spherule filled with endospores in lung tissue, surrounded by granulomatous inflammation with multinucleated giant cells.

Panel C: Clinical photograph of erythema nodosum on the lower extremities of a patient with acute pulmonary coccidioidomycosis, demonstrating the raised, tender, erythematous nodules.

Panel D: Comparison microscopy images showing the distinctive morphologic features of Paracoccidioides (pilot's wheel multiple budding), Talaromyces marneffei (yeast with central septum), and Emergomyces species. </image>

VI. Candida

Candida species are commensal yeasts that colonize the mucosal surfaces of the gastrointestinal tract, oropharynx, and female genital tract in a substantial proportion of healthy individuals. Morphologically, Candida can exist as budding yeasts, pseudohyphae (elongated yeast cells that remain attached), or true hyphae, with the ability to transition between forms contributing to virulence. Candida albicans remains the most common species causing human disease and can be identified in the laboratory by its ability to produce germ tubes when incubated in serum at 37 degrees Celsius. Other clinically important species include Candida glabrata (notable for reduced susceptibility to fluconazole), Candida parapsilosis (associated with central venous catheters and parenteral nutrition), Candida tropicalis, Candida krusei (intrinsically resistant to fluconazole), and the emerging pathogen Candida auris (often multidrug-resistant and associated with healthcare outbreaks).

Candida causes a spectrum of clinical syndromes ranging from superficial mucosal infections to life-threatening invasive disease. Oropharyngeal candidiasis (thrush) presents as white, curd-like plaques on the buccal mucosa, tongue, and oropharynx that can be scraped off to reveal an erythematous base; it occurs in infants, denture wearers, patients using inhaled corticosteroids, those with diabetes mellitus, and immunocompromised individuals, particularly those with HIV/AIDS. Esophageal candidiasis is an AIDS-defining illness, typically occurring when CD4 counts fall below 100 cells per microliter, presenting with odynophagia and dysphagia; presumptive treatment is appropriate, with endoscopy reserved for non-responders. Vulvovaginal candidiasis is extremely common, affecting up to 75 percent of women at least once, presenting with pruritus, thick white discharge, and vaginal erythema; it is not necessarily associated with immunocompromise and often occurs with antibiotic use, pregnancy, or diabetes. Cutaneous candidiasis includes diaper dermatitis with satellite pustules, intertrigo in skin folds, and chronic mucocutaneous candidiasis (a rare primary immunodeficiency affecting T-cell responses to Candida).

Candidemia and invasive candidiasis represent serious healthcare-associated infections with significant morbidity and mortality. Risk factors include central venous catheters, total parenteral nutrition, broad-spectrum antibiotic use, recent abdominal surgery, neutropenia, and intensive care unit admission. Candidemia can lead to metastatic complications including endophthalmitis (requiring dilated funduscopic examination in all patients with candidemia), endocarditis (particularly in patients with prosthetic valves or intravenous drug use), osteomyelitis, and hepatosplenic candidiasis (chronic disseminated candidiasis occurring in neutropenic patients, presenting as fever that persists after neutrophil recovery with characteristic target lesions in liver and spleen on imaging). Candida auris has emerged as a particularly concerning pathogen due to frequent multidrug resistance, persistence in healthcare environments, and propensity for healthcare transmission.

Diagnosis of mucosal candidiasis is often clinical, supported by KOH preparation showing yeasts and pseudohyphae. Invasive candidiasis requires blood cultures, though sensitivity is only 50-70 percent. The serum 1,3-beta-D-glucan assay is sensitive but not specific for Candida (elevated in many fungal infections). The T2Candida panel provides rapid molecular identification directly from blood samples. Treatment of oropharyngeal candidiasis employs topical nystatin or clotrimazole troches, or oral fluconazole for refractory cases or immunocompromised patients. Esophageal candidiasis requires systemic therapy with fluconazole as first-line treatment. Candidemia treatment has shifted to echinocandins (caspofungin, micafungin, anidulafungin) as first-line therapy, with removal of central venous catheters being a critical component of management. Fluconazole remains an option for stable patients with susceptible isolates. Candida auris infections typically require echinocandins due to frequent azole and polyene resistance, and infection control measures are essential to prevent transmission.

