Mycology Flashcards
What are the oxygen requirements of fungi?
Mostly obligate aerobes, sometimes facultative aerobes
What are the features of molds?
- Growth occurs by production of multicellular branching cylindrical tubules called hyphae
- The mass of intertwined hyphae is called a mycelium
- Some hyphae are divided into cells by cross-walls called septa
What are examples of clinically significant molds?
- Zygomycetes
- Aspergillus
- Dermatophytes
What are the features of yeasts?
- Spherical or ellipsoid unicellular organisms
- Reproduce by budding
- Some species produce buds that do not detach and become elongated, producing chains of yeast cells called pseudohyphae
What are dimorphic fungi?
Fungi that are capable of growing as both yeasts (in vivo, at 37ºC) and molds (22ºC, e.g. in the lab)
How can hyphae and pseudohyphae be distinguished?
In pseudohyphae, there is a slight tapering of the cell near the hypha (appearing like a chain of sausages), while in true hyphae, it is a cleaner straight line
What are examples of clinically significant yeasts?
- Candida albicans
- Creptococcus neoformans
What are examples of clinically significant dimorphic fungi?
- Blastomyces dermatidis
- Coccidioides immitis
- Histoplasma capsulatum
- Sporothrix shenckii
- Paracoccidioides brasiliens
How do fungi reproduce?
Spores (both sexual and asexual)
Define
Sporangiospores
A collection of asexual spores within a sac-like structure called a sporangium
What are the features of the fungal cell membrane and cell wall?
- The cell wall contains chitin (unbranched polymer of N-acetylglucosamine), glucans, and mannans
- The plasma membrane contains ergosterol instead of cholesterol
What are the types of reactions to fungi?
- Infection
- Hypersensitivity
- Intoxication from mycotoxins
Which fungus is prominent in hypersensitivity reactions?
Aspergillus fumigatus
How do fungi cause hypersensitivity reactions?
Spores and toxins of fungi contain potent surface antigens that stimulate strong type I hypersensitivity (allergic) reactions, manifesting as eosinophilia and IgE-mediated bronchoconstriction
Can mycotoxins be inactivated by cooking?
No, as most of them are heat stable
Which species produce aflatoxin?
- Aspergillus flavus
- Aspergillus parasiticus
Which foods are typically contaminated with aflatoxin?
- Peanuts
- Corn
- Grains
- Tree nuts
What is the pathogenesis of aflatoxicosis?
- Aflatoxin is metabolized in the liver to epoxides, which are carcinogenic
- Aflatoxin B1 causes mutations in the tumor suppressor gene p53, leading to hepatocellular carcinoma
How are mycoses classified?
- Superficial infections
- Cutaneous mycoses
- Subcutaneous mycoses
- Systemic mycoses
How are mycoses diagnosed?
- Clinical presentation: the first indication of a potential systemic mycosis is the failure to respond to antibacterial antibiotics
- Laboratory diagnosis: microscopic examination, serology, PCR, fungal culture
What are the types of specimens collected for diagnosis of a mycosis?
- Ringworm: skin scales, nail clippings, scalp scrapings with hair stubs and skin scales (folded into paper squares for transport)
- Candidiasis: mucous membrane swabs (transported in clear transport media)
- Subcutaneous infections: scrapings and crusts, aspirated pus, biopsies
- Systemic infections: specimens collected from the affected sites (e.g. bronchoalveolar lavage fluid, urine, wounds, genital swabs)
How are mycotic specimens prepared for microscopic examination?
- Wet mount after partial digestion of the tissue by 10–20% KOH
- Addition of calcofluor white aids in identification of the fungal cell wall in fluorescence microscopy
- Special stains include: methylene blue, lactophenol blue, PAS stain, ink
What are the components of a fungal cell culture?
- Sabouraud’s dextrose agar, which supports fungi and restricts growth of bacteria
- Supplementation with chloramphenicol to inhibit bacteria and cycloheximide to inhibit saprophytic molds
What are the classes of antifungal agents?
- Polyene derivatives
- Azoles
- Griseofulvin
- Flucytosine (5-fluorocytosine)
- Allylamines
- Echinocandins
What is the mechanism of action of most antifungal drugs?
Targeting the synthesis or incorporation of ergosterol into the cell membrane
What are the fungicidal antifungals?
