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16 Cards in this Set
- Front
- Back
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.Fungi are eukaryotes found almost exclusively in soil, water, or on plants
1. Of the 200,000 or more species, about 200 can cause human disease |
f.
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.Conidia are
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.asexual reproductive elements
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.Spores are
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.sexual reproductive elements
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Fungi differ from animal cells in two ways that are exploited for medical use
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1. fungi contain a rigid cell wall containing chitin that can be visualized after 10% KOH treatment of infected tissue
2. instead of cholesterol, fungal membranes contain ergosterol which is a target for several anti-fungal drugs |
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Two basic morphological forms
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yeast and molds
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yeast are
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single celled
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molds
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are multicellular
a. develop tube-like extensions called hyphae b. hyphae can be septate or non-septate c. an intertwined mass of hyphae is called a mycelium |
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several pathogenic fungi grow in both forms: dimorphic
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a. typically in the yeast form in tissue and at 37 C in culture
b. mold form in culture at room temperature |
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Fungi cause 3 types of problems
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A. Allergies
B. Mycotoxicoses (eg, mushroom poisoning) C. Mycoses (true infections) |
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A. Human pathogens fall into 4 phyla
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1. Zygomycetes
2. Ascomycetes 3. Basidiomycetes 4. Deuteromycetes (fungi imperfecti) a. lack a sexual stage |
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Better working classification (increasing severity)
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1. Superficial mycoses
a. only the outermost layer of skin b. often from poor hygiene 2. Cutaneous mycoses a. tineas (“ring worm,”, athlete’s foot) 3. Subcutaneous mycoses a. often from wound to the skin b. can involve the lymphatics 4. Systemic mycoses a. mostly from inhalation of conidia or spores b. can cause mild to severe disease in healthy individuals c. most virulent of the fungi d. can cause chronic infection resembling tuberculosis 5. Opportunistic mycoses a. found predominantly in immunocompromised patients b. severe infections that do not respond to antibiotics |
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In general, fungi produce sub-acute, self-limiting infections in normal healthy individuals, but are often opportunistic pathogens that can cause severe systemic disease in immunocompromised patients
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B. In the last decade, the incidence of mycotic infections has rapidly increased
1. predominantly nosocomial complications of at-risk hospitalized patients 2. increased survival times of AIDS, chemotherapy, and transplantation patients 3. improved technologies: intravascular catheters, better immunosuppressive drugs |
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C. Fungi must cross an initial barrier to infect
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1. mechanical breaks
2. inhalation of spores or conidia 3. invasive hyphae formation |
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D. Normal healthy individuals have high immunity to many fungi
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1. Opportunistic molds are sensitive to neutrophils
2. T-cell immunity is of primary importance 3. Antibodies can play a role in preventing infection 4. The immune response leads to much of the cellular harm seen in fungal infections |
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Diagnostics
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A. Clinical appearance
B. Microscopic examination of samples treated with 10% KOH: look for hyphae C. Culture on Sabouraud’s medium 1. low pH, high glucose inhibits bacterial growth 2. antibiotics are sometimes incorporated to further inhibit bacterial growth 3. necessary because fungi are slow-growing, bacteria would overwhelm culture dish D. Yeast growth at 37 C, biochemical assays used in identification E. Mold growth at room temperature, mycelium’s appearance for identification F. Serology for some systemic infections, but cross-reactivity can be a problem |
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VI. Treatment
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A. Antifungals will be covered in a separate lecture, but general principles
1. Because fungal metabolism is slow, treatment is for long periods 2. Many treatment failures/relapses 3. Relatively few antifungals because they are eukaryotes, treatments are fairly toxic 4. Fungal membrane (ergosterol), cell wall, and genome replication are common targets B. Treatment guidelines 1. In general, localized or superficial/cutaneous infections are left to observation or topical creams 2. Disseminated infections are treated with IV or oral drugs 3. Immunocompromised patients are left on prophylaxis for life to limit recurrences |