<image> Panel A: Microscopic image of Candida albicans demonstrating the morphologic forms including budding yeast cells, pseudohyphae (elongated attached cells), and the germ tube test positive result showing filamentous extension from yeast cell.

Panel B: Clinical photograph of oropharyngeal candidiasis (thrush) showing white, creamy plaques on the tongue and buccal mucosa that can be scraped to reveal erythematous base.

Panel C: Funduscopic photograph demonstrating Candida endophthalmitis with characteristic fluffy white chorioretinal lesions extending into the vitreous.

Panel D: Abdominal CT scan showing hepatosplenic candidiasis with multiple target lesions in the liver and spleen in a patient recovering from chemotherapy-induced neutropenia. </image>

VII. Cryptococcus

Cryptococcus species are encapsulated yeasts that cause significant disease, particularly in immunocompromised individuals. The polysaccharide capsule composed of glucuronoxylomannan (GXM) is the major virulence factor, inhibiting phagocytosis, interfering with antigen presentation, and suppressing T-cell responses. Cryptococcus neoformans is found worldwide in soil contaminated with pigeon and other bird droppings, as the high nitrogen content from uric acid supports its growth. Cryptococcus gattii is associated with certain tree species including eucalyptus and has traditionally been found in tropical and subtropical regions, though outbreaks have occurred in the Pacific Northwest of North America; notably, C. gattii can cause disease in immunocompetent individuals. Both species enter through the respiratory tract following inhalation of desiccated yeasts or basidiospores.

Cryptococcosis manifests across a clinical spectrum determined largely by host immune status. Pulmonary cryptococcosis in immunocompetent individuals is often asymptomatic or causes mild pneumonia that resolves spontaneously. In immunocompromised patients, pulmonary disease can progress to severe pneumonia with diffuse infiltrates. The organism demonstrates pronounced neurotropism, and cryptococcal meningitis is the most clinically significant manifestation, occurring primarily in patients with AIDS (typically CD4 counts below 100 cells per microliter), solid organ transplant recipients, and patients with cirrhosis, malignancy, or receiving corticosteroids. Cryptococcal meningitis presents insidiously with headache (often severe), fever, and altered mental status developing over days to weeks; classic meningeal signs are often absent. Disseminated disease can involve skin, producing umbilicated papules resembling molluscum contagiosum, as well as bones, prostate, and other organs.

Diagnosis of cryptococcosis utilizes several complementary methods. India ink examination of cerebrospinal fluid creates negative staining that reveals the characteristic encapsulated yeast against a dark background; sensitivity is approximately 50-70 percent in HIV-associated disease but lower in other populations. The cryptococcal antigen test (CrAg) detecting capsular polysaccharide in CSF and serum is highly sensitive (exceeding 95 percent) and specific, providing rapid diagnosis and allowing titers to be followed during treatment. CSF analysis typically reveals elevated opening pressure (often markedly so), lymphocytic pleocytosis (though cell counts may be low in AIDS), elevated protein, and low glucose. Culture on standard media grows within days to weeks. Histopathology with mucicarmine staining highlights the capsule in tissue sections.

Treatment of cryptococcal meningitis follows a three-phase approach. Induction therapy consists of liposomal amphotericin B combined with flucytosine for at least two weeks; this combination has demonstrated superior outcomes compared to amphotericin alone. Consolidation therapy follows with fluconazole 400-800 mg daily for eight weeks. Maintenance therapy continues with fluconazole 200 mg daily to prevent relapse, with duration depending on immune status; in HIV patients, maintenance continues until sustained immune reconstitution is achieved with antiretroviral therapy (CD4 above 100 cells per microliter for more than three months). Management of elevated intracranial pressure is critical and often overlooked; serial lumbar punctures with removal of sufficient CSF to normalize opening pressure (or reduce by 50 percent) should be performed daily if needed, as elevated ICP is the major cause of early mortality. Ventriculoperitoneal shunting may be required for refractory elevated pressure. Immune reconstitution inflammatory syndrome (IRIS) can occur in HIV patients starting antiretroviral therapy, presenting as clinical worsening despite microbiologic improvement.

<image> Panel A: India ink preparation of cerebrospinal fluid showing Cryptococcus neoformans yeasts with prominent clear halos representing the polysaccharide capsule against the dark ink background.

Panel B: Environmental illustration showing pigeon droppings as the natural reservoir for C. neoformans and eucalyptus trees associated with C. gattii, with inhalation as the route of infection.