The polyene derivatives: amphotericin B and nystatin
What are the polyene derivatives?
- Amphotericin B
- Nystatin
What are the features of amphotericin B?
- The most effective drug for severe systemic mycoses due to its broad spectrum and low incidence of resistance
- Administered IV only
- Forms complexes with already formed ergosterol in the cell membrane, causing cell lysis (fungicidal)
What are the features of nystatin/
- Similar mechanism of action to amphotericin B (fungicidal)
- Used to treat local candidal infections of the mouth and vagina
- Too toxic for parenteral administration
What are the azoles?
- Ketoconazole
- Fluconazole
- Voriconazole
- Itraconazole
- Posaconazole
What are the features of the azoles?
- Interfere with the synthesis of ergosterol (fungistatic)
- Oral drugs used to treat a wide range of systemic and localized mycoses
What are the features of griseofulvin?
- Administered orally
- Interferes with fungal mitosis (fungistatic)
- Used to treat dermatophytoses only—has no effect on other fungi
What are the features of flucytosine (5-fluorocytosine)?
- Disrupts nucleic acid and protein synthesis (fungistatic)
- Administered orally, usually in conjunction with amphotericin B to treat cryptococcosis and candidiasis
What are the allylamines?
Terbinafine
What are the features of terbinafine?
Inhibits ergosterol synthesis at the earliest stage (fungistatic)
What are the features of the echinocandins?
Inhibit ergosterol synthesis (fungistatic)
What are the features of superficial mycoses?
- They affect the skin (stratum corneum) only
- There is no tissue invasion, little tissue destruction, and therefore no immune response
- The organisms are commensal microbiotal fungi in overgrowth
What diseases do Malassezia spp. cause?
- Pityriasis versicolor
- Seborrheic dermatitis (dandruff)
- Catheter-associated infections
What are the morphologic features of Malassezia spp.?
- Lipophilic yeasts that are round in shape
- The single cell yeasts become hyphated molds, appearing in a “spaghetti and meatballs” shape. The hyphae are septate
- Note: this is not true dimorphism as it does not relate to temperature
What species cause pityriasis versicolor?
- Malassezia furfur
- Malassezia globosa
What are the features of pityriasis veriscolor?
- Discrete, serpentine maculae on the chest, upper back, arms, or abdomen
- The maculae are hyperpigmented in light-skinned patients and hypopigmented in dark-skinned patients
- Depigmentation is caused by carboxylic acids produced by the yeasts
- Maculae may coalesce, but show limited scaling, inflammation, or irritation
- More common in the tropics (elevated temperature and humidity)
How is pityriasis versicolor diagnosed?
- Skin scrapings are examined under a microscope using KOH and ink stains
- Short unbranched, nonpigmented hyphae and spherical cells are observed
- The lesions fluoresce under Wood’s lamp (UV irradiation), giving a pale green color
What species cause seborrheic dermatitis (dandruff)?
Malassezia furfur
What are the features of seborrheic dermatitis (dandruff)?
- Skin hyperproliferation
- Red lesions on the scalp covered with greasy scales, causing itching
How are Malassezia infections treated?
- Some cases resolve spontaneously
- Topical azoles (as a cream or shampoo) are applied for 2 weeks for cosmetic reasons
- In severe cases, oral azoles are used
- There is a high recurrence rate
What are the features of cutaneous mycoses?
- Affect the skin and dermis only
- Do trigger an immune response, resulting in localized symptoms
Which genera are collectively known as dermatophytes?
- Microsporum
- Trichophyton
- Epidermophyton
What is the common name of dermatophytes?
Ringworm
How are dermatophytes transmitted?
- Contact with contaminated soil (geophilic species)
- Direct contact (anthropophilic species)
- Contact with infected animals (zoophilic species)
- Any underlying skin conditions may increase risk of infection as the skin is the most important barrier to infection
- Heat and humidity enhance infection
What are the diseases caused by dermatophytes?
- Tinea pedis (athlete’s foot)
- Tinea unuguium (nail)
- Tinea corporis (body)
- Tinea cruris (groin area)
- Tinea manus (hand)
- Tinea capitis (head)
- Tinea barbae (bearded region)
What are the features of tinea pedia (athlete’s foot)?