Panel C: Brain MRI showing dilated ventricles (hydrocephalus) and gelatinous pseudocysts in the basal ganglia in a patient with cryptococcal meningoencephalitis, alongside a photograph of lumbar puncture demonstrating opening pressure measurement.

Panel D: Clinical photograph showing disseminated cutaneous cryptococcosis with umbilicated papules resembling molluscum contagiosum on the face of an immunocompromised patient. </image>

VIII. Aspergillus

Aspergillus species are ubiquitous molds found in soil, decaying vegetation, and organic debris throughout the environment. The genus includes several hundred species, with Aspergillus fumigatus being responsible for the majority of human infections; other pathogenic species include A. flavus, A. niger, A. terreus (notable for amphotericin B resistance), and A. nidulans. Morphologically, Aspergillus produces septate hyphae that branch at acute angles (approximately 45 degrees), a characteristic finding that helps distinguish it from the Mucorales. The organisms produce vast numbers of airborne conidia (asexual spores) that are inhaled daily by everyone; disease occurs only when host defenses are compromised or when specific anatomic or immunologic conditions allow fungal proliferation.

The clinical spectrum of Aspergillus disease depends on the host immune status and underlying pulmonary architecture. Allergic bronchopulmonary aspergillosis (ABPA) occurs in patients with asthma or cystic fibrosis, representing a hypersensitivity response to Aspergillus antigens colonizing the airways; it presents with worsening asthma, recurrent infiltrates, central bronchiectasis, elevated IgE, and Aspergillus-specific IgE and IgG antibodies. Aspergilloma ("fungus ball") develops in pre-existing pulmonary cavities (from tuberculosis, sarcoidosis, or other causes), consisting of a mass of hyphae, mucus, and debris within the cavity; patients may be asymptomatic or present with hemoptysis that can be life-threatening. Chronic pulmonary aspergillosis encompasses a spectrum from simple aspergilloma to chronic cavitary disease with progressive lung destruction, occurring in patients with underlying structural lung disease but without severe immunocompromise.

Invasive aspergillosis (IA) is the most serious manifestation, occurring predominantly in severely immunocompromised patients. Major risk factors include prolonged neutropenia (especially exceeding 10 days), hematopoietic stem cell transplantation (particularly with graft-versus-host disease), solid organ transplantation, high-dose corticosteroids, and other conditions causing impaired phagocyte function. IA typically presents as fever unresponsive to broad-spectrum antibacterial therapy in a neutropenic patient, along with pulmonary symptoms including cough, pleuritic chest pain, and hemoptysis. The organism is angioinvasive, invading blood vessel walls and causing thrombosis, hemorrhage, and infarction. Characteristic imaging findings include the halo sign (ground-glass opacity surrounding a nodule, representing hemorrhage around the infected focus) early in disease, followed by the air-crescent sign (crescent of air between the necrotic tissue and cavity wall) as neutropenia resolves. Invasive sinusitis and dissemination to brain, skin, and other organs can occur. Mortality remains substantial despite treatment, ranging from 30 to 80 percent depending on the underlying condition and degree of immunosuppression.

Diagnosis of invasive aspergillosis requires a high index of suspicion given the non-specific presentation. Serum galactomannan, a cell wall component, serves as a biomarker, with sensitivity improved when measured twice weekly in high-risk patients; it can also be measured in bronchoalveolar lavage fluid with enhanced sensitivity. The 1,3-beta-D-glucan assay is elevated but non-specific. CT imaging demonstrating characteristic findings (nodules with halo sign, air-crescent sign, wedge-shaped infarcts) supports the diagnosis. Bronchoscopy with BAL for culture, galactomannan, and cytology improves diagnostic yield. Tissue biopsy demonstrating septate hyphae with acute-angle branching invading tissue provides definitive diagnosis, though this may not be feasible in thrombocytopenic patients. Treatment of invasive aspergillosis relies on voriconazole as first-line therapy, with isavuconazole as an alternative with fewer drug interactions and better tolerability. Liposomal amphotericin B serves as salvage therapy. Combination antifungal therapy is sometimes used in refractory cases. ABPA treatment combines systemic corticosteroids with itraconazole to reduce antigenic burden. Aspergilloma management is primarily surgical for life-threatening hemoptysis, with antifungal therapy playing a supportive role.

<image> Panel A: Microscopic image of Aspergillus demonstrating septate hyphae with characteristic acute-angle (45 degree) dichotomous branching pattern in tissue stained with Gomori methenamine silver (GMS) stain.

Panel B: CT scan of the chest showing the halo sign with a pulmonary nodule surrounded by ground-glass opacity representing hemorrhage, and a separate image showing the air-crescent sign in a patient with resolving invasive aspergillosis.

Panel C: Chest radiograph and CT showing an aspergilloma ("fungus ball") within a pre-existing cavitary lesion in the upper lobe, demonstrating the characteristic mobile intracavitary mass with the air-crescent sign.

Panel D: Microscopic image of Aspergillus conidiophore head showing the characteristic "broom-like" or columnar arrangement of phialides bearing chains of conidia, as seen on culture. </image>

IX. Mucormycosis and Other Opportunistic Fungi

Mucormycosis (formerly zygomycosis) comprises infections caused by fungi in the order Mucorales, including Rhizopus, Mucor, Rhizomucor, Cunninghamella, and Lichtheimia (formerly Absidia) species. These organisms are ubiquitous in the environment, found in soil, decaying organic matter, and bread mold. Morphologically, they produce broad (10-20 micrometers), ribbon-like hyphae that are pauciseptate (few or no septations) and branch at wide angles (approximately 90 degrees), distinguishing them from Aspergillus. The infection occurs through inhalation of sporangiospores or direct inoculation through wounds. Unlike most other fungi, these organisms do not respond to voriconazole, and this agent may actually promote mucormycosis in susceptible patients.

The clinical presentation of mucormycosis depends on the portal of entry and underlying risk factors. Rhinocerebral mucormycosis is the most recognized form, occurring classically in patients with diabetic ketoacidosis or other forms of hyperglycemia with acidosis, as the elevated glucose and iron availability in acidosis promote fungal growth and impair neutrophil function. The infection begins in the nasal turbinates and paranasal sinuses, with rapid extension through bone and soft tissue to involve the orbit, palate, and brain via direct invasion and angioinvasion. A characteristic finding is the black eschar (necrotic tissue) on the nasal turbinates or palate resulting from vascular invasion and infarction. Pulmonary mucormycosis occurs in neutropenic patients and resembles invasive aspergillosis clinically and radiographically; angioinvasion leads to nodules, consolidation, and the reversed halo sign. Cutaneous mucormycosis follows traumatic inoculation or develops in burn patients. Gastrointestinal mucormycosis occurs in neonates and malnourished individuals. Disseminated disease carries mortality exceeding 90 percent.

Treatment of mucormycosis requires an aggressive multimodal approach combining antifungal therapy, surgical debridement, and correction of underlying predisposing factors. Surgical debridement is essential, as antifungal agents cannot adequately penetrate necrotic, devitalized tissue; this often requires radical procedures including orbital exenteration for rhinocerebral disease. First-line antifungal therapy consists of high-dose liposomal amphotericin B (5-10 mg/kg/day), as conventional amphotericin deoxycholate causes unacceptable nephrotoxicity at required doses. Isavuconazole and posaconazole have activity against Mucorales and serve as step-down therapy after initial response to amphotericin; importantly, other azoles (fluconazole, voriconazole) and echinocandins have no activity. Control of diabetic ketoacidosis, discontinuation of deferoxamine (an iron chelator that paradoxically provides iron to the fungi), and reduction of immunosuppression when possible are critical adjuncts. Despite aggressive therapy, mortality remains 40-80 percent depending on the site and extent of infection.