- Most prevalent dermatophytosis
- Chronic infection of the toe webs
- Initially, there is itching between the toes and the development of small vesicles that rupture and discharge a thin fluid
- The skin of the toe webs becomes macerated and peels, and the cracks that appear are prone to secondary bacterial infection
- Rarely, severe maceration and loss of foot volume can occur
What are the features of tinea unguium?
- Oftentimes follows prolonged tinea pedis or tinea manus
- The nails become yellow, brittle, thickened, and crumbly
- This is painless, unlike paronychia associated with candidiasis
What are the features of tinea corporis?
- Ring-like lesions with a clearing, scaly center surrounded by a red advancing border that may be dry or vesicular
- Fungal metabolites, enzymes, and antigens diffuse through the epidermis leading to erythema, vesicle formation, and pruritus
What are the features of tinea capitis?
- Infection begins in the skin of the scalp and spreads down the keratinized wall of the hair follice
- Tinea favus is an acute inflammatory infection of the hair follice, which may lead to permanint hair loss and black spots on the affected area
How are dermatophytes diagnosed?
- Microscopic examination
- Culture
How are dermatophyte samples prepared and examined microscopically?
- Specimens are scrapings from the skin, nails, and hair
- Specimens are prepared in a wet mount using 10% KOH
- The fungi appear as branching hyphae
- Hyphae or spores are detected either in the hair (endothrix) or around the hair (ectothrix)
How are dermatophytes identified in culture?
- Specimens are grown on Sabouraud’s dextrose agar at room temperature for 4 weeks
- The arising colonies are examined microscopically after staining with lactophenol cotton blue
How are species of dermatophyte distinguished?
Their microscopic morphology:
- Microsporum: thick-walled, multicellular, spindle-shaped
- Trichophyton: large, smooth, thin wall, pencil-shaped, septate hyphae
- Epidermophyton floccosum: bifurcated hyphae with 2–4 cells; multiple smooth club-shaped spores (macroconidia)
How are dermatophytes treated?
- Local antifungal cream (e.g. miconazole)
- Oral terbinafine for weeks to months
What are the features of subcutaneous mycoses?
- The causative agents typically reside in soil or on vegetation
- Infection occurs often by traumatic implantation, with lesions expanding slowly from the area of inoculation
- Inflammation and cell damage are triggered
What is the causative agent of sporotrichosis?
Sporothrix schenkii
What disease does Sporotrhix schenkii cause?
Sporotrichosis
How is sporotrichosis transmitted?
A variety of plants, but especially rose bushes
What are the features of sporotrichosis?
Granulomatous infection—either skin nodules or pulmonary sporotrichosis
What is the causative agent of fungal mycetoma (Madura foot; maduromycosis)
Madurella mycetomatis, a mold with septate hyphae
What disease does Madurella mycetomatis cause?
Fungal mycetoma (Madura foot; maduromycosis)
What are the features of fungal mycetoma?
- Affects subcutaneous tissues of the feet
- Swelling follows trauma, with purplish discoloration and multiples sinuses that drain pus containing yellow, white, red, or black granules
- In fungal mycetoma, the abscesses are not painful
How is fungal mycetoma diagnosed?
- Culture on SDA
- Clinical presentation: black granules are typical of fungal mycetoma
- Microscopic examination: septate hyphae with intercalary chlamydospores—spores in the middle of hyphae
How is fungal mycetoma treated?
- Chemotherapy: itraconazole, ketoconazole, amphotericin B
- Surgical debridement or excision
In which group do opportunistic mycoses occur?
- Immunosuppressed patients
- Patients with immunodeficiencies (e.g. AIDS patients with CD4+ T cell counts of <100 µL–1)
- Pregnant women
- Patients who have undergone extensive broad-spectrum antibiotic therapy
What are the predisposing and risk factors for candidiasis?
- Certain diseases, e.g. AIDS, diabetes mellitus
- Persistent use of antibiotics or corticosteroids
- General debility (e.g. ICU patients)
- Urinary, venous, or arterial catheters
What is the most clinically significant Candida species?
Candida albicans
What are the diseases caused by Candida?
- Cutaneous/mucosal candidiasis: thrush, vulvovaginitis, skin infection, diaper pseudorash
- Candidemia
Where are the typical locations of skin infection with Candida?