Pneumocystis jirovecii (formerly Pneumocystis carinii, though it remains the cause of "PCP") is a unique organism now classified as a fungus based on genetic analysis, though it has characteristics of both fungi and protozoa and does not grow on standard fungal culture media. Pneumocystis causes pneumonia almost exclusively in immunocompromised individuals, particularly those with HIV/AIDS and CD4 counts below 200 cells per microliter, organ transplant recipients, and patients receiving chemotherapy or prolonged corticosteroid therapy. PCP presents insidiously with progressive dyspnea, nonproductive cough, fever, and hypoxia that is often out of proportion to physical examination and chest radiograph findings. Chest imaging typically shows bilateral diffuse interstitial infiltrates with a ground-glass pattern, often with perihilar predominance; atypical presentations including nodules, cysts, and pneumothorax can occur. Elevated serum lactate dehydrogenase (LDH) is characteristic but non-specific. Diagnosis requires demonstration of organisms in induced sputum or bronchoalveolar lavage using Giemsa stain (shows trophozoites), methenamine silver or calcofluor white stains (shows cyst walls), or direct fluorescent antibody testing. Treatment consists of trimethoprim-sulfamethoxazole (TMP-SMX) as first-line therapy; adjunctive corticosteroids are indicated for moderate to severe disease (PaO2 less than 70 mmHg or A-a gradient greater than 35 mmHg) to reduce inflammatory lung injury. Alternative agents for sulfa-allergic patients include pentamidine, atovaquone, dapsone-trimethoprim, and clindamycin-primaquine. Primary prophylaxis with TMP-SMX is indicated for HIV patients with CD4 counts below 200 cells per microliter and continues until immune reconstitution is achieved.

<image> Panel A: Microscopic image showing the characteristic broad, ribbon-like, pauciseptate hyphae of Mucorales with irregular, wide-angle (90 degree) branching on GMS stain, contrasted with the narrower, regularly septate hyphae of Aspergillus.

Panel B: Clinical photograph demonstrating rhinocerebral mucormycosis with black necrotic eschar on the hard palate and nasal septum, indicating vascular invasion and tissue infarction.

Panel C: CT scan showing pulmonary mucormycosis with the reversed halo sign (area of ground-glass opacity surrounded by a ring of consolidation), alongside a diagram showing the pathophysiology of mucormycosis in diabetic ketoacidosis with elevated iron and glucose.

Panel D: Bronchoalveolar lavage specimen showing Pneumocystis jirovecii cysts with GMS silver stain demonstrating the characteristic cup or helmet shapes, and a chest radiograph showing bilateral diffuse interstitial infiltrates typical of PCP. </image>

X. Antifungal Agents

The polyene antifungals, including amphotericin B and nystatin, represent the oldest class of antifungal agents and maintain a critical role in treating serious fungal infections. These agents bind to ergosterol in the fungal cell membrane, creating pores that disrupt membrane integrity and cause leakage of intracellular contents, ultimately leading to cell death. Amphotericin B deoxycholate (conventional amphotericin) has the broadest spectrum of any antifungal, covering most yeasts, molds, and dimorphic fungi, but its use is limited by significant toxicity including nephrotoxicity (occurring in the majority of patients), infusion-related reactions (fever, chills, rigors), and electrolyte disturbances (hypokalemia, hypomagnesemia). Lipid formulations of amphotericin (liposomal amphotericin, amphotericin B lipid complex) markedly reduce nephrotoxicity while maintaining efficacy and have largely replaced conventional amphotericin for most indications. Nystatin is too toxic for parenteral use and is employed only topically for oropharyngeal and vulvovaginal candidiasis.

The azole antifungals inhibit the enzyme lanosterol 14-alpha-demethylase (CYP51), blocking a critical step in ergosterol synthesis and leading to accumulation of toxic sterol intermediates. Fluconazole has excellent oral bioavailability and CSF penetration, covering Candida species (except C. krusei and with variable activity against C. glabrata) and Cryptococcus; it serves as first-line therapy for cryptococcal consolidation and maintenance, and as an option for candidemia in stable patients with susceptible isolates. Itraconazole has activity against dimorphic fungi (Histoplasma, Blastomyces, Sporothrix) and is used for mild to moderate endemic mycoses and ABPA; absorption is variable and requires gastric acid. Voriconazole is first-line therapy for invasive aspergillosis, with activity against most molds and yeasts; notable toxicities include visual disturbances, hepatotoxicity, and photosensitivity. Posaconazole and isavuconazole have the broadest spectrum among azoles, including activity against Mucorales, making them valuable for prophylaxis in high-risk patients and treatment of mucormycosis. All azoles inhibit hepatic cytochrome P450 enzymes (CYP3A4, CYP2C19) to varying degrees, creating numerous important drug interactions.

The echinocandins (caspofungin, micafungin, anidulafungin) inhibit synthesis of 1,3-beta-D-glucan, a critical component of the fungal cell wall not present in mammalian cells, resulting in excellent tolerability. These agents are fungicidal against Candida species and fungistatic against Aspergillus; they have become first-line therapy for invasive candidiasis and candidemia due to reliable activity against most Candida species including fluconazole-resistant strains. Notably, echinocandins have no activity against Cryptococcus (which has minimal glucan in its cell wall) or Mucorales. All three agents require intravenous administration as they are not absorbed orally. Drug interactions are minimal, though caspofungin has some interactions with hepatic inducers. Anidulafungin is unique in undergoing non-enzymatic degradation rather than hepatic metabolism, potentially advantageous in patients with liver disease or multiple medications.

Additional antifungal agents address specific clinical niches. Flucytosine (5-fluorocytosine) is a fluorinated pyrimidine analog that inhibits DNA and RNA synthesis after conversion to 5-fluorouracil by fungal cytosine deaminase; it has activity against Candida and Cryptococcus but develops resistance rapidly when used alone, thus it is employed in combination with amphotericin B for cryptococcal meningitis induction therapy. Bone marrow suppression is the major toxicity, requiring monitoring of blood counts. Terbinafine inhibits squalene epoxidase, blocking an early step in ergosterol synthesis; it is highly effective against dermatophytes and is first-line oral therapy for onychomycosis and tinea capitis, with excellent keratin penetration and prolonged tissue levels. Griseofulvin disrupts microtubule function and was historically used for dermatophyte infections but has been largely supplanted by terbinafine and azoles. The selection of antifungal therapy depends on the causative organism, site of infection, host factors (renal and hepatic function, immune status, drug allergies), potential drug interactions, local resistance patterns, and cost considerations.

<image> Panel A: Diagram of the fungal cell showing the targets of major antifungal drug classes: polyenes binding ergosterol in the membrane, azoles inhibiting ergosterol synthesis pathway, echinocandins blocking cell wall glucan synthesis, and flucytosine affecting nucleic acid synthesis.

Panel B: Flowchart illustrating antifungal drug selection by clinical scenario, showing first-line agents for invasive candidiasis, cryptococcal meningitis, invasive aspergillosis, mucormycosis, and endemic mycoses.

Panel C: Table-style illustration comparing the spectrum of activity of major antifungal classes against Candida, Cryptococcus, Aspergillus, Mucorales, and dimorphic fungi, with checkmarks and X marks indicating coverage.

Panel D: Clinical decision tree for antifungal selection incorporating host factors (renal function, hepatic function, drug interactions) and organism susceptibility patterns, with notation of key resistance patterns including fluconazole-resistant Candida and voriconazole-resistant Aspergillus terreus. </image>

Summary

• Fungi are eukaryotic organisms with cell walls containing chitin and cell membranes containing ergosterol, both of which serve as targets for antifungal therapy
• Dermatophytes (Trichophyton, Microsporum, Epidermophyton) cause tinea infections of skin, hair, and nails; diagnosis uses KOH preparation and culture; treatment involves topical or systemic azoles and terbinafine
• Sporotrichosis is the "rose gardener's disease" caused by Sporothrix schenckii, presenting with lymphocutaneous nodules following traumatic inoculation; treat with itraconazole
• Endemic dimorphic fungi include Histoplasma (Ohio-Mississippi River valleys, bird/bat droppings, intracellular in macrophages), Blastomyces (broad-based budding), and Coccidioides (Southwest US, spherules with endospores)
• Candida is normal flora that causes mucosal disease (thrush, esophagitis, vaginitis) and invasive disease (candidemia, disseminated candidiasis); echinocandins are first-line for invasive disease
• Cryptococcus is an encapsulated yeast causing meningitis primarily in AIDS patients; diagnosis relies on cryptococcal antigen testing; treatment requires amphotericin B plus flucytosine induction followed by fluconazole, with aggressive management of elevated intracranial pressure
• Aspergillus causes allergic disease (ABPA), aspergilloma, and invasive aspergillosis in neutropenic patients; septate hyphae branch at 45 degree angles; voriconazole is first-line for invasive disease
• Mucormycosis affects patients with diabetic ketoacidosis and neutropenia; broad pauciseptate hyphae branch at 90 degree angles; treatment requires surgical debridement and high-dose amphotericin B
• Pneumocystis jirovecii causes pneumonia in AIDS patients with CD4 counts below 200; treat and prevent with TMP-SMX

Key Terms

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