- Axillae
- Groin
- Intergluteal folds (diaper pseudorash in newborns)
- Inframammary folds
What are the features of skin candidiasis?
- Infected areas become red and may develop “weeping” vesicles
- Paronychia: erythematous swelling of the nail fold
- Interdigital candidiasis: forms a web between fingers. Prominent in those who skin hands in water extensively, e.g. homemakers, bartenders, cooks
What is thrush?
Mucosal candidiasis on the tongue, lips, gums or palate, causing whitish pseudomembranous lesions
What is vulvovaginitis?
Mucosal candidiasis of the vagina, characterized by irritation, pruritus, and vaginal discharge
What are the features of candidemia?
- Caused by indwelling catheters, surgery, intravenous drug abuse, aspiration, or damage to the skin or GI tract
- Lesions may occur anywhere, especially the kidney, skin, respiratory tract, eye, heart, and meninges
How are Candida samples identified microscopically?
- Specimens include swabs and scrapings from superficial lesions, tissue biopsies, and exudates
- Appear as oval, buding yeast cells; some form pseudohyphae
How are Candida samples identified in culture?
Appear as creamy colonies on SDA agar:
- Gram-positive
- Produce germ tubes in human serum at 37ºC for 30 minutes
- Form chlamydospores on cornmeal agar
- Ferment glucose and maltose with acid and gas formation
What is the causative agent of cryptococcosis?
Cryptococcus neoformans
What disease does Cryptococcus neoformans cause?
Cryptococcosis
How is cryptococcosis transmitted?
- Inhalation of desiccated yeast cells from dry pigeon feces, trees, soil, etc.
- The yeasts migrate to the central nervous system where they cause meningoencephalitis, or to the lungs
How is cryptococcosis diagnosed?
- Microscopic examination of CSF samples in wet mounts with or without India ink
- Culture: either SDA or bird seed culture
- PCR
What are the diseases caused by Aspergillus spp.?
- Allergic aspergillosis
- Invasive aspergillosis
What are the features of allergic aspergillosis?
- Immediate asthmatic reaction upon second exposure after sensitization (mostly for A. fumigatus)
- In others, the conidia germinate and hyphae colonize the bronchial tree without invading the lung parenchyma—allergic bronchopulmonary aspergillosis
- Normal hosts exposed to massive doses of conidia can develop extrinsic allergic alveolitis
- People with preexisting pulmonary lesions develop cavitary lesions filled with aspergillomas/fungomas
What are the features of invasive aspergillosis?
- Occurs in AIDS patients, people taking corticosteroids, or people with lymphomas and leukemias
- Hyphae invade the lumens and walls of blood vessels, causing thrombosis, infarction, and necrosis
- From the lungs, the disease may spread to the GI tract, kidney, liver, or brain, causing abscesses and necrotic lesions
What is the causative agent of mucormycosis (zygomycosis)?
Molds of the order Mucorales, especially Rhizopus, Mucor, and Absidia
What is the pathogenesis of mucormycosis (zygomycosis)?
Air contaminants enter the nose, eyes, heart, and brain of susceptible individuals
How is mucormycosis (zygomycosis) diagnosed?
Direct examiantion or culture of nasal discharge, tissue, or sputum by direct smear
How is mucormycosis (zygomycosis) treated?
- Aggressive surgical debridement
- Rapid administration of amphotericin B
- Control of the underlying disease (e.g. diabetes)
What is the causative agent of pneumocystis pneumonia?
Pneumocystis jiroveci
How is pneumocystis pneumonia diagnosed?
- BAL, lung biopsy, or induced sputum samples are stained and examined for presence of cysts or trophozoites
- Stained with eosin methylene blue or silver stains
- Cannot be culture in vitro
How is pneumocystis pneumonia treated?
Trimethoprim-sulfamethoxazole (cotrimoxazole)
What are the endemic mycoses?
- Coccidiodiomycosis
- Histoplasmosis
- Blastomycosis
- Paracoccidiodiomycosis
What are the features of the endemic mycoses?
- Most infections are asymptomatic or mild and resolve without treatment
- A small number of patients develop pulmonary disease
What are the types of fungi, by growth form?
- Molds
- Yeasts
What are the types of fungi, by growth location?
- Saprophytic: live on dead and decaying tissue
- Parasitic: live on living organisms